Glucokinase
Encyclopedia
Glucokinase is an enzyme
that facilitates phosphorylation
of glucose
to glucose-6-phosphate
. Glucokinase occurs in cells
in the liver
, pancreas
, gut
, and brain
of humans and most other vertebrate
s. In each of these organs it plays an important role in the regulation of carbohydrate
metabolism
by acting as a glucose sensor
, triggering shifts in metabolism or cell function in response to rising or falling levels of glucose, such as occur after a meal or when fasting
. Mutation
s of the gene
for this enzyme can cause unusual forms of diabetes
or hypoglycemia
.
Glucokinase (GK) is a hexokinase
isozyme
, related homologously
and by evolution
to at least three other hexokinases. All of the hexokinases can mediate phosphorylation of glucose to glucose-6-phosphate (G6P), which is the first step of both glycogen
synthesis and glycolysis
. However, glucokinase is coded
by a separate gene
and its distinctive kinetic
properties allow it to serve a different set of functions. Glucokinase has a lower affinity for glucose than the other hexokinases do, and its activity is localized to a few cell types, leaving the other three hexokinases as more important preparers of glucose for glycolysis and glycogen synthesis for most tissues and organs. Because of this reduced affinity, the activity of glucokinase, under usual physiological condition
s, varies substantially according to the concentration of glucose.
2.7.1.1 (previously 2.7.1.2). The common name, glucokinase, is derived from its relative specificity for glucose under physiologic conditions.
Some biochemists
have argued that the name glucokinase should be abandoned as misleading, as this enzyme can phosphorylate other hexoses in the right conditions, and there are distantly related enzymes in bacteria with more absolute specificity for glucose that better deserve the name and the EC
2.7.1.2. Nevertheless, glucokinase remains the name preferred in the contexts of medicine
and mammalian physiology
.
Another mammalian glucose kinase, ADP-specific glucokinase
, was discovered in 2004. The gene is distinct and similar to that of primitive organisms. It is dependent on ADP
rather than ATP (suggesting the possibility of more effective function during hypoxia
), and the metabolic role and importance remain to be elucidated.
of physiologic importance of glucokinase is glucose
, and the most important product
is glucose-6-phosphate
(G6P). The other necessary substrate, from which the phosphate is derived, is adenosine triphosphate
(ATP), which is converted to adenosine diphosphate
(ADP) when the phosphate is removed. The reaction catalyzed by glucokinase is:
ATP participates in the reaction in a form complexed to magnesium
(Mg) as a cofactor
. Furthermore, under certain conditions, glucokinase, like other hexokinases, can induce phosphorylation of other hexose
s (6 carbon sugar
s) and similar molecules. Therefore the general glucokinase reaction is more accurately described as:
Among the hexose substrates are mannose
, fructose
, and glucosamine
, but the affinity of glucokinase for these requires concentrations not found in cells for significant activity.
properties distinguish glucokinase from the other hexokinases, allowing it to function in a special role as glucose sensor.
These two features allow it to regulate a "supply-driven" metabolic pathway. That is, the rate of reaction is driven by the supply of glucose, not by the demand for end products.
Another distinctive property of glucokinase is its moderate cooperativity
with glucose, with a Hill coefficient (nH) of about 1.7. Glucokinase has only a single binding site for glucose and is the only monomeric regulatory enzyme known to display substrate cooperativity. The nature of the cooperativity has been postulated to involve a "slow transition" between two different enzyme states with different rates of activity. If the dominant state depends upon glucose concentration, it would produce an apparent cooperativity similar to that observed.
Because of this cooperativity, the kinetic interaction of glucokinase with glucose does not follow classical Michaelis-Menten kinetics
. Rather than a Km for glucose, it is more accurate to describe a half-saturation level S0.5, which is the concentration at which the enzyme is 50% saturated and active.
The S0.5 and nH extrapolate to an "inflection point" of the curve describing enzyme activity as a function of glucose concentration at about 4 mmol/L. In other words, at a glucose concentration of about 72 mg/dl, which is near the low end of the normal range, glucokinase activity is most sensitive to small changes in glucose concentration.
The kinetic relationship with the other substrate, MgATP, can be described by classical Michaelis-Menten kinetics, with an affinity at about 0.3-0.4 mmol/L, well below a typical intracellular concentration of 2.5 mmol/L. The fact that there is nearly always an excess of ATP available implies that ATP concentration rarely influences glucokinase activity.
The maximum specific activity (Kcat, also known as the turnover rate) of glucokinase when saturated with both substrates is 62/s.
A "minimal mathematical model" has been devised based on the above kinetic information to predict the beta cell glucose phosphorylation rate (BGPR) of normal ("wild type") glucokinase and the known mutations. The BGPR for wild type glucokinase is about 28% at a glucose concentration of 5 mmol/l, indicating that the enzyme is running at 28% of capacity at the usual threshold glucose for triggering insulin release.
s surround the glucose binding site. All except cys 230 are essential for the catalytic process, forming multiple disulfide bridges during interaction with the substrates and regulators. At least in the beta cells, the ratio of active to inactive glucokinase molecules is at least partly determined by the balance of oxidation of sulfhydryl groups or reduction of disulfide bridges.
These sulfhydryl groups are quite sensitive to the oxidation status of the cells, making glucokinase one of the components most vulnerable to oxidative stress, especially in the beta cells.
Glucokinase is a monomer
ic protein of 465 amino acid
s and a molecular weight of about 50 kD. There are at least two clefts, one for the active site
, binding glucose and MgATP, and the other for a putative allosteric activator
that has not yet been identified.
This is about half the size of the other mammalian hexokinases, which retain a degree of dimeric structure. Several sequences and the three-dimensional structure of the key active sites. The ATP binding domain, for example, are shared with hexokinases, bacterial glucokinases, and other proteins, and the common structure is termed an actin fold.
on chromosome 7. This single autosomal
gene has 10 exon
s. Genes for glucokinase in other animals are homologous to human GCK.
A distinctive feature of the gene is that it begins with two promoter regions. The first exon
from the 5' end contains two tissue-specific promoter regions. Transcription
can begin at either promoter (depending on the tissue) so that the same gene can produce a slightly different molecule in liver and in other tissues. The two isoforms of glucokinase differ only by 13-15 amino acid
s at the N-terminal end
of the molecule, which produces only a minimal difference in structure. The two isoforms have the same kinetic and functional characteristics.
