Galactose
Encyclopedia
Galactose sometimes abbreviated Gal, is a type of sugar
that is less sweet
than glucose
. It is a C-4 epimer
of glucose
.
Galactan is a polymer
of the sugar galactose found in hemicellulose
. Galactan can be converted to galactose by hydrolysis
.
at the end of the chain.
Four isomers are cyclic, two of them with a pyranose
(six-membered) ring, two with a furanose
(five-membered) ring. Galactofuranose occurs in bacteria, fungi and protozoa.
. When combined with glucose
(monosacccharide), through a dehydration reaction
, the result is the disaccharide
lactose
. The hydrolysis
of lactose to glucose and galactose is catalyzed
by the enzyme
s lactase
and β-galactosidase
. The latter is produced by the lac operon
in Escherichia coli
.
Lactose is found primarily in milk and milk products. Galactose metabolism
, which converts galactose into glucose, is carried out by the three principal enzymes in a mechanism known as the Leloir pathway
. The enzymes are listed in the order of the metabolic pathway: galactokinase (GALK), galactose-1-phosphate uridyltransferase (GALT), and UDP-galactose-4’-epimerase (GALE).
In the human body, glucose is changed into galactose via hexoneogenesis to enable the mammary gland
s to secrete lactose. However, most galactose in breast milk
is synthesized from galactose taken up from the blood, and only 35±6% is made by de novo synthesis.
Glycerol also contributes some to the mammary galactose production.
The main pathway of galactose metabolism is the Leloir pathway
; humans and other species, however, have been noted to contain several alternate pathways, such as the De Ley Doudoroff pathway. The Leloir pathway consists of the latter stage of a two-part process that converts β-D-galactose to UDP-glucose
. The initial stage is the conversion of β-D-galactose to α-D-galactose by the enzyme, mutarotase (GALM). The Leloir pathway then carries out the conversion of α-D-galactose to UDP-glucose via three principle enzymes. Galactokinase (GALK) phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P. Galactose-1-phosphate uridyltransferase (GALT) then transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose. Finally, UDP galactose-4’-epimerase (GALE) interconverts UDP-galactose and UDP-glucose, thereby completing the pathway. a 517 b 516 c 519
s, sugar beet
s, and other gum
s and mucilage
s. It is also synthesized
by the body, where it forms part of glycolipid
s and glycoprotein
s in several tissue
s.
, rats
, and Drosophila
to D-galactose causes the acceleration of senescence
and has been used as an aging model.
Two studies have suggested a possible link between galactose in milk and ovarian cancer
. Other studies show no correlation, even in the presence of defective galactose metabolism. More recently, pooled analysis done by the Harvard School of Public Health
showed no specific correlation between lactose-containing foods and ovarian cancer, and showed statistically insignificant increases in risk for consumption of lactose at ≥30 g/d. More research is necessary to ascertain possible risks.
Some ongoing studies suggest galactose may have a role in treatment of focal segmental glomerulosclerosis
(a kidney disease resulting in kidney failure and proteinuria). This effect is likely to be a result of binding of galactose to FSGS factor.
Galactose is a component of the antigens present on blood cells that determine blood type within the ABO blood group system
.
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...
that is less sweet
Sweetness
Sweetness is one of the five basic tastes and is almost universally regarded as a pleasurable experience. Foods rich in simple carbohydrates such as sugar are those most commonly associated with sweetness, although there are other natural and artificial compounds that are sweet at much lower...
than 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...
. It is a C-4 epimer
Epimer
In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center. Diastereomers are a class of stereoisomers that are non-superposable, non-mirror images of one another....
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...
.
Galactan is a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
of the sugar galactose found in hemicellulose
Hemicellulose
A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...
. Galactan can be converted to galactose by hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
.
Structure and isomerism
Galactose exists in both open-chain and cyclic form. The open-chain form has a carbonylCarbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
at the end of the chain.
Four isomers are cyclic, two of them with a pyranose
Pyranose
Pyranose is a collective term for carbohydrates that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds...
(six-membered) ring, two with a furanose
Furanose
A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom...
(five-membered) ring. Galactofuranose occurs in bacteria, fungi and protozoa.
Relationship to lactose
Galactose is a monosaccharideMonosaccharide
Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...
. When combined with 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...
(monosacccharide), through a dehydration reaction
Dehydration reaction
In chemistry and the biological sciences, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reactions...
, the result is the disaccharide
Disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...
lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
. The hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of lactose to glucose and galactose is catalyzed
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
by the 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 lactase
Lactase
Lactase , a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers...
and β-galactosidase
Beta-galactosidase
β-galactosidase, also called beta-gal or β-gal, is a hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides. Substrates of different β-galactosidases include ganglioside GM1, lactosylceramides, lactose, and various glycoproteins...
. The latter is produced by the lac operon
Lac operon
The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. It consists of three adjacent structural genes, lacZ, lacY and lacA. The lac operon is regulated by several factors including the availability of glucose and of...
in Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
.
