AMP-activated protein kinase
Encyclopedia
5' AMP-activated protein kinase or AMPK or 5' adenosine monophosphate-activated protein kinase is an enzyme
that plays a role in cellular energy homeostasis. It consists of three proteins (subunits
) that together make a functional enzyme
, conserved from yeast to humans. It is expressed in a number of tissues, including the liver
, brain
, and skeletal muscle
. The net effect of AMPK activation is stimulation of hepatic fatty acid oxidation and ketogenesis
, inhibition
of cholesterol
synthesis, lipogenesis
, and triglyceride
synthesis, inhibition of adipocyte lipolysis
and lipogenesis, stimulation of skeletal muscle fatty acid oxidation and muscle glucose uptake, and modulation of insulin secretion by pancreatic beta-cells.
It should not be confused with cyclic AMP-activated protein kinase (protein kinase A), which, although being of similar nature, may have opposite effects.
giving AMPK its ability to sensitively detect shifts in the AMP
:ATP
ratio. The four CBS domains create two binding sites for AMP commonly referred to as Bateman domains. Binding of one AMP
to a Bateman domain cooperatively increases the binding affinity
of the second AMP to the other Bateman domain. As AMP binds both Bateman domains the γ subunit undergoes a conformational change which exposes the catalytic domain found on the α subunit. It is in this catalytic domain where AMPK becomes activated when phosphorylation
takes place at threonine
-172 by an upstream
AMPK kinase
(AMPKK). The α, β, and γ subunits can also be found in different isoforms: the γ subunit can exist as either the γ1, γ2 or γ3 isoform; the β subunit can exist as either the β1 or β2 isoform; and the α subunit can exist as either the α1 or α2 isoform. Although the most common isoforms expressed in most cells are the α1, β1, and γ1 isoforms, it has been demonstrated that the α2, β2, γ2, and γ3 isoforms are also expressed in cardiac and skeletal muscle
.
The following humans gene encode AMPK subunits:
systems including the cellular uptake of glucose
, the β-oxidation of fatty acids and the biogenesis
of glucose transporter 4 (GLUT4
) and mitochondria. The energy-sensing capability of AMPK can be attributed to its ability to detect and react to fluctuations in the AMP:ATP ratio that take place during rest
and exercise (muscle stimulation). During muscle
stimulation, AMP increases while ATP decreases, which changes AMPK into a good substrate for activation via an upstream kinase complex, AMPKK, or better, where binding of AMP renders activated AMPK that is phosphorylated at Thr-172 a worse substrate for protein phosphatase 2Calpha. AMPKK is a complex of three proteins, STE-related adaptor (STRAD
), mouse protein 25 (MO25
), and LKB1
(a serine
/threonine kinase). During a bout of exercise, AMPK activity increases while the muscle cell experiences metabolic stress
brought about by an extreme cellular demand for ATP. Upon activation, AMPK increases cellular energy levels by inhibiting anabolic energy consuming pathways (fatty acid synthesis
, protein
synthesis, etc.) and stimulating energy producing, catabolic pathways (fatty acid oxidation, glucose transport, etc.).
A recent JBC
paper on mice
at Johns Hopkins
has shown that when the activity of brain AMPK was pharmacologically inhibited, the mice ate less and lost weight. When AMPK activity was pharmacologically raised (AICAR
see below) the mice ate more and gained weight. Research in Britain has shown that the appetite-stimulating hormone ghrelin
also affects AMPK levels.
The antidiabetic drug metformin
(Glucophage) acts by stimulating AMPK, leading to reduced glucose production in the liver
and reduced insulin resistance in the muscle. (Metformin usually causes weight loss and reduced appetite, not weight gain and increased appetite, which is opposite of what might be expected given the Johns Hopkins mouse study results. )
A number of recent studies suggest that the botanical alkaloid berberine
, also activates AMPK and glucose transport in muscles.
(Thr-172) on the α subunit. The conformational change of the γ subunit of AMPK can be accomplished under increased concentrations of AMP. Increased concentrations of AMP will give rise to the conformational change on the γ subunit of AMPK as two AMP bind the two Bateman domains located on that subunit. It is this conformational change brought about by increased concentrations of AMP that exposes the active site (Thr-172) on the α subunit. This critical role of AMP is further substantiated in experiments that demonstrate AMPK activation via an AMP analogue 5-amino-4-imidazolecarboxamide ribotide (ZMP) which is derived from the familiar 5-amino-4-imidazolecarboxamide riboside (AICAR
). The second condition that must be met is the phosphorylation and consequent activation of AMPK on its activating loop at Thr-172 of the α subunit brought about by an upstream kinase (AMPKK). The complex formed between LKB1 (STK 11), mouse protein 25 (MO25), and the pseudokinase STE-related adaptor protein (STRAD) has of late been identified as the major upstream kinase responsible for phosphorylation of AMPK on its activating loop at Thr-172. Although AMPK must be phosphorylated by the LKB1/MO25/STRAD complex, it can also be regulated by allosteric modulators
which directly increase general AMPK activity and modify AMPK to make it a better substrate
for AMPKK and a worse substrate for phosphatase
s. It has recently been found that 3-phosphoglycerate (glycolysis
intermediate
) acts to further pronounce AMPK activation via AMPKK.
Muscle contraction
is the main method carried out by the body that can provide the conditions mentioned above needed for AMPK activation. As muscles contract, ATP is hydrolyzed, forming ADP
. ADP then helps to replenish cellular ATP by donating a phosphate
group to another ADP, forming an ATP and an AMP. As more AMP is produced during muscle contraction, the AMP:ATP ratio dramatically increases, leading to the allosteric activation of AMPK. This fact is further authenticated with studies, such as those sited above, that used electrical stimuli as a means to contract muscle to facilitate AMPK activation. For over a decade it has been known that calmodulin
-dependent protein kinase kinase-beta (CaMKKbeta) can phosphorylate and thereby activate AMPK, but it was not the main AMPKK in liver
. Richter et al. found that CaMKK inhibitors strongly inhibited AMPK phosphorylation in mouse
soleus and EDL muscles after 2 minutes of contraction, but much less as time of contraction increased. CaMKK inhibitors had no effect on 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) phosphorylation and activation of AMPK. AICAR is taken into the cell
and converted to ZMP, an AMP analog
that has been shown to activate AMPK. Recent LKB1 knockout studies have shown that without LKB1, electrical and AICAR stimulation of muscle
results in very little phosphorylation of AMPK and of ACC, providing evidence that LKB1-STRAD-MO25 is the major AMPKK in muscle.
s, also known as adipocytokines, are secreted by adipose tissue
to take on several different but important physiological roles in the body including the regulation of appetite
, metabolism, fatty acid catabolism, coagulation
and systemic inflammation
, for example. Collectively, the adipokines are in essence cytokine
s (cell-to-cell signaling proteins) which, when secreted, act on other cells, usually resulting in a biochemical and metabolic response. Two particular adipokines, adiponectin
and leptin
, have even been demonstrated to regulate AMPK.