The first promoter from the 5' end, referred to as the "upstream" or neuroendocrine promoter, is active in pancreatic islet cells, neural tissue, and enterocytes (small intestine
cells) to produce the "neuroendocrine isoform" of glucokinase. The second promoter, the "downstream" or liver promoter, is active in hepatocyte
s and directs production of the "liver isoform". The two promoters have little or no sequence homology and are separated by a 30 kbp
sequence of unknown function. The two promoters are functionally exclusive and governed by distinct sets of regulatory factors, so that glucokinase expression can be regulated separately in different tissue types. The two promoters correspond to two broad categories of glucokinase function: In liver, glucokinase acts as the gateway for the "bulk processing" of available glucose, while, in the neuroendocrine cells, it acts as a sensor, triggering cell responses that affect body-wide carbohydrate metabolism.
, and brain. All play crucial roles in responding to rising or falling levels of blood glucose.
s and bat
s, though some reptile
s, bird
s, amphibians, and fish
have it. Whether glucokinase occurs similarly in the pancreas and other organs has not yet been determined. It has been postulated that the presence of glucokinase in liver reflects the ease with which carbohydrates can be included in the animals' diet
s.
(G6P) by glucokinase is the first step of both glycogen
synthesis and glycolysis
in the liver.
When ample glucose is available, glycogen synthesis proceeds at the periphery of the hepatocytes until the cells are replete with glycogen. Excess glucose is then increasingly converted into triglyceride
s for export and storage in adipose tissue. Glucokinase activity in the cytoplasm rises and falls with available glucose.
G6P, the product of glucokinase, is the principal substrate of glycogen synthesis, and glucokinase has a close functional and regulatory association with glycogen synthesis. When maximally active, GK and glycogen synthase
appears to be located in the same peripheral areas of hepatocyte cytoplasm in which glycogen synthesis occurs. The supply of G6P affects the rate of glycogen synthesis not only as the primary substrate, but by direct stimulation of glycogen synthase and inhibition of glycogen phosphorylase
.
Glucokinase activity can be rapidly amplified or damped in response to changes in the glucose supply, typically resulting from eating and fasting. Regulation occurs at several levels and speeds, and is influenced by many factors that affect mainly two general mechanisms:
-1c (SREBP1c) is thought to be the most important direct activator of glucokinase gene transcription in hepatocytes. SREBP1c is a basic helix-loop-helix zipper
(bHLHZ) transactivator. This class of transactivators bind to the "E box" sequence of genes for a number of regulatory enzymes. The liver promoter in the first exon of the glucokinase gene includes such an E box, which appears to be the principal insulin-response element of the gene in hepatocytes. SREBP1c must be present for transcription of glucokinase in hepatocytes. SREBP1c increases in response to a high-carbohydrate diet, presumed as a direct effect of frequent insulin elevation. Increased transcription can be detected in less than an hour after hepatocytes are exposed to rising insulin levels.
Fructose-2,6-bisphosphate (F2,6P2) also stimulates GK transcription, it seems by way of Akt2 rather than SREBP1c. It is not known whether this effect is one of the downstream effects of activation of insulin receptors or independent of insulin action. Levels of F2,6P2 play other amplifying roles in glycolysis in hepatocytes.
Other transacting factors suspected of playing a role in liver cell transcription regulation include:
is by far the most important of the hormones that have direct or indirect effects on glucokinase expression and activity in the liver. Insulin appears to affect both glucokinase transcription and activity through multiple direct and indirect pathways. While rising portal vein glucose levels increase glucokinase activity, the concomitant rise of insulin amplifies this effect by induction
of glucokinase synthesis. Glucokinase transcription begins to rise within an hour of rising insulin levels. Glucokinase transcription becomes nearly undetectable in prolonged starvation, severe carbohydrate deprivation, or untreated insulin-deficient diabetes.
The mechanisms by which insulin induces glucokinase may involve both of the major intracellular pathways of insulin action, the extracellular signal-regulated kinase (ERK 1/2) cascade, and the phosphoinositide 3-kinase (PI3-K) cascade. The latter may operate via the FOXO1 transactivator.
However, as would be expected given its antagonistic effect on glycogen synthesis, glucagon
and its intracellular second messenger cAMP
suppresses glucokinase transcription and activity, even in the presence of insulin.
Other hormones such as triiodothyronine
(T3) and glucocorticoid
s provide permissive or stimulatory effects on glucokinase in certain circumstances. Biotin
and retinoic acid
increase GCK mRNA transcription as well as GK activity. Fatty acid
s in significant amounts amplify GK activity in the liver, while long chain acyl CoA inhibits it.
), which operates to maintain an inactive reserve of GK, which can be made quickly available in response to rising levels of portal vein glucose.
GKRP moves between nucleus
and cytoplasm
of the hepatocytes and may be tethered to the microfilament cytoskeleton
. It forms reversible 1:1 complexes with GK, and can move it from the cytoplasm into the nucleus. It acts as a competitive inhibitor with glucose, such that the enzyme activity is reduced to near-zero while bound. GK:GKRP complexes are sequestered in the nucleus while glucose and fructose levels are low. Nuclear sequestration may serve to protect GK from degradation by cytoplasmic protease
s. GK can be rapidly released from GKRP in response to rising levels of glucose. Unlike GK in beta cells, GK in hepatocytes is not associated with mitochondria.
Fructose
in tiny (micromolar) amounts (after phosphorylation by ketohexokinase to fructose-1-phosphate
(F1P)) accelerates release of GK from GKRP. This sensitivity to the presence of small amounts of fructose allows GKRP, GK, and ketohexokinase to act as a "fructose sensing system," which signals that a mixed carbohydrate meal is being digested, and accelerates the utilization of glucose. However, fructose 6-phosphate
(F6P) potentiates binding of GK by GKRP. F6P decreases phosphorylation of glucose by GK when glycogenolysis
or gluconeogenesis
are underway. F1P and F6P both bind to the same site on GKRP. It is postulated that they produce 2 different conformations of GKRP, one able to bind GK and the other not.
s, glucokinase activity serves as a principal control for the secretion of insulin
in response to rising levels of blood glucose. As G6P is consumed, increasing amounts of ATP initiate a series of processes that result in release of insulin. One of the immediate consequences of increased cellular respiration is a rise in the NADH and NADPH concentrations (collectively referred to as NAD(P)H). This shift in the redox status of the beta cells results in rising intracellular calcium
levels, closing of the KATP channels
, depolarization of the cell membrane, merging of the insulin secretory granules with the membrane, and release of insulin into the blood.
It is as a signal for insulin release that glucokinase exerts the largest effect on blood sugar levels and overall direction of carbohydrate metabolism. Glucose, in turn, influences both the immediate activity and the amount of glucokinase produced in the beta cells.