Lactose is found primarily in milk and milk products. Galactose 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...
, which converts galactose into glucose, is carried out by the three principal enzymes in a mechanism known as the Leloir pathway
Leloir pathway
The Leloir pathway is a metabolic pathway for the catabolism of D-galactose. It is named after Luis Federico Leloir.In the first step α-D-galactose is phosphorylated by a kinase to galactose 1-phosphate. Also part of this pathway is a mutarotase that facilitates the conversion of β-D-galactose to...
. The enzymes are listed in the order of the metabolic pathway: galactokinase (GALK), galactose-1-phosphate uridyltransferase (GALT), and UDP-galactose-4’-epimerase (GALE).
In the human body, glucose is changed into galactose via hexoneogenesis to enable the mammary gland
Mammary gland
A mammary gland is an organ in mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders...
s to secrete lactose. However, most galactose in breast milk
Breast milk
Breast milk, more specifically human milk, is the milk produced by the breasts of a human female for her infant offspring...
is synthesized from galactose taken up from the blood, and only 35±6% is made by de novo synthesis.
Glycerol also contributes some to the mammary galactose production.
Galactose metabolism
Glucose is the primary metabolic fuel for humans. It is more stable than galactose and is less susceptible to the formation of nonspecific glycoconjugates, molecules with at least one sugar attached to a protein or lipid. Many speculate that it is for this reason that a pathway for rapid conversion from galactose to glucose has been highly conserved among many species.a 4 b 21 c 22 d 22The main pathway of galactose metabolism is the Leloir pathway
Leloir pathway
The Leloir pathway is a metabolic pathway for the catabolism of D-galactose. It is named after Luis Federico Leloir.In the first step α-D-galactose is phosphorylated by a kinase to galactose 1-phosphate. Also part of this pathway is a mutarotase that facilitates the conversion of β-D-galactose to...
; humans and other species, however, have been noted to contain several alternate pathways, such as the De Ley Doudoroff pathway. The Leloir pathway consists of the latter stage of a two-part process that converts β-D-galactose to UDP-glucose
Uridine diphosphate glucose
Uridine diphosphate glucose is a nucleotide sugar. It is involved in glycosyltransferase reactions in metabolism.-Functions:...
. The initial stage is the conversion of β-D-galactose to α-D-galactose by the enzyme, mutarotase (GALM). The Leloir pathway then carries out the conversion of α-D-galactose to UDP-glucose via three principle enzymes. Galactokinase (GALK) phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P. Galactose-1-phosphate uridyltransferase (GALT) then transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose. Finally, UDP galactose-4’-epimerase (GALE) interconverts UDP-galactose and UDP-glucose, thereby completing the pathway. a 517 b 516 c 519
Sources
Galactose is found in dairy productDairy product
Dairy products are generally defined as foods produced from cow's or domestic buffalo's milk. They are usually high-energy-yielding food products. A production plant for such processing is called a dairy or a dairy factory. Raw milk for processing comes mainly from cows, and, to a lesser extent,...
s, sugar beet
Sugar beet
Sugar beet, a cultivated plant of Beta vulgaris, is a plant whose tuber contains a high concentration of sucrose. It is grown commercially for sugar production. Sugar beets and other B...
s, and other gum
Natural gum
Natural gums are polysaccharides of natural origin, capable of causing a large viscosity increase in solution, even at small concentrations. In the food industry they are used as thickening agents, gelling agents, emulsifying agents, and stabilizers...
s and mucilage
Mucilage
Mucilage is a thick, gluey substance produced by most plants and some microorganisms. It is a polar glycoprotein and an exopolysaccharide.It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as...
s. It is also synthesized
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...
by the body, where it forms part of glycolipid
Glycolipid
Glycolipids are lipids with a carbohydrate attached. Their role is to provide energy and also serve as markers for cellular recognition.-Metabolism:...
s and glycoprotein
Glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
s in several tissue
Biological tissue
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...
s.
Clinical significance
Chronic systemic exposure of miceMICE
-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...
, rats
RATS
RATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...
, and Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
to D-galactose causes the acceleration of senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
and has been used as an aging model.
Two studies have suggested a possible link between galactose in milk and ovarian cancer
Ovarian cancer
Ovarian cancer is a cancerous growth arising from the ovary. Symptoms are frequently very subtle early on and may include: bloating, pelvic pain, difficulty eating and frequent urination, and are easily confused with other illnesses....
. Other studies show no correlation, even in the presence of defective galactose metabolism. More recently, pooled analysis done by the Harvard School of Public Health
Harvard School of Public Health
The Harvard School of Public Health is one of the professional graduate schools of Harvard University, located in the Longwood Area of the Boston, Massachusetts neighborhood of Mission Hill, which is next to Harvard Medical School. HSPH is considered a significant school focusing on health in the...
showed no specific correlation between lactose-