It has been known for some time now that among many of the metabolic roles of leptin, one of its main functions in skeletal muscle is the upregulation of fatty acid oxidation. Recently, a study revealed that leptin is able to do this by way of the AMPK signaling pathway. A similar study showed that much like leptin, adiponectin also stimulates the oxidation of fatty acids via the AMPK pathway, and that it also stimulates the uptake of glucose in skeletal muscle. As of yet, the metabolic roles of leptin and adiponectin pertaining to biochemical adaptations to long-term endurance
training remain unclear. Certainly future studies will involve an investigation of leptin and adiponectin activities and their respective relationships with the AMPK signaling pathway immediately following a high-intensity endurance training protocol
.
, such as increased mitochondrial biogenesis and capacity, increased muscle glycogen
, and an increase in enzyme
s which specialize in glucose uptake in cells such as GLUT4 and hexokinase
II are thought to be mediated in part by AMPK when it is activated. Additionally, recent discoveries can conceivably suggest a direct AMPK role in increasing blood supply to exercised/trained muscle cells by stimulating and stabilizing both vasculogenesis
and angiogenesis
. Taken together, these adaptation
s most likely transpire as a result of both temporary and maintained increases in AMPK activity brought about by increases in the AMP:ATP ratio during single bouts of exercise and long-term training.
During a single acute exercise bout, AMPK allows the contracting muscle cells to adapt to the energy challenges by increasing expression of hexokinase II, translocation of GLUT4 to the plasma membrane, for glucose uptake, and by stimulating glycolysis. If bouts of exercise continue through a long-term training
regimen, AMPK and other signals will facilitate contracting muscle adaptations by escorting muscle cell activity to a metabolic transition resulting in an oxidative dependent approach to energy
metabolism
as opposed to a glycolytic approach. AMPK accomplishes this transition to the oxidative mode of metabolism by upregulating and activating oxidative enzymes such as GLUT4, hexokinase II, PPARalpha
, PGC-1
, UCP3
, cytochrome C
and TFAM
.
AMPK activity increases with exercise and the LKB1/MO25/STRAD complex
is considered to be the major upstream
AMPKK of the 5’-AMP-activated protein kinase phosphorylating the α subunit of AMPK at Thr-172. This fact is puzzling considering that although AMPKK protein
abundance has been shown to increase in skeletal tissue
with endurance
training, its level of activity has been shown to decrease with endurance training in both trained and untrained tissue. Currently, the activity of AMPKK immediately following a 2-hr bout of exercise of an endurance trained rat is unclear. It is possible that there exists a direct link between the observed decrease in AMPKK activity in endurance trained skeletal muscle and the apparent decrease in the AMPK response to exercise with endurance training.
homologue of AMPK, aak-2, has been shown by Michael Ristow
and colleagues to be required for extension of life span in states of glucose restriction mediating a process named mitohormesis.
, which provides more energy
for the cell. One of the key pathways in AMPK’s regulation of fatty acid oxidation is the phosphorylation and inactivation of acetyl-CoA carboxylase
. Acetyl-CoA carboxylase (ACC) converts acetyl-CoA to malonyl-CoA
, an inhibitor
of carnitine parmitoyltransferase 1 (CPT-1
). CPT-1 transports fatty acids into the mitochondria for oxidation. Inactivation of ACC, therefore, results in increased fatty acid transport and subsequent oxidation. It is also thought that the decrease in malonyl-CoA occurs as a result of malonyl-CoA decarboxylase
(MCD), which may be regulated by AMPK. MCD is an antagonist
to ACC, decarboxylating malonyl-CoA to acetyl-CoA, resulting in decreased malonyl-CoA and increased CPT-1 and fatty acid oxidation.
AMPK also plays an important role in lipid metabolism
in the liver
. It has long been known that hepatic ACC has been regulated in the liver by phosphorylation
. AMPK also phosphorylates and inactivates 3-hydroxy-3-methylglutaryl-CoA reductase
(HMGCR), a key enzyme in cholesterol synthesis. HMGR converts 3-hydroxy-3-methylglutaryl-CoA, which is made from acetyl-CoA, into mevalonic acid
, which then travels down several more metabolic steps to become cholesterol
. AMPK, therefore, helps regulate fatty acid oxidation and cholesterol synthesis.
is a hormone
which helps regulate glucose
levels in the body. When blood glucose is high, insulin is released from the Islets of Langerhans
. Insulin, among other things, will then facilitate the uptake of glucose into cells via increased expression and translocation
of glucose transporter GLUT-4. Under conditions of exercise, however, blood sugar
levels are not necessarily high, and insulin is not necessarily activated, yet muscles are still able to bring in glucose. AMPK seems to be responsible in part for this exercise-induced glucose uptake. Goodyear et al. observed that with exercise, the concentration of GLUT-4 was increased in the plasma membrane, but decreased in the microsomal membranes, suggesting that exercise facilitates the translocation of vesicular GLUT-4 to the plasma membrane. While acute exercise increases GLUT-4 translocation, endurance training will increase the total amount of GLUT-4 protein available. It has been shown that both electrical contraction and AICAR treatment increase AMPK activation, glucose uptake, and GLUT-4 translocation in perfused rat hindlimb muscle, linking exercise-induced glucose uptake to AMPK. Chronic AICAR injections, simulating some of the effects of endurance training
, also increase the total amount of GLUT-4 protein
in the muscle cell.
Two proteins are essential for the regulation of GLUT-4 expression at a transcriptional level – myocyte enhancer factor 2 (MEF2
) and GLUT-4 enhancer factor (GEF). Mutations in the DNA
binding regions for either of these proteins results in ablation
of transgene
GLUT-4 expression. These results prompted a study in 2005 which showed that AMPK directly phosphorylates GEF, but it doesn’t seem to directly activate MEF2. AICAR treatment has been shown, however, to increase transport of both proteins into the nucleus
, as well as increase the binding of both to the GLUT-4 promoter region.
There is another protein involved in carbohydrate metabolism
that is worthy of mention along with GLUT-4. The enzyme hexokinase
phosphorylates a six-carbon sugar, most notably glucose
, which is the first step in glycolysis
. When glucose is transported into the cell it is phosphorylated by hexokinase. This phosphorylation keeps glucose from leaving the cell
, and by changing the structure of glucose through phosphorylation, it decreases the concentration of glucose molecules, allowing a gradient for more glucose to be transported into the cell. Hexokinase II transcription
is increased in both red and white skeletal muscle
upon treatment with AICAR. With chronic injections of AICAR, total protein content of hexokinase II increases in rat
skeletal muscle.