A second important rapid regulator of glucokinase activity in beta cells occurs by direct protein-protein interaction between glucokinase and the "bifunctional enzyme" (phosphofructokinase-2/fructose-2,6-bisphosphatase),
which also plays a role in the regulation of glycolysis. This physical association stabilizes glucokinase in a catalytically favorable conformation (somewhat opposite the effect of GKRP binding) that enhances its activity.
In as little as 15 minutes, glucose can stimulate GCK transcription and glucokinase synthesis by way of insulin. Insulin is produced by the beta cells, but some of it acts on beta cell B-type insulin receptor
s, providing an autocrine positive-feedback amplification of glucokinase activity. Further amplification occurs by insulin action (via A-type receptors) to stimulate its own transcription.
Transcription of the GCK gene is initiated through the "upstream," or neuroendocrine, promoter. This promoter, in contrast to the liver promoter, has elements homologous to other insulin-induced gene promoters. Among the probable transacting factors are Pdx-1 and PPARγ. Pdx-1 is a homeodomain transcription factor involved in the differentiation of the pancreas. PPARγ is a nuclear receptor that responds to glitazone drugs by enhancing insulin sensitivity.
s, but the evidence is less consistent, and some researchers have found no evidence of glucokinase activity in these cells. Alpha cells occur in pancreatic islets, mixed with beta and other cells. While beta cells respond to rising glucose levels by secreting insulin, alpha cells respond by reducing glucagon
secretion. When blood glucose concentration falls to hypoglycemic
levels, alpha cells release glucagon. Glucagon is a protein hormone that blocks the effect of insulin on hepatocytes, inducing glycogenolysis, gluconeogenesis, and reduced glucokinase activity in hepatocytes. The degree to which glucose suppression of glucagon is a direct effect of glucose via glucokinase in alpha cells, or an indirect effect mediated by insulin or other signals from beta cells, is still uncertain.
s use glucose for fuel, certain glucose-sensing neurons alter their firing rates in response to rising or falling levels of glucose. These glucose-sensing neurons are concentrated primarily in the ventromedial nucleus
and arcuate nucleus
of the hypothalamus
, which regulate many aspects of glucose homeostasis (especially the response to hypoglycemia), fuel utilization, satiety and appetite
, and weight
maintenance. These neurons are most sensitive to glucose changes in the 0.5-3.5 mmol/L glucose range.
Glucokinase has been found in the brain in largely the same areas that contain glucose-sensing neurons, including both of the hypothalamic nuclei. Inhibition of glucokinase abolishes the ventromedial nucleus response to a meal. However, brain glucose levels are lower than plasma levels, typically 0.5-3.5 mmol/L. Although this range is matches the sensitivity of the glucose-sensing neurons, it is below the optimal inflection sensitivity for glucokinase. The presumption, based on indirect evidence and speculation, is that neuronal glucokinase is somehow exposed to plasma glucose levels even in the neurons.
and stomach, its function and regulation have not been worked out. It has been suggested that here, also, glucokinase serves as a glucose sensor, allowing these cells to provide one of the earliest metabolic responses to incoming carbohydrates. It is suspected that these cells are involved in incretin
functions.
Around 200 mutation
s of the human glucokinase gene
GCK have been discovered, that can change the efficiency of glucose binding and phosphorylation, increasing or decreasing the sensitivity of beta cell insulin secretion in response to glucose, and producing clinically significant hyperglycemia
or hypoglycemia
.
, type 2 (MODY2).
Homozygosity for GCK alleles with reduced function can cause severe congenital insulin deficiency, resulting in persistent neonatal diabetes.
, or fasting or reactive hypoglycemia appearing at an older age.
Homozygosity for gain of function mutations has not been found.
are researching molecules that activate glucokinase in hope that it will be useful in the treatment of type 2 diabetes
.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
that facilitates phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
of glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
to glucose-6-phosphate
Glucose-6-phosphate
Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. This compound is very common in cells as the vast majority of glucose entering a cell will become phosphorylated in this way....
. Glucokinase occurs in cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
in the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, pancreas
Pancreas
The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
, gut
Gut (zoology)
In zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
, and brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
of humans and most other vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s. In each of these organs it plays an important role in the regulation of carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
by acting as a glucose sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
, triggering shifts in metabolism or cell function in response to rising or falling levels of glucose, such as occur after a meal or when fasting
Fasting
Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. An absolute fast is normally defined as abstinence from all food and liquid for a defined period, usually a single day , or several days. Other fasts may be only partially restrictive,...
. Mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
s of the gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
for this enzyme can cause unusual forms of diabetes
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
or hypoglycemia
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...
.
Glucokinase (GK) is a hexokinase
Hexokinase
A hexokinase is an enzyme that phosphorylates a six-carbon sugar, a hexose, to a hexose phosphate. In most tissues and organisms, glucose is the most important substrate of hexokinases, and glucose-6-phosphate the most important product....
isozyme
Isozyme
Isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters Isozymes (also known as isoenzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes...
, related homologously
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
and by evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
to at least three other hexokinases. All of the hexokinases can mediate phosphorylation of glucose to glucose-6-phosphate (G6P), which is the first step of both glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
synthesis and glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
. However, glucokinase is coded
Genetic code
The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....
by a separate gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
and its distinctive kinetic
Enzyme kinetics
Enzyme kinetics is the study of the chemical reactions that are catalysed by enzymes. In enzyme kinetics, the reaction rate is measured and the effects of varying the conditions of the reaction investigated...
properties allow it to serve a different set of functions. Glucokinase has a lower affinity for glucose than the other hexokinases do, and its activity is localized to a few cell types, leaving the other three hexokinases as more important preparers of glucose for glycolysis and glycogen synthesis for most tissues and organs. Because of this reduced affinity, the activity of glucokinase, under usual physiological condition
Physiological condition
Physiological condition or, more often "physiological conditions" is a term used in biology, biochemistry, and medicine. It refers to conditions of the external or internal milieu that may occur in nature for that organism or cell system, in contrast to artificial laboratory conditions...
s, varies substantially according to the concentration of glucose.
Nomenclature
Alternative names for this enzyme are: human hexokinase IV, hexokinase D, and ATP:D-hexose 6-phosphotransferase, ECEC number
The Enzyme Commission number is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze....
2.7.1.1 (previously 2.7.1.2). The common name, glucokinase, is derived from its relative specificity for glucose under physiologic conditions.
Some biochemists
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
have argued that the name glucokinase should be abandoned as misleading, as this enzyme can phosphorylate other hexoses in the right conditions, and there are distantly related enzymes in bacteria with more absolute specificity for glucose that better deserve the name and the EC
EC number
The Enzyme Commission number is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze....
2.7.1.2. Nevertheless, glucokinase remains the name preferred in the contexts of medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
and mammalian physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
.