. After pyruvate is formed from glucose
during glycolysis
in the cytoplasm
, it is transported into the mitochondria, where it is oxidized to acetyl-CoA
and enters the citric acid cycle
. Oxidative phosphorylation
is the process by which NADH and FADH2 are oxidized and oxygen
is reduced. This process creates a proton
gradient which is used to drive ATP synthase
and produce ATP
. This process creates energy for cellular properties, and since ATP is indispensable in the contraction process, so then, are mitochondria.
Mitochondrial enzymes, such as cytochrome c
, succinate dehydrogenase, malate dehydrogenase
, α-ketoglutarate dehydrogenase, and citrate synthase
, increase in expression and activity in response to exercise. AICAR stimulation of AMPK increases cytochrome c and δ-aminolevulinate synthase (ALAS
), a rate-limiting enzyme involved in the production of heme
. Malate dehydrogenase
and succinate dehydrogenase also increase, as well as citrate synthase activity, in rats treated with AICAR injections. Conversely, in LKB1 knockout mice, there are decreases in cytochrome c and citrate synthase activity, even if the mice are “trained” by voluntary exercise.
[Peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α
) is a transcriptional regulator for genes involved in mitochondrial biogenesis
, fatty acid oxidation, and gluconeogenesis
.
To do this, it enhances the activity of transcription factors like nuclear respiratory factor 1 (NRF-1
), myocyte
enhancer factor 2 (MEF2), host cell factor (HCF), and others. It also has a positive feedback loop, enhancing its own expression.
Both MEF2 and cAMP
response element (CRE) are essential for contraction-induced PGC-1α promoter activity. AMPK is required for increased PGC-1α expression in skeletal muscle in response to creatine
depletion. LKB1 knockout mice show a decrease in PGC-1α, as well as mitochondrial proteins.
hormone
regulate some similar processes. Knowing these similarities, Winder and Hardie et al. designed an experiment to see if AMPK was influenced by thyroid hormone
. They found that all of the subunits of AMPK were increased in skeletal muscle
, especially in the soleus and red quadriceps, with thyroid hormone treatment. There was also an increase in phospho-ACC, a marker of AMPK activity.
in relation to exercise and long-term training. Similar to short-term acute training scale, long-term endurance training studies also reveal increases in oxidative metabolic enzymes and increases in GLUT-4, mitochondrial size and quantity, and an increased dependency on the oxidation of fatty acids; however, Winder et al. reported in 2002 that despite observing these increased oxidative biochemical adaptations to long-term endurance training (similar to those mentioned above), the AMPK response (activation of AMPK with the onset of exercise) to acute bouts of exercise decreased in red
quadriceps (RQ) with training (3 – see Fig.1). Conversely, the study did not observe the same results in white
quadriceps (WQ) and soleus (SOL) muscles that they did in RQ. The trained rat
s used for that endurance study
ran on treadmill
s 5 days/wk in two 1-h sessions, morning
and afternoon
. The rats were also running up to 31m/min (grade 15%). Finally, following training, the rats were sacrifice
d either at rest or following 10 min. of exercise.
Because the AMPK response to exercise decreases with increased training duration, many questions arise that would challenge the AMPK role with respect to biochemical adaptation
s to exercise and endurance training. This is due in part to the marked increases in the biogenesis
and upregulation of mitochondria, GLUT-4, UCP-3
, Hexokinase
II and other metabolic and mitochondrial enzymes despite decreases in AMPK activity with training. Questions also arise because skeletal muscle
cells
which express these decreases in AMPK activity in response to endurance training also seem to be maintaining an oxidative dependent approach to energy metabolism, which is likewise thought to be regulated to some extent by AMPK activity.
If the AMPK response to exercise is responsible in part for biochemical adaptations to training, how then can these adaptations to training be maintained if the AMPK response to exercise is being attenuated with training? It is hypothesized that these adaptive roles to training are maintained by AMPK activity and that the increases in AMPK activity in response to exercise in trained skeletal muscle have not yet been observed due to biochemical adaptations that the training itself stimulated in the muscle tissue to reduce the metabolic need for AMPK activation. In other words, AMPK will not become activated until it is "apparent" that the cell is in need of greater adaptation
to exercise. Until energy stores (ATP) are depleted (ATP low + AMP high), AMPK will remain inactivated. Biochemical preparations for a high-intensity energy challenge must be exhausted before AMPK is to be activated in order to mediate further metabolic adaptations to exercise.
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 plays a role in cellular energy homeostasis. It consists of three proteins (subunits
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...
) that together make a functional enzyme
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...
, conserved from yeast to humans. It is expressed in a number of tissues, including 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...
, 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,...
, and skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
. The net effect of AMPK activation is stimulation of hepatic fatty acid oxidation and ketogenesis
Ketogenesis
Ketogenesis is the process by which ketone bodies are produced as a result of fatty acid breakdown.-Production:Ketone bodies are produced mainly in the mitochondria of liver cells. Its synthesis occurs in response to low glucose levels in the blood, and after exhaustion of cellular carbohydrate...
, inhibition
Enzyme inhibitor
An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...
of cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
synthesis, lipogenesis
Lipogenesis
Lipogenesis is the process by which acetyl-CoA is converted to fats. The former is an intermediate stage in metabolism of simple sugars, such as glucose, a source of energy of living organisms. Through lipogenesis, the energy can be efficiently stored in the form of fats...
, and 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....
synthesis, inhibition of adipocyte lipolysis
Lipolysis
Lipolysis is the breakdown of lipids and involves the hydrolysis of triglycerides into free fatty acids followed by further degradation into acetyl units by beta oxidation. The process produces Ketones, which are found in large quantities in ketosis, a metabolic state that occurs when the liver...
and lipogenesis, stimulation of skeletal muscle fatty acid oxidation and muscle glucose uptake, and modulation of insulin secretion by pancreatic beta-cells.
It should not be confused with cyclic AMP-activated protein kinase (protein kinase A), which, although being of similar nature, may have opposite effects.
Structure
The heterotrimeric protein AMPK is formed by α, β, and γ subunits. Each of these three subunits takes on a specific role in both the stability and activity of AMPK. Specifically, the γ subunit includes four particular Cystathionine beta synthase (CBS) domainsCBS domain
The CBS domain is a protein domain found in a range of proteins in all species from bacteria to man. It was first identified as a conserved sequence region in 1997 and named after cystathionine beta synthase, one of the proteins it is found in...
giving AMPK its ability to sensitively detect shifts in the AMP
Adenosine monophosphate
Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...