Another mammalian glucose kinase, ADP-specific glucokinase
ADP-specific glucokinase
In enzymology, an ADP-specific glucokinase also known as ADP-dependent glucokinase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are ADP and D-glucose, whereas its two products are AMP and D-glucose 6-phosphate.This enzyme belongs to the family of...
, was discovered in 2004. The gene is distinct and similar to that of primitive organisms. It is dependent on ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
rather than ATP (suggesting the possibility of more effective function during hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
), and the metabolic role and importance remain to be elucidated.
Substrates and products
The principal substrateSubstrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
of physiologic importance of glucokinase is glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, and the most important product
Product (chemistry)
Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...
is glucose-6-phosphate
Glucose-6-phosphate
Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. This compound is very common in cells as the vast majority of glucose entering a cell will become phosphorylated in this way....
(G6P). The other necessary substrate, from which the phosphate is derived, is adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(ATP), which is converted to adenosine diphosphate
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
(ADP) when the phosphate is removed. The reaction catalyzed by glucokinase is:
ATP participates in the reaction in a form complexed to magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
(Mg) as a cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....
. Furthermore, under certain conditions, glucokinase, like other hexokinases, can induce phosphorylation of other hexose
Hexose
In organic chemistry, a hexose is a monosaccharide with six carbon atoms, having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2....
s (6 carbon sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s) and similar molecules. Therefore the general glucokinase reaction is more accurately described as:
-
- Hexose + MgATP2- → hexose-PPhosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
OOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
32- + MgADP- + H+
- Hexose + MgATP2- → hexose-P
Among the hexose substrates are mannose
Mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....
, fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
, and glucosamine
Glucosamine
Glucosamine is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of the polysaccharides chitosan and chitin, which compose the exoskeletons of crustaceans and other arthropods, cell walls in fungi and...
, but the affinity of glucokinase for these requires concentrations not found in cells for significant activity.
Kinetics
Two important kineticEnzyme kinetics
Enzyme kinetics is the study of the chemical reactions that are catalysed by enzymes. In enzyme kinetics, the reaction rate is measured and the effects of varying the conditions of the reaction investigated...
properties distinguish glucokinase from the other hexokinases, allowing it to function in a special role as glucose sensor.
- Glucokinase has a lower affinity for glucose than the other hexokinases. Glucokinase changes conformation and/or function in parallel with rising glucose concentrations in the physiologically important range of 4-10 mmol/L (72-180 mg/dl). It is half-saturated at a glucose concentration of about 8 mmol/L (144 mg/dl).
- Glucokinase is not inhibited by its product, glucose-6-phosphate. This allows continued signal output (e.g., to trigger insulinInsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
release) amid significant amounts of its product
These two features allow it to regulate a "supply-driven" metabolic pathway. That is, the rate of reaction is driven by the supply of glucose, not by the demand for end products.
Another distinctive property of glucokinase is its moderate cooperativity
Cooperativity
Cooperativity is a phenomenon displayed by enzymes or receptors that have multiple binding sites where the affinity of the binding sites for a ligand is increased, positive cooperativity, or decreased, negative cooperativity, upon the binding of a ligand to a binding site...
with glucose, with a Hill coefficient (nH) of about 1.7. Glucokinase has only a single binding site for glucose and is the only monomeric regulatory enzyme known to display substrate cooperativity. The nature of the cooperativity has been postulated to involve a "slow transition" between two different enzyme states with different rates of activity. If the dominant state depends upon glucose concentration, it would produce an apparent cooperativity similar to that observed.
Because of this cooperativity, the kinetic interaction of glucokinase with glucose does not follow classical Michaelis-Menten kinetics
Michaelis-Menten kinetics
In biochemistry, Michaelis–Menten kinetics is one of the simplest and best-known models of enzyme kinetics. It is named after German biochemist Leonor Michaelis and Canadian physician Maud Menten. The model takes the form of an equation describing the rate of enzymatic reactions, by relating...
. Rather than a Km for glucose, it is more accurate to describe a half-saturation level S0.5, which is the concentration at which the enzyme is 50% saturated and active.
The S0.5 and nH extrapolate to an "inflection point" of the curve describing enzyme activity as a function of glucose concentration at about 4 mmol/L. In other words, at a glucose concentration of about 72 mg/dl, which is near the low end of the normal range, glucokinase activity is most sensitive to small changes in glucose concentration.
The kinetic relationship with the other substrate, MgATP, can be described by classical Michaelis-Menten kinetics, with an affinity at about 0.3-0.4 mmol/L, well below a typical intracellular concentration of 2.5 mmol/L. The fact that there is nearly always an excess of ATP available implies that ATP concentration rarely influences glucokinase activity.
The maximum specific activity (Kcat, also known as the turnover rate) of glucokinase when saturated with both substrates is 62/s.
A "minimal mathematical model" has been devised based on the above kinetic information to predict the beta cell glucose phosphorylation rate (BGPR) of normal ("wild type") glucokinase and the known mutations. The BGPR for wild type glucokinase is about 28% at a glucose concentration of 5 mmol/l, indicating that the enzyme is running at 28% of capacity at the usual threshold glucose for triggering insulin release.
Mechanism
The sulfhydryl groups of several cysteineCysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
s surround the glucose binding site. All except cys 230 are essential for the catalytic process, forming multiple disulfide bridges during interaction with the substrates and regulators. At least in the beta cells, the ratio of active to inactive glucokinase molecules is at least partly determined by the balance of oxidation of sulfhydryl groups or reduction of disulfide bridges.
These sulfhydryl groups are quite sensitive to the oxidation status of the cells, making glucokinase one of the components most vulnerable to oxidative stress, especially in the beta cells.
Structure
Glucokinase is a monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
ic protein of 465 amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s and a molecular weight of about 50 kD. There are at least two clefts, one for the active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
, binding glucose and MgATP, and the other for a putative allosteric activator
Activator
Activator may mean:* Activator , a DNA-binding protein that regulates one or more genes by increasing the rate of transcription* Activator , a type of effector that increases the rate of enzyme mediated reactions...
that has not yet been identified.
This is about half the size of the other mammalian hexokinases, which retain a degree of dimeric structure. Several sequences and the three-dimensional structure of the key active sites. The ATP binding domain, for example, are shared with hexokinases, bacterial glucokinases, and other proteins, and the common structure is termed an actin fold.
Genetics
Human glucokinase is coded for by the GCK geneGene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
on chromosome 7. This single autosomal
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
gene has 10 exon
Exon
An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...
s. Genes for glucokinase in other animals are homologous to human GCK.