:ATP
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...
ratio. The four CBS domains create two binding sites for AMP commonly referred to as Bateman domains. Binding of one AMP
Adenosine monophosphate
Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...
to a Bateman domain cooperatively increases the binding affinity
Chemical affinity
In chemical physics and physical chemistry, chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds...
of the second AMP to the other Bateman domain. As AMP binds both Bateman domains the γ subunit undergoes a conformational change which exposes the catalytic domain found on the α subunit. It is in this catalytic domain where AMPK becomes activated when phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
takes place at threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
-172 by an upstream
Upstream (bioprocess)
The upstream part of a bioprocess refers to the first step in which biomolecules are grown, usually by bacterial or mammalian cell lines , in bioreactors. When they reach the desired density they are harvested and moved to the downstream section of the bioprocess....
AMPK kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
(AMPKK). The α, β, and γ subunits can also be found in different isoforms: the γ subunit can exist as either the γ1, γ2 or γ3 isoform; the β subunit can exist as either the β1 or β2 isoform; and the α subunit can exist as either the α1 or α2 isoform. Although the most common isoforms expressed in most cells are the α1, β1, and γ1 isoforms, it has been demonstrated that the α2, β2, γ2, and γ3 isoforms are also expressed in cardiac and skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
.
The following humans gene encode AMPK subunits:
- α – PRKAA1PRKAA15'-AMP-activated protein kinase catalytic subunit alpha-1 is an enzyme that in humans is encoded by the PRKAA1 gene.-Interactions:Protein kinase, AMP-activated, alpha 1 has been shown to interact with TSC2.-Further reading:...
, PRKAA2PRKAA25'-AMP-activated protein kinase catalytic subunit alpha-2 is an enzyme that in humans is encoded by the PRKAA2 gene.-Further reading:... - β – PRKAB1PRKAB1-Interactions:PRKAB1 has been shown to interact with PRKAG2 and PRKAG1.The 5'-AMP-activated protein kinase beta subunit interaction domain is a conserved domain found in the beta subunit of the 5-AMP-activated protein kinase complex, and its yeast homologues Sip1, Sip2 and Gal83, which are found...
, PRKAB2PRKAB25'-AMP-activated protein kinase subunit beta-2 is an enzyme that in humans is encoded by the PRKAB2 gene.-Interactions:PRKAB2 has been shown to interact with PRKAG2 and PRKAG1.-Further reading:... - γ – PRKAG1PRKAG15'-AMP-activated protein kinase subunit gamma-1 is an enzyme that in humans is encoded by the PRKAG1 gene.-Interactions:PRKAG1 has been shown to interact with PRKAB2 and PRKAB1.-Further reading:...
, PRKAG2PRKAG25'-AMP-activated protein kinase subunit gamma-2 is an enzyme that in humans is encoded by the PRKAG2 gene.- Function :AMP-activated protein kinase is a heterotrimeric protein composed of a catalytic alpha subunit, a noncatalytic beta subunit, and a noncatalytic regulatory gamma subunit. Various...
, PRKAG3PRKAG35'-AMP-activated protein kinase subunit gamma-3 is an enzyme that in humans is encoded by the PRKAG3 gene.-Further reading:...
Function
AMPK acts as a metabolic master switch regulating several intracellularIntracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
systems including the cellular uptake 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...
, the β-oxidation of fatty acids and the biogenesis
Biogenesis
Biogenesis is the law that living things come only from other living things, e.g. a spider lays eggs, which develop into spiders. It may also refer to biochemical processes of production in living organisms.-Spontaneous generation:...
of glucose transporter 4 (GLUT4
GLUT4
Glucose transporter type 4, also known as GLUT4, is a protein that in humans is encoded by the GLUT4 gene. GLUT4 is the insulin-regulated glucose transporter found in adipose tissues and striated muscle that is responsible for insulin-regulated glucose translocation into the cell...
) and mitochondria. The energy-sensing capability of AMPK can be attributed to its ability to detect and react to fluctuations in the AMP:ATP ratio that take place during rest
Relaxation technique
A relaxation technique is any method, process, procedure, or activity that helps a person to relax; to attain a state of increased calmness; or otherwise reduce levels of anxiety, stress or anger...
and exercise (muscle stimulation). During muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
stimulation, AMP increases while ATP decreases, which changes AMPK into a good substrate for activation via an upstream kinase complex, AMPKK, or better, where binding of AMP renders activated AMPK that is phosphorylated at Thr-172 a worse substrate for protein phosphatase 2Calpha. AMPKK is a complex of three proteins, STE-related adaptor (STRAD
Strad
Strad may refer to:* Stradivarius, a stringed instrument* Steradian, the measurement of solid angles* The Strad, a classical music magazine...
), mouse protein 25 (MO25
CAB39
Calcium-binding protein 39 is a protein that in humans is encoded by the CAB39 gene.The protein encoded by this gene associates with STK11 and STRAD . CAB39 enhances formation of STK11/STRAD complexes and stimulates STK11 catalytic activity...
), and LKB1
STK11
Serine/threonine kinase 11 also known as liver kinase B1 or renal carcinoma antigen NY-REN-19 is a protein kinase that in humans is encoded by the STK11 gene.-Expression:...
(a serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
/threonine kinase). During a bout of exercise, AMPK activity increases while the muscle cell experiences metabolic stress
Stress (medicine)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...
brought about by an extreme cellular demand for ATP. Upon activation, AMPK increases cellular energy levels by inhibiting anabolic energy consuming pathways (fatty acid synthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
, protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
synthesis, etc.) and stimulating energy producing, catabolic pathways (fatty acid oxidation, glucose transport, etc.).
A recent JBC
Journal of Biological Chemistry
The Journal of Biological Chemistry is a weekly peer-reviewed scientific journal that was established in 1905. Since 1925 it is published by the American Society for Biochemistry and Molecular Biology. It covers research in any area of biochemistry or molecular biology. The editor-in-chief is...
paper on mice
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
at Johns Hopkins
Johns Hopkins
Johns Hopkins was a wealthy American entrepreneur, philanthropist and abolitionist of 19th-century Baltimore, Maryland, now most noted for his philanthropic creation of the institutions that bear his name, namely the Johns Hopkins Hospital, and the Johns Hopkins University and its associated...
has shown that when the activity of brain AMPK was pharmacologically inhibited, the mice ate less and lost weight. When AMPK activity was pharmacologically raised (AICAR
AICA ribonucleotide
5-Aminoimidazole-4-carboxamide ribotide is an intermediate in the generation of inosine monophosphate....
see below) the mice ate more and gained weight. Research in Britain has shown that the appetite-stimulating hormone ghrelin
Ghrelin
Ghrelin is a 28 amino acid peptide and hormone that is produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone...
also affects AMPK levels.
The antidiabetic drug metformin
Metformin
Metformin is an oral antidiabetic drug in the biguanide class. It is the first-line drug of choice for the treatment of type 2 diabetes, in particular, in overweight and obese people and those with normal kidney function. Its use in gestational diabetes has been limited by safety concerns...