A distinctive feature of the gene is that it begins with two promoter regions. The first exon
Exon
An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...
from the 5' end contains two tissue-specific promoter regions. Transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
can begin at either promoter (depending on the tissue) so that the same gene can produce a slightly different molecule in liver and in other tissues. The two isoforms of glucokinase differ only by 13-15 amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s at the N-terminal end
N-terminal end
The N-terminus refers to the start of a protein or polypeptide terminated by an amino acid with a free amine group . The convention for writing peptide sequences is to put the N-terminus on the left and write the sequence from N- to C-terminus...
of the molecule, which produces only a minimal difference in structure. The two isoforms have the same kinetic and functional characteristics.
The first promoter from the 5' end, referred to as the "upstream" or neuroendocrine promoter, is active in pancreatic islet cells, neural tissue, and enterocytes (small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
cells) to produce the "neuroendocrine isoform" of glucokinase. The second promoter, the "downstream" or liver promoter, is active in hepatocyte
Hepatocyte
A hepatocyte is a cell of the main tissue of the liver. Hepatocytes make up 70-80% of the liver's cytoplasmic mass.These cells are involved in:* Protein synthesis* Protein storage* Transformation of carbohydrates...
s and directs production of the "liver isoform". The two promoters have little or no sequence homology and are separated by a 30 kbp
Base pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
sequence of unknown function. The two promoters are functionally exclusive and governed by distinct sets of regulatory factors, so that glucokinase expression can be regulated separately in different tissue types. The two promoters correspond to two broad categories of glucokinase function: In liver, glucokinase acts as the gateway for the "bulk processing" of available glucose, while, in the neuroendocrine cells, it acts as a sensor, triggering cell responses that affect body-wide carbohydrate metabolism.
Distribution among organ systems
Glucokinase has been discovered in specific cells in four types of mammalian tissue: liver, pancreas, small intestineSmall intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
, and brain. All play crucial roles in responding to rising or falling levels of blood glucose.
- The predominant cells of the liver are the hepatocyteHepatocyteA hepatocyte is a cell of the main tissue of the liver. Hepatocytes make up 70-80% of the liver's cytoplasmic mass.These cells are involved in:* Protein synthesis* Protein storage* Transformation of carbohydrates...
s, and GK is found exclusively in these cells. During digestionDigestionDigestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
of a carbohydrate meal, when blood glucose is plentiful and insulinInsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
levels are high, hepatocytes remove glucose from the blood and store it as glycogenGlycogenGlycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
. After completion of digestion and absorption, the liver manufactures glucose from both non-glucose substrates (gluconeogenesisGluconeogenesisGluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids....
) and glycogen (glycogenolysisGlycogenolysisGlycogenolysis is the conversion of glycogen polymers to glucose monomers. Glycogen is catabolized by removal of a glucose monomer through cleavage with inorganic phosphate to produce glucose-1-phosphate...
), and exports it into the blood, to maintain adequate blood glucose levels during fasting. Because GK activity rises rapidly as the glucose concentration rises, it serves as a central metabolic switch to shift hepatic carbohydrate metabolism between fed and fasting states. Phosphorylation of glucose to glucose-6-phosphate by GK facilitates storage of glucose as glycogen and disposal by glycolysis. The separate liver promoter allows glucokinase to be regulated differently in hepatocytes than in the neuroendocrine cells. - Neuroendocrine cells of the pancreas, gut, and brain share some common aspects of glucokinase production, regulation, and function. These tissues are collectively referred to as "neuroendocrine" cells in this context.
- Beta cellBeta cellBeta cells are a type of cell in the pancreas located in the so-called islets of Langerhans. They make up 65-80% of the cells in the islets.-Function:...
s and alpha cellAlpha cellAlpha cells are endocrine cells in the islets of Langerhans of the pancreas. They make up 33-46% of the human islet cells and are responsible for synthesizing and secreting the peptide hormone glucagon, which elevates the glucose levels in the blood....
s of the pancreatic isletIsletAn islet is a very small island.- Types :As suggested by its origin as islette, an Old French diminutive of "isle", use of the term implies small size, but little attention is given to drawing an upper limit on its applicability....
s- Beta cells release insulinInsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
in response to rising levels of glucose. Insulin enables many types of cells to import and use glucose, and signals the liver to synthesize glycogen. Alpha cells produce less glucagonGlucagonGlucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
in response to rising glucose levels, and more glucagon if blood glucose is low. Glucagon serves as a signal to the liver to break down glycogen and release glucose into the blood. Glucokinase in beta cells serves as a glucose sensor, amplifying insulin secretion as blood glucose rises.
- Beta cells release insulin
- Glucose-sensitive neuronNeuronA neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s of the hypothalamusHypothalamusThe Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
- In response to rising or falling levels of glucose, cells in the hypothalamus polarize or depolarize. Among the neuroendocrine reactions of the central nervous systemCentral nervous systemThe central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
to hypoglycemiaHypoglycemiaHypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...
is activation of the adrenergicAdrenergicAn adrenergic agent is a drug, or other substance, which has effects similar to, or the same as, epinephrine . Thus, it is a kind of sympathomimetic agent...
responses of the autonomic nervous systemAutonomic nervous systemThe autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
. Glucokinase likely serves as a glucose signal here as well. Glucokinase has also been found in cells of the anterior pituitary.
- In response to rising or falling levels of glucose, cells in the hypothalamus polarize or depolarize. Among the neuroendocrine reactions of the central nervous system
- EnterocyteEnterocyteEnterocytes, or intestinal absorptive cells, are simple columnar epithelial cells found in the small intestines and colon. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase surface area for the digestion and transport of molecules from the intestinal...
s of the small intestine- This is the least-understood of the glucokinase sensor systems. It seems likely that responses to incoming glucose during digestion play a role in the incretinIncretinIncretins are a group of gastrointestinal hormones that cause an increase in the amount of insulin released from the beta cells of the islets of Langerhans after eating, even before blood glucose levels become elevated. They also slow the rate of absorption of nutrients into the blood stream by...
amplification of insulin secretion during a meal, or in the generation of satiety signals from gut to brain.
- This is the least-understood of the glucokinase sensor systems. It seems likely that responses to incoming glucose during digestion play a role in the incretin
- Beta cell
Distribution among species
Liver glucokinase occurs widely but not universally throughout vertebrate species. The gene structure and amino acid sequence are highly conserved among most mammals (e.g., rat and human glucokinase is more than 80% homologous). However, there are some unusual exceptions: For example, it has not been discovered in catCat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
s and bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s, though some reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, amphibians, and fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
have it. Whether glucokinase occurs similarly in the pancreas and other organs has not yet been determined. It has been postulated that the presence of glucokinase in liver reflects the ease with which carbohydrates can be included in the animals' diet
Diet (nutrition)
In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...
s.