(Glucophage) acts by stimulating AMPK, leading to reduced glucose production 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...
and reduced insulin resistance in the muscle. (Metformin usually causes weight loss and reduced appetite, not weight gain and increased appetite, which is opposite of what might be expected given the Johns Hopkins mouse study results. )
A number of recent studies suggest that the botanical alkaloid berberine
Berberine
Berberine is a quaternary ammonium salt from the protoberberine group of isoquinoline alkaloids. It is found in such plants as Berberis Berberine is a quaternary ammonium salt from the protoberberine group of isoquinoline alkaloids. It is found in such plants as Berberis Berberine is a quaternary...
, also activates AMPK and glucose transport in muscles.
Activation
Triggering the activation of AMPK can be carried out provided that two conditions are met. First, the γ subunit of AMPK must undergo a conformational change so as to expose the active siteActive 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...
(Thr-172) on the α subunit. The conformational change of the γ subunit of AMPK can be accomplished under increased concentrations of AMP. Increased concentrations of AMP will give rise to the conformational change on the γ subunit of AMPK as two AMP bind the two Bateman domains located on that subunit. It is this conformational change brought about by increased concentrations of AMP that exposes the active site (Thr-172) on the α subunit. This critical role of AMP is further substantiated in experiments that demonstrate AMPK activation via an AMP analogue 5-amino-4-imidazolecarboxamide ribotide (ZMP) which is derived from the familiar 5-amino-4-imidazolecarboxamide riboside (AICAR
AICA ribonucleotide
5-Aminoimidazole-4-carboxamide ribotide is an intermediate in the generation of inosine monophosphate....
). The second condition that must be met is the phosphorylation and consequent activation of AMPK on its activating loop at Thr-172 of the α subunit brought about by an upstream kinase (AMPKK). The complex formed between LKB1 (STK 11), mouse protein 25 (MO25), and the pseudokinase STE-related adaptor protein (STRAD) has of late been identified as the major upstream kinase responsible for phosphorylation of AMPK on its activating loop at Thr-172. Although AMPK must be phosphorylated by the LKB1/MO25/STRAD complex, it can also be regulated by allosteric modulators
Allosteric regulation
In biochemistry, allosteric regulation is the regulation of an enzyme or other protein by binding an effector molecule at the protein's allosteric site . Effectors that enhance the protein's activity are referred to as allosteric activators, whereas those that decrease the protein's activity are...
which directly increase general AMPK activity and modify AMPK to make it a better substrate
Substrate (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...
for AMPKK and a worse substrate for phosphatase
Phosphatase
A phosphatase is an enzyme that removes a phosphate group from its substrate by hydrolysing phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl group . This action is directly opposite to that of phosphorylases and kinases, which attach phosphate groups to their...
s. It has recently been found that 3-phosphoglycerate (glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
intermediate
Reaction intermediate
A reaction intermediate or an intermediate is a molecular entity that is formed from the reactants and reacts further to give the directly observed products of a chemical reaction. Most chemical reactions are stepwise, that is they take more than one elementary step to complete...
) acts to further pronounce AMPK activation via AMPKK.
Muscle contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
is the main method carried out by the body that can provide the conditions mentioned above needed for AMPK activation. As muscles contract, ATP is hydrolyzed, forming 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....
. ADP then helps to replenish cellular ATP by donating a phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
group to another ADP, forming an ATP and an AMP. As more AMP is produced during muscle contraction, the AMP:ATP ratio dramatically increases, leading to the allosteric activation of AMPK. This fact is further authenticated with studies, such as those sited above, that used electrical stimuli as a means to contract muscle to facilitate AMPK activation. For over a decade it has been known that calmodulin
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...
-dependent protein kinase kinase-beta (CaMKKbeta) can phosphorylate and thereby activate AMPK, but it was not the main AMPKK in 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...
. Richter et al. found that CaMKK inhibitors strongly inhibited AMPK phosphorylation in mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
soleus and EDL muscles after 2 minutes of contraction, but much less as time of contraction increased. CaMKK inhibitors had no effect on 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) phosphorylation and activation of AMPK. AICAR is taken into the cell
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....
and converted to ZMP, an AMP analog
Analog (chemistry)
In chemistry, a structural analog , also known as chemical analog or simply analog, is a compound having a structure similar to that of another one, but differing from it in respect of a certain component. It can differ in one or more atoms, functional groups, or substructures, which are replaced...
that has been shown to activate AMPK. Recent LKB1 knockout studies have shown that without LKB1, electrical and AICAR stimulation of muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
results in very little phosphorylation of AMPK and of ACC, providing evidence that LKB1-STRAD-MO25 is the major AMPKK in muscle.
Regulation by adipocytokines
AdipokineAdipokine
The adipokines or adipocytokines are cytokines secreted by adipose tissue.Members include:* chemerin* interleukin-6...
s, also known as adipocytokines, are secreted by adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
to take on several different but important physiological roles in the body including the regulation of 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...
, metabolism, fatty acid catabolism, coagulation
Coagulation
Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
and systemic inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
, for example. Collectively, the adipokines are in essence cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
s (cell-to-cell signaling proteins) which, when secreted, act on other cells, usually resulting in a biochemical and metabolic response. Two particular adipokines, adiponectin
Adiponectin
Adiponectin is a protein which in humans is encoded by the ADIPOQ gene.- Structure :...
and leptin
Leptin
Leptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
, have even been demonstrated to regulate AMPK.
It has been known for some time now that among many of the metabolic roles of leptin, one of its main functions in skeletal muscle is the upregulation of fatty acid oxidation. Recently, a study revealed that leptin is able to do this by way of the AMPK signaling pathway. A similar study showed that much like leptin, adiponectin also stimulates the oxidation of fatty acids via the AMPK pathway, and that it also stimulates the uptake of glucose in skeletal muscle. As of yet, the metabolic roles of leptin and adiponectin pertaining to biochemical adaptations to long-term endurance
Endurance
Endurance is the ability for a human or animal to exert itself and remain active for a long period of time, as well as its ability to resist, withstand, recover from, and have immunity to trauma, wounds, or fatigue. In humans, it is usually used in aerobic or anaerobic exercise...
training remain unclear. Certainly future studies will involve an investigation of leptin and adiponectin activities and their respective relationships with the AMPK signaling pathway immediately following a high-intensity endurance training protocol
Protocol (natural sciences)
In the natural sciences a protocol is a predefined written procedural method in the design and implementation of experiments. Protocols are written whenever it is desirable to standardize a laboratory method to ensure successful replication of results by others in the same laboratory or by other...
.
Exercise/training
Many biochemical adaptations of skeletal muscle that take place during a single bout of exercise or an extended duration of trainingTraining
The term training refers to the acquisition of knowledge, skills, and competencies as a result of the teaching of vocational or practical skills and knowledge that relate to specific useful competencies. It forms the core of apprenticeships and provides the backbone of content at institutes of...
, such as increased mitochondrial biogenesis and capacity, increased muscle 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...
, and an increase in enzyme
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...
s which specialize in glucose uptake in cells such as GLUT4 and 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....
II are thought to be mediated in part by AMPK when it is activated. Additionally, recent discoveries can conceivably suggest a direct AMPK role in increasing blood supply to exercised/trained muscle cells by stimulating and stabilizing both vasculogenesis
Vasculogenesis
Vasculogenesis is the process of blood vessel formation occurring by a de novo production of endothelial cells.-Process:Though similar to angiogenesis, the two are different in one aspect: The term angiogenesis denotes the formation of new blood vessels from pre-existing ones, whereas...
and angiogenesis
Angiogenesis
Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...
. Taken together, these adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
s most likely transpire as a result of both temporary and maintained increases in AMPK activity brought about by increases in the AMP:ATP ratio during single bouts of exercise and long-term training.
During a single acute exercise bout, AMPK allows the contracting muscle cells to adapt to the energy challenges by increasing expression of hexokinase II, translocation of GLUT4 to the plasma membrane, for glucose uptake, and by stimulating glycolysis. If bouts of exercise continue through a long-term training
Training
The term training refers to the acquisition of knowledge, skills, and competencies as a result of the teaching of vocational or practical skills and knowledge that relate to specific useful competencies. It forms the core of apprenticeships and provides the backbone of content at institutes of...
regimen, AMPK and other signals will facilitate contracting muscle adaptations by escorting muscle cell activity to a metabolic transition resulting in an oxidative dependent approach to energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
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...
as opposed to a glycolytic approach. AMPK accomplishes this transition to the oxidative mode of metabolism by upregulating and activating oxidative enzymes such as GLUT4, hexokinase II, PPARalpha
Peroxisome proliferator-activated receptor alpha
Peroxisome proliferator-activated receptor alpha , also known as NR1C1 , is a nuclear receptor protein that in humans is encoded by the PPARA gene.- Function :...
, PGC-1
PPARGC1A
Peroxisome proliferator-activated receptor gamma coactivator 1-alpha is a protein that in humans is encoded by the PPARGC1A gene....
, UCP3
UCP3
Mitochondrial uncoupling protein 3 is a protein that in humans is encoded by the UCP3 gene.-Function:Mitochondrial uncoupling proteins are members of the larger family of mitochondrial anion carrier proteins . UCPs separate oxidative phosphorylation from ATP synthesis with energy dissipated as...
, cytochrome C
Cytochrome c
The Cytochrome complex, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins. Cytochrome c is a highly soluble protein, unlike other cytochromes, with a solubility of about 100 g/L and is an...
and TFAM
TFAM
Transcription factor A, mitochondrial is a protein that in humans is encoded by the TFAM gene.-Further reading:...
.
AMPK activity increases with exercise and the LKB1/MO25/STRAD complex
Protein complex
A multiprotein complex is a group of two or more associated polypeptide chains. If the different polypeptide chains contain different protein domain, the resulting multiprotein complex can have multiple catalytic functions...
is considered to be the major upstream
Upstream (bioprocess)
The upstream part of a bioprocess refers to the first step in which biomolecules are grown, usually by bacterial or mammalian cell lines , in bioreactors. When they reach the desired density they are harvested and moved to the downstream section of the bioprocess....
AMPKK of the 5’-AMP-activated protein kinase phosphorylating the α subunit of AMPK at Thr-172. This fact is puzzling considering that although AMPKK protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
abundance has been shown to increase in skeletal tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
with endurance
Endurance
Endurance is the ability for a human or animal to exert itself and remain active for a long period of time, as well as its ability to resist, withstand, recover from, and have immunity to trauma, wounds, or fatigue. In humans, it is usually used in aerobic or anaerobic exercise...
training, its level of activity has been shown to decrease with endurance training in both trained and untrained tissue. Currently, the activity of AMPKK immediately following a 2-hr bout of exercise of an endurance trained rat is unclear. It is possible that there exists a direct link between the observed decrease in AMPKK activity in endurance trained skeletal muscle and the apparent decrease in the AMPK response to exercise with endurance training.
Maximum life span
The C. elegansCaenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...
homologue of AMPK, aak-2, has been shown by Michael Ristow
Michael Ristow
Michael Ristow is a German medical researcher who has published influential articles on the metabolic basis of human diseases, including type 2 diabetes, obesity and cancer, as well as general aging processes...
and colleagues to be required for extension of life span in states of glucose restriction mediating a process named mitohormesis.
Lipid metabolism
One of the effects of exercise is an increase in fatty acid metabolismFatty acid metabolism
Fatty acids are an important source of energy and adenosine triphosphate for many cellular organisms. Excess fatty acids, glucose, and other nutrients can be stored efficiently as fat. Triglycerides yield more than twice as much energy for the same mass as do carbohydrates or proteins. All cell...
, which provides more energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
for the cell. One of the key pathways in AMPK’s regulation of fatty acid oxidation is the phosphorylation and inactivation of acetyl-CoA carboxylase
Acetyl-CoA carboxylase
Acetyl-CoA carboxylase is a biotin-dependent enzyme that catalyzes the irreversible carboxylation of acetyl-CoA to produce malonyl-CoA through its two catalytic activities, biotin carboxylase and carboxyltransferase...
. Acetyl-CoA carboxylase (ACC) converts acetyl-CoA to malonyl-CoA
Malonyl-CoA
Malonyl-CoA is a coenzyme A derivative.-Functions:It plays a key role in chain elongation in fatty acid biosynthesis and polyketide biosynthesis....
, an inhibitor
Enzyme inhibitor
An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...
of carnitine parmitoyltransferase 1 (CPT-1
Carnitine palmitoyltransferase I
Carnitine palmitoyltransferase I also known as carnitine acyltransferase I or CAT1 is a mitochondrial enzyme. It is part of a family of enzymes called carnitine acyltransferases. Three isoforms of CPT1 are currently known: CPT1A, CPT1B, and CPT1C...
). CPT-1 transports fatty acids into the mitochondria for oxidation. Inactivation of ACC, therefore, results in increased fatty acid transport and subsequent oxidation. It is also thought that the decrease in malonyl-CoA occurs as a result of malonyl-CoA decarboxylase
Malonyl-CoA decarboxylase
Malonyl-CoA decarboxylase is an enzyme associated with Malonyl-CoA decarboxylase deficiency.It catalyzes the conversion of malonyl-CoA into acetyl-CoA and carbon dioxide.To some degree, it reverses the action of Acetyl-CoA carboxylase....