Function and regulation
Most of the glucokinase in a mammal is found in the liver, and glucokinase provides approximately 95% of the hexokinase activity in hepatocytes. Phosphorylation of glucose to glucose-6-phosphateGlucose-6-phosphate
Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. This compound is very common in cells as the vast majority of glucose entering a cell will become phosphorylated in this way....
(G6P) by glucokinase is the first step of both glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
synthesis and glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
in the liver.
When ample glucose is available, glycogen synthesis proceeds at the periphery of the hepatocytes until the cells are replete with glycogen. Excess glucose is then increasingly converted into triglyceride
Triglyceride
A triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
s for export and storage in adipose tissue. Glucokinase activity in the cytoplasm rises and falls with available glucose.
G6P, the product of glucokinase, is the principal substrate of glycogen synthesis, and glucokinase has a close functional and regulatory association with glycogen synthesis. When maximally active, GK and glycogen synthase
Glycogen synthase
Glycogen synthase is an enzyme involved in converting glucose to glycogen. It takes short polymers of glucose and converts them into long polymers....
appears to be located in the same peripheral areas of hepatocyte cytoplasm in which glycogen synthesis occurs. The supply of G6P affects the rate of glycogen synthesis not only as the primary substrate, but by direct stimulation of glycogen synthase and inhibition of glycogen phosphorylase
Glycogen phosphorylase
Glycogen phosphorylase is one of the phosphorylase enzymes . Glycogen phosphorylase catalyzes the rate-limiting step in the degradation of glycogen in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond...
.
Glucokinase activity can be rapidly amplified or damped in response to changes in the glucose supply, typically resulting from eating and fasting. Regulation occurs at several levels and speeds, and is influenced by many factors that affect mainly two general mechanisms:
- Glucokinase activity can be amplified or reduced in minutes by actions of the glucokinase regulatory protein (GKRP). The actions of this protein are influenced by small molecules such as glucose and fructose.
- The amount of glucokinase can be increased by synthesis of new protein. Insulin is the principal signal for increased transcription, operating mainly by way of a transcription factor called sterol regulatory element binding proteinSterol regulatory element binding proteinSterol Regulatory Element-Binding Proteins are transcription factors that bind to the sterol regulatory element DNA sequence TCACNCCAC. Mammalian SREBPs are encoded by the genes SREBF1 and SREBF2. SREBPs belong to the basic-helix-loop-helix leucine zipper class of transcription factors...
-1c (SREBP1c). This occurs within an hour after a rise in insulin levels, as after a carbohydrate meal.
Transcriptional
Insulin acting via the sterol regulatory element binding proteinSterol regulatory element binding protein
Sterol Regulatory Element-Binding Proteins are transcription factors that bind to the sterol regulatory element DNA sequence TCACNCCAC. Mammalian SREBPs are encoded by the genes SREBF1 and SREBF2. SREBPs belong to the basic-helix-loop-helix leucine zipper class of transcription factors...
-1c (SREBP1c) is thought to be the most important direct activator of glucokinase gene transcription in hepatocytes. SREBP1c is a basic helix-loop-helix zipper
Basic helix-loop-helix leucine zipper transcription factors
Basic helix-loop-helix leucine zipper transcription factors are, as their name indicates, transcription factors containing both Basic helix-loop-helix and leucine zipper motifs.An example is Microphthalmia-associated transcription factor....
(bHLHZ) transactivator. This class of transactivators bind to the "E box" sequence of genes for a number of regulatory enzymes. The liver promoter in the first exon of the glucokinase gene includes such an E box, which appears to be the principal insulin-response element of the gene in hepatocytes. SREBP1c must be present for transcription of glucokinase in hepatocytes. SREBP1c increases in response to a high-carbohydrate diet, presumed as a direct effect of frequent insulin elevation. Increased transcription can be detected in less than an hour after hepatocytes are exposed to rising insulin levels.
Fructose-2,6-bisphosphate (F2,6P2) also stimulates GK transcription, it seems by way of Akt2 rather than SREBP1c. It is not known whether this effect is one of the downstream effects of activation of insulin receptors or independent of insulin action. Levels of F2,6P2 play other amplifying roles in glycolysis in hepatocytes.
Other transacting factors suspected of playing a role in liver cell transcription regulation include:
- Hepatic nuclear factor-4-alpha (HNF4αHepatocyte nuclear factor 4 alphaHepatocyte nuclear factor 4 alpha also known as NR2A1 is a nuclear receptor that in humans is encoded by the HNF4A gene.- Function :...
) is an orphan nuclear receptor important in the transcription of many genes for enzymes of carbohydrate and lipid metabolism. It activates GCK transcription. - Upstream stimulatory factor 1 (USF1USF1Upstream stimulatory factor 1 is a protein that in humans is encoded by the USF1 gene.-Interactions:USF1 has been shown to interact with USF2, FOSL1 and GTF2I.-External links:...
) is another basic helix-loop-helix zipperBasic helix-loop-helix leucine zipper transcription factorsBasic helix-loop-helix leucine zipper transcription factors are, as their name indicates, transcription factors containing both Basic helix-loop-helix and leucine zipper motifs.An example is Microphthalmia-associated transcription factor....
(bHLHZ) transactivator. - Hepatic nuclear factor 6 (HNF6) is a homeodomain transcriptional regulator of the "one-cut class." HNF6 is also involved in regulation of transcription of gluconeogenicGluconeogenesisGluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids....
enzymes such as glucose-6-phosphatase and phosphoenolpyruvate carboxykinasePhosphoenolpyruvate carboxykinasePhosphoenolpyruvate carboxykinase is an enzyme in the lyase family used in the metabolic pathway of gluconeogenesis. It converts oxaloacetate into phosphoenolpyruvate and carbon dioxide.It is found in two forms, cytosolic and mitochondrial....
.
Hormonal and dietary
InsulinInsulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
is by far the most important of the hormones that have direct or indirect effects on glucokinase expression and activity in the liver. Insulin appears to affect both glucokinase transcription and activity through multiple direct and indirect pathways. While rising portal vein glucose levels increase glucokinase activity, the concomitant rise of insulin amplifies this effect by induction
Enzyme induction and inhibition
Enzyme induction is a process in which a molecule induces the expression of an enzyme.Enzyme inhibition can refer to* the inhibition of the expression of the enzyme by another molecule...
of glucokinase synthesis. Glucokinase transcription begins to rise within an hour of rising insulin levels. Glucokinase transcription becomes nearly undetectable in prolonged starvation, severe carbohydrate deprivation, or untreated insulin-deficient diabetes.
The mechanisms by which insulin induces glucokinase may involve both of the major intracellular pathways of insulin action, the extracellular signal-regulated kinase (ERK 1/2) cascade, and the phosphoinositide 3-kinase (PI3-K) cascade. The latter may operate via the FOXO1 transactivator.
However, as would be expected given its antagonistic effect on glycogen synthesis, glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
and its intracellular second messenger cAMP
Cyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...
suppresses glucokinase transcription and activity, even in the presence of insulin.
Other hormones such as triiodothyronine
Triiodothyronine
Triiodothyronine, C15H12I3NO4, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate....
(T3) and glucocorticoid
Glucocorticoid
Glucocorticoids are a class of steroid hormones that bind to the glucocorticoid receptor , which is present in almost every vertebrate animal cell...
s provide permissive or stimulatory effects on glucokinase in certain circumstances. Biotin
Biotin
Biotin, also known as Vitamin H or Coenzyme R, is a water-soluble B-complex vitamin discovered by Bateman in 1916. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring...
and retinoic acid
Retinoic acid
Retinoic acid is a metabolite of vitamin A that mediates the functions of vitamin A required for growth and development. Retinoic acid is required in chordate animals which includes all higher animals from fishes to humans...
increase GCK mRNA transcription as well as GK activity. Fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s in significant amounts amplify GK activity in the liver, while long chain acyl CoA inhibits it.
Hepatic
Glucokinase can be rapidly activated and inactivated in hepatocytes by a novel regulatory protein (glucokinase regulatory proteinGlucokinase regulatory protein
The glucokinase regulatory protein also known as glucokinase regulator is a protein produced in hepatocytes . GKRP binds and moves glucokinase , thereby controlling both activity and intracellular location of this key enzyme of glucose metabolism.GKRP is a 68 kD protein of 626 amino acids...
), which operates to maintain an inactive reserve of GK, which can be made quickly available in response to rising levels of portal vein glucose.
GKRP moves between nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
of the hepatocytes and may be tethered to the microfilament cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
. It forms reversible 1:1 complexes with GK, and can move it from the cytoplasm into the nucleus. It acts as a competitive inhibitor with glucose, such that the enzyme activity is reduced to near-zero while bound. GK:GKRP complexes are sequestered in the nucleus while glucose and fructose levels are low. Nuclear sequestration may serve to protect GK from degradation by cytoplasmic protease
Protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
s. GK can be rapidly released from GKRP in response to rising levels of glucose. Unlike GK in beta cells, GK in hepatocytes is not associated with mitochondria.
Fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
in tiny (micromolar) amounts (after phosphorylation by ketohexokinase to fructose-1-phosphate
Fructose-1-phosphate
Fructose-1-phosphate is a derivative of fructose. It is generated by hepatic fructokinase.It is converted by aldolase B into glyceraldehyde and dihydroxyacetone phosphate ....
(F1P)) accelerates release of GK from GKRP. This sensitivity to the presence of small amounts of fructose allows GKRP, GK, and ketohexokinase to act as a "fructose sensing system," which signals that a mixed carbohydrate meal is being digested, and accelerates the utilization of glucose. However, fructose 6-phosphate
Fructose 6-phosphate
Fructose 6-phosphate is fructose sugar phosphorylated on carbon 6 . The β-D-form of this compound is very common in cells. The vast majority of glucose and fructose entering a cell will become converted to this at some point...
(F6P) potentiates binding of GK by GKRP. F6P decreases phosphorylation of glucose by GK when glycogenolysis
Glycogenolysis
Glycogenolysis is the conversion of glycogen polymers to glucose monomers. Glycogen is catabolized by removal of a glucose monomer through cleavage with inorganic phosphate to produce glucose-1-phosphate...
or gluconeogenesis
Gluconeogenesis
Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids....
are underway. F1P and F6P both bind to the same site on GKRP. It is postulated that they produce 2 different conformations of GKRP, one able to bind GK and the other not.
Pancreatic
Although most of the glucokinase in the body is in the liver, smaller amounts in the beta and alpha cells of the pancreas, certain hypothalamic neurons, and specific cells (enterocytes) of the gut play an increasingly appreciated role in regulation of carbohydrate metabolism. In the context of glucokinase function, these cell types are collectively referred to as neuroendocrine tissues, and they share some aspects of glucokinase regulation and function, especially the common neuroendocrine promoter. Of the neuroendocrine cells, the beta cells of the pancreatic islets are the most-studied and best-understood. It is likely that many of the regulatory relationships discovered in the beta cells will also exist in the other neuroendocrine tissues with glucokinase.A signal for insulin
In islet beta cellBeta cell
Beta cells are a type of cell in the pancreas located in the so-called islets of Langerhans. They make up 65-80% of the cells in the islets.-Function:...
s, glucokinase activity serves as a principal control for the secretion of insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
in response to rising levels of blood glucose. As G6P is consumed, increasing amounts of ATP initiate a series of processes that result in release of insulin. One of the immediate consequences of increased cellular respiration is a rise in the NADH and NADPH concentrations (collectively referred to as NAD(P)H). This shift in the redox status of the beta cells results in rising intracellular calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
levels, closing of the KATP channels
Potassium channel
In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...
, depolarization of the cell membrane, merging of the insulin secretory granules with the membrane, and release of insulin into the blood.
It is as a signal for insulin release that glucokinase exerts the largest effect on blood sugar levels and overall direction of carbohydrate metabolism. Glucose, in turn, influences both the immediate activity and the amount of glucokinase produced in the beta cells.
Regulation in beta cells
Glucose immediately amplifies glucokinase activity by the cooperativity effect.A second important rapid regulator of glucokinase activity in beta cells occurs by direct protein-protein interaction between glucokinase and the "bifunctional enzyme" (phosphofructokinase-2/fructose-2,6-bisphosphatase),
which also plays a role in the regulation of glycolysis. This physical association stabilizes glucokinase in a catalytically favorable conformation (somewhat opposite the effect of GKRP binding) that enhances its activity.
In as little as 15 minutes, glucose can stimulate GCK transcription and glucokinase synthesis by way of insulin. Insulin is produced by the beta cells, but some of it acts on beta cell B-type insulin receptor
Insulin receptor
In molecular biology, the insulin receptor is a transmembrane receptor that is activated by insulin. It belongs to the large class of tyrosine kinase receptors....
s, providing an autocrine positive-feedback amplification of glucokinase activity. Further amplification occurs by insulin action (via A-type receptors) to stimulate its own transcription.
Transcription of the GCK gene is initiated through the "upstream," or neuroendocrine, promoter. This promoter, in contrast to the liver promoter, has elements homologous to other insulin-induced gene promoters. Among the probable transacting factors are Pdx-1 and PPARγ. Pdx-1 is a homeodomain transcription factor involved in the differentiation of the pancreas. PPARγ is a nuclear receptor that responds to glitazone drugs by enhancing insulin sensitivity.