(MCD), which may be regulated by AMPK. MCD is an antagonist
Receptor antagonist
A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses...
to ACC, decarboxylating malonyl-CoA to acetyl-CoA, resulting in decreased malonyl-CoA and increased CPT-1 and fatty acid oxidation.
AMPK also plays an important role in lipid metabolism
Lipid metabolism
Lipid metabolism refers to the processes that involve the intercourse and degradation of lipids.The types of lipids involved include:* Bile salts* Cholesterols* Eicosanoids* Glycolipids* Ketone bodies* Fatty acids - see also fatty acid metabolism...
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...
. It has long been known that hepatic ACC has been regulated in the liver by phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
. AMPK also phosphorylates and inactivates 3-hydroxy-3-methylglutaryl-CoA reductase
HMG-CoA reductase
HMG-CoA reductase is the rate-controlling enzyme of the mevalonate pathway, the metabolic pathway that produces cholesterol and other isoprenoids...
(HMGCR), a key enzyme in cholesterol synthesis. HMGR converts 3-hydroxy-3-methylglutaryl-CoA, which is made from acetyl-CoA, into mevalonic acid
Mevalonic acid
Mevalonic acid is a key organic compound in biochemistry. The anion of mevalonic acid, the predominant form in biological media, is known as mevalonate. This compound is of major pharmaceutical importance...
, which then travels down several more metabolic steps to become cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
. AMPK, therefore, helps regulate fatty acid oxidation and cholesterol synthesis.
Glucose transport
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 a hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
which helps regulate 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...
levels in the body. When blood glucose is high, insulin is released from the Islets of Langerhans
Islets of Langerhans
The islets of Langerhans are the regions of the pancreas that contain its endocrine cells. Discovered in 1869 by German pathological anatomist Paul Langerhans at the age of 22, the islets of Langerhans constitute approximately 1 to 2% of the mass of the pancreas...
. Insulin, among other things, will then facilitate the uptake of glucose into cells via increased expression and translocation
Protein targeting
Protein targeting or protein sorting is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its exterior via secretion...
of glucose transporter GLUT-4. Under conditions of exercise, however, blood sugar
Blood sugar
The blood sugar concentration or blood glucose level is the amount of glucose present in the blood of a human or animal. Normally in mammals, the body maintains the blood glucose level at a reference range between about 3.6 and 5.8 mM , or 64.8 and 104.4 mg/dL...
levels are not necessarily high, and insulin is not necessarily activated, yet muscles are still able to bring in glucose. AMPK seems to be responsible in part for this exercise-induced glucose uptake. Goodyear et al. observed that with exercise, the concentration of GLUT-4 was increased in the plasma membrane, but decreased in the microsomal membranes, suggesting that exercise facilitates the translocation of vesicular GLUT-4 to the plasma membrane. While acute exercise increases GLUT-4 translocation, endurance training will increase the total amount of GLUT-4 protein available. It has been shown that both electrical contraction and AICAR treatment increase AMPK activation, glucose uptake, and GLUT-4 translocation in perfused rat hindlimb muscle, linking exercise-induced glucose uptake to AMPK. Chronic AICAR injections, simulating some of the effects of endurance training
Endurance training
Endurance training is the deliberate act of exercising to increase stamina and endurance. Exercises for endurance tends to be aerobic in nature versus anaerobic movements. Aerobic exercise develops slow twitch muscles...
, also increase the total amount of GLUT-4 protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
in the muscle cell.
Two proteins are essential for the regulation of GLUT-4 expression at a transcriptional level – myocyte enhancer factor 2 (MEF2
Mef2
In the field of molecular biology, myocyte enhancer factor-2 proteins are a family of transcription factors which through control of gene expression are important regulators of cellular differentiation and consequently play a critical role in embryonic development. In adult organisms, Mef2...
) and GLUT-4 enhancer factor (GEF). Mutations in the DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
binding regions for either of these proteins results in ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
of transgene
Transgene
A transgene is a gene or genetic material that has been transferred naturally or by any of a number of genetic engineering techniques from one organism to another....
GLUT-4 expression. These results prompted a study in 2005 which showed that AMPK directly phosphorylates GEF, but it doesn’t seem to directly activate MEF2. AICAR treatment has been shown, however, to increase transport of both proteins into the 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...
, as well as increase the binding of both to the GLUT-4 promoter region.
There is another protein involved in carbohydrate metabolism
Carbohydrate metabolism
Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms....
that is worthy of mention along with GLUT-4. The enzyme 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....
phosphorylates a six-carbon sugar, most notably 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...
, which is the first step in glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
. When glucose is transported into the cell it is phosphorylated by hexokinase. This phosphorylation keeps glucose from leaving the cell
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....
, and by changing the structure of glucose through phosphorylation, it decreases the concentration of glucose molecules, allowing a gradient for more glucose to be transported into the cell. Hexokinase II 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...
is increased in both red and white skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
upon treatment with AICAR. With chronic injections of AICAR, total protein content of hexokinase II increases in rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...
skeletal muscle.
Mitochondria
Mitochondria are often called the powerhouse of the cellCell (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....
. After pyruvate is formed from 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...
during glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
in the 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...
, it is transported into the mitochondria, where it is oxidized to acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
and enters the citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
. Oxidative phosphorylation
Oxidative phosphorylation
Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...
is the process by which NADH and FADH2 are oxidized and oxygen
Oxygen
Oxygen 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...
is reduced. This process creates a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
gradient which is used to drive ATP synthase
ATP synthase
right|thumb|300px|Molecular model of ATP synthase by X-ray diffraction methodATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate . ATP is the most commonly used "energy currency" of cells from most organisms...
and produce ATP
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...
. This process creates energy for cellular properties, and since ATP is indispensable in the contraction process, so then, are mitochondria.
Mitochondrial enzymes, such as cytochrome c
Cytochrome c
The Cytochrome complex, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins. Cytochrome c is a highly soluble protein, unlike other cytochromes, with a solubility of about 100 g/L and is an...
, succinate dehydrogenase, malate dehydrogenase
Malate dehydrogenase
Malate dehydrogenase is an enzyme in the citric acid cycle that catalyzes the conversion of malate into oxaloacetate and vice versa...
, α-ketoglutarate dehydrogenase, and citrate synthase
Citrate synthase
The enzyme citrate synthase exists in nearly all living cells and stands as a pace-making enzyme in the first step of the Citric Acid Cycle . Citrate synthase is localized within eukaryotic cells in the mitochondrial matrix, but is encoded by nuclear DNA rather than mitochondrial...
, increase in expression and activity in response to exercise. AICAR stimulation of AMPK increases cytochrome c and δ-aminolevulinate synthase (ALAS
Aminolevulinic acid synthase
ALA synthase catalyzes the synthesis of D-Aminolevulinic acid the first common precursor in the biosynthesis of all tetrapyrroles. The enzyme is expressed in all non-plant eukaryotes and the α-class of proteobacteria. Other organisms produce ALA through a three enzyme pathway known as the Shemin...
), a rate-limiting enzyme involved in the production of heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
. Malate dehydrogenase
Malate dehydrogenase
Malate dehydrogenase is an enzyme in the citric acid cycle that catalyzes the conversion of malate into oxaloacetate and vice versa...
and succinate dehydrogenase also increase, as well as citrate synthase activity, in rats treated with AICAR injections. Conversely, in LKB1 knockout mice, there are decreases in cytochrome c and citrate synthase activity, even if the mice are “trained” by voluntary exercise.
[Peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α
PPARGC1A
Peroxisome proliferator-activated receptor gamma coactivator 1-alpha is a protein that in humans is encoded by the PPARGC1A gene....
) is a transcriptional regulator for genes involved in mitochondrial biogenesis
Mitochondrial biogenesis
Mitochondrial biogenesis is the process by which new mitochondria are formed in the cell. Mitochondrial biogenesis is activated by numerous different signals during times of cellular stress or in response to environmental stimuli...
, fatty acid oxidation, and 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....
.
To do this, it enhances the activity of transcription factors like nuclear respiratory factor 1 (NRF-1
NRF1
Nuclear respiratory factor 1, also known as Nrf1, Nrf-1, NRF1 and NRF-1, encodes a protein that homodimerizes and functions as a transcription factor which activates the expression of some key metabolic genes regulating cellular growth and nuclear genes required for respiration, heme biosynthesis,...
), myocyte
Myocyte
A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long, long chains of sarcomeres, the contractile units of the cell....
enhancer factor 2 (MEF2), host cell factor (HCF), and others. It also has a positive feedback loop, enhancing its own expression.
Both MEF2 and cAMP
Cyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...
response element (CRE) are essential for contraction-induced PGC-1α promoter activity. AMPK is required for increased PGC-1α expression in skeletal muscle in response to creatine
Creatine
Creatine is a nitrogenous organic acid that occurs naturally in vertebrates and helps to supply energy to all cells in the body, primarily muscle. This is achieved by increasing the formation of Adenosine triphosphate...
depletion. LKB1 knockout mice show a decrease in PGC-1α, as well as mitochondrial proteins.
Thyroid hormone
AMPK and thyroidThyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
regulate some similar processes. Knowing these similarities, Winder and Hardie et al. designed an experiment to see if AMPK was influenced by thyroid hormone
Thyroid hormone
The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...
. They found that all of the subunits of AMPK were increased in skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
, especially in the soleus and red quadriceps, with thyroid hormone treatment. There was also an increase in phospho-ACC, a marker of AMPK activity.
Controversy
A seemingly paradoxical role of AMPK occurs when we take a closer look at the energy-sensing enzymeEnzyme
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...
in relation to exercise and long-term training. Similar to short-term acute training scale, long-term endurance training studies also reveal increases in oxidative metabolic enzymes and increases in GLUT-4, mitochondrial size and quantity, and an increased dependency on the oxidation of fatty acids; however, Winder et al. reported in 2002 that despite observing these increased oxidative biochemical adaptations to long-term endurance training (similar to those mentioned above), the AMPK response (activation of AMPK with the onset of exercise) to acute bouts of exercise decreased in red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...
quadriceps (RQ) with training (3 – see Fig.1). Conversely, the study did not observe the same results in white
White
White is a color, the perception of which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness compared to the surroundings. A white visual stimulation will be void of hue and grayness.White light can be...
quadriceps (WQ) and soleus (SOL) muscles that they did in RQ. The trained rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...
s used for that endurance study
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
ran on treadmill
Treadmill
A treadmill is an exercise machine for running or walking while staying in one place. The word treadmill traditionally refers to a type of mill which was operated by a person or animal treading steps of a wheel to grind grain...
s 5 days/wk in two 1-h sessions, morning
Morning
The word morning originally referred to the sunrise. Morning precedes midday, afternoon, and night in the sequence of a day.Morning is the part of the day usually reckoned from dawn to noon...
and afternoon
Afternoon
Afternoon is the time of day from noon to about 18:00.The term should not be confused with "after noon" , which is a translation of the Latin "post meridiem" as used in the 12-hour clock, meaning a time of day from noon to midnight.In Australia and New Zealand, the word "arvo" is a slang term for...
. The rats were also running up to 31m/min (grade 15%). Finally, following training, the rats were sacrifice
Sacrifice
Sacrifice is the offering of food, objects or the lives of animals or people to God or the gods as an act of propitiation or worship.While sacrifice often implies ritual killing, the term offering can be used for bloodless sacrifices of cereal food or artifacts...
d either at rest or following 10 min. of exercise.
Because the AMPK response to exercise decreases with increased training duration, many questions arise that would challenge the AMPK role with respect to biochemical adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
s to exercise and endurance training. This is due in part to the marked increases in the biogenesis
Biogenesis
Biogenesis is the law that living things come only from other living things, e.g. a spider lays eggs, which develop into spiders. It may also refer to biochemical processes of production in living organisms.-Spontaneous generation:...
and upregulation of mitochondria, GLUT-4, UCP-3
UCP3
Mitochondrial uncoupling protein 3 is a protein that in humans is encoded by the UCP3 gene.-Function:Mitochondrial uncoupling proteins are members of the larger family of mitochondrial anion carrier proteins . UCPs separate oxidative phosphorylation from ATP synthesis with energy dissipated as...
, 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....
II and other metabolic and mitochondrial enzymes despite decreases in AMPK activity with training. Questions also arise because skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
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....
which express these decreases in AMPK activity in response to endurance training also seem to be maintaining an oxidative dependent approach to energy metabolism, which is likewise thought to be regulated to some extent by AMPK activity.
If the AMPK response to exercise is responsible in part for biochemical adaptations to training, how then can these adaptations to training be maintained if the AMPK response to exercise is being attenuated with training? It is hypothesized that these adaptive roles to training are maintained by AMPK activity and that the increases in AMPK activity in response to exercise in trained skeletal muscle have not yet been observed due to biochemical adaptations that the training itself stimulated in the muscle tissue to reduce the metabolic need for AMPK activation. In other words, AMPK will not become activated until it is "apparent" that the cell is in need of greater adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
to exercise. Until energy stores (ATP) are depleted (ATP low + AMP high), AMPK will remain inactivated. Biochemical preparations for a high-intensity energy challenge must be exhausted before AMPK is to be activated in order to mediate further metabolic adaptations to exercise.