Association with insulin secretory granules
Much, but not all, of the glucokinase found in the cytoplasm of beta cells is associated with insulin secretory granules and with mitochondria. The proportion thus "bound" falls rapidly in response to rising glucose and insulin secretion. It has been suggested that binding serves a purpose similar to the hepatic glucokinase regulatory protein—protecting glucokinase from degradation so that it is rapidly available as the glucose rises. The effect is to amplify the glucokinase response to glucose more rapidly than transcription could do so.Suppression of glucagon in alpha cells
It has also been proposed that glucokinase plays a role in the glucose sensing of the pancreatic alpha cellAlpha cell
Alpha cells are endocrine cells in the islets of Langerhans of the pancreas. They make up 33-46% of the human islet cells and are responsible for synthesizing and secreting the peptide hormone glucagon, which elevates the glucose levels in the blood....
s, but the evidence is less consistent, and some researchers have found no evidence of glucokinase activity in these cells. Alpha cells occur in pancreatic islets, mixed with beta and other cells. While beta cells respond to rising glucose levels by secreting insulin, alpha cells respond by reducing glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
secretion. When blood glucose concentration falls to hypoglycemic
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...
levels, alpha cells release glucagon. Glucagon is a protein hormone that blocks the effect of insulin on hepatocytes, inducing glycogenolysis, gluconeogenesis, and reduced glucokinase activity in hepatocytes. The degree to which glucose suppression of glucagon is a direct effect of glucose via glucokinase in alpha cells, or an indirect effect mediated by insulin or other signals from beta cells, is still uncertain.
Hypothalamic
While all neuronNeuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s use glucose for fuel, certain glucose-sensing neurons alter their firing rates in response to rising or falling levels of glucose. These glucose-sensing neurons are concentrated primarily in the ventromedial nucleus
Ventromedial nucleus
The ventromedial nucleus is a nucleus of the hypothalamus.-Division:It has four subdivisions:* anterior * dorsomedial * ventrolateral * central ....
and arcuate nucleus
Arcuate nucleus
The arcuate nucleus is an aggregation of neurons in the mediobasal hypothalamus, adjacent to the third ventricle and the median eminence...
of the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, which regulate many aspects of glucose homeostasis (especially the response to hypoglycemia), fuel utilization, satiety and appetite
Appetite
The appetite is the desire to eat food, felt as hunger. Appetite exists in all higher life-forms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the brain. Decreased desire to eat is...
, and weight
Body weight
The term body weight is used in daily English speech as well as in the contexts of biological and medical sciences to describe the mass of an organism's body. Body weight is measured in kilograms throughout the world, although in some countries it is still measured in pounds or stones and pounds...
maintenance. These neurons are most sensitive to glucose changes in the 0.5-3.5 mmol/L glucose range.
Glucokinase has been found in the brain in largely the same areas that contain glucose-sensing neurons, including both of the hypothalamic nuclei. Inhibition of glucokinase abolishes the ventromedial nucleus response to a meal. However, brain glucose levels are lower than plasma levels, typically 0.5-3.5 mmol/L. Although this range is matches the sensitivity of the glucose-sensing neurons, it is below the optimal inflection sensitivity for glucokinase. The presumption, based on indirect evidence and speculation, is that neuronal glucokinase is somehow exposed to plasma glucose levels even in the neurons.
Enterocytes and incretin
While glucokinase has been shown to occur in certain cells (enterocytes) of the small intestineSmall intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
and stomach, its function and regulation have not been worked out. It has been suggested that here, also, glucokinase serves as a glucose sensor, allowing these cells to provide one of the earliest metabolic responses to incoming carbohydrates. It is suspected that these cells are involved in incretin
Incretin
Incretins are a group of gastrointestinal hormones that cause an increase in the amount of insulin released from the beta cells of the islets of Langerhans after eating, even before blood glucose levels become elevated. They also slow the rate of absorption of nutrients into the blood stream by...
functions.
Clinical significance
Because insulin is one of, if not the most important, regulators of glucokinase synthesis, diabetes of all types diminishes glucokinase synthesis and activity by a variety of mechanisms. Glucokinase activity is sensitive to oxidative stress of cells, especially the beta cells.Around 200 mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
s of the human glucokinase gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
GCK have been discovered, that can change the efficiency of glucose binding and phosphorylation, increasing or decreasing the sensitivity of beta cell insulin secretion in response to glucose, and producing clinically significant hyperglycemia
Hyperglycemia
Hyperglycemia or Hyperglycæmia, or high blood sugar, is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a glucose level higher than 13.5mmol/l , but symptoms may not start to become noticeable until even higher values such as 15-20 mmol/l...
or hypoglycemia
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...
.
Diabetes
Over 190 of these mutations reduce the functional efficiency of the glucokinase molecule. Heterozygosity for alleles with reduced enzyme activity results in a higher threshold for insulin release and persistent, mild hyperglycemia. This condition is referred to as maturity onset diabetes of the youngMaturity onset diabetes of the young
Maturity onset diabetes of the young refers to any of several hereditary forms of diabetes caused by mutations in an autosomal dominant gene disrupting insulin production...
, type 2 (MODY2).
Homozygosity for GCK alleles with reduced function can cause severe congenital insulin deficiency, resulting in persistent neonatal diabetes.
Hyperinsulinemic hypoglycemia
As of 2004, 5 mutations have been found to enhance insulin secretion. Heterozygosity for gain of function mutations reduces the threshold glucose that triggers insulin release. This creates hypoglycemia of varying patterns, including transient or persistent congenital hyperinsulinismCongenital hyperinsulinism
Congenital hyperinsulinism is a medical term referring to a variety of congenital disorders in which hypoglycemia is caused by excessive insulin secretion. Congenital forms of hyperinsulinemic hypoglycemia can be transient or persistent, mild or severe. These conditions are present at birth and...
, or fasting or reactive hypoglycemia appearing at an older age.
Homozygosity for gain of function mutations has not been found.
As a drug target
Several laboratories sponsored by pharmaceutical companiesPharmaceutical company
The pharmaceutical industry develops, produces, and markets drugs licensed for use as medications. Pharmaceutical companies are allowed to deal in generic and/or brand medications and medical devices...
are researching molecules that activate glucokinase in hope that it will be useful in the treatment of type 2 diabetes
Diabetes mellitus type 2
Diabetes mellitus type 2formerly non-insulin-dependent diabetes mellitus or adult-onset diabetesis a metabolic disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. Diabetes is often initially managed by increasing exercise and...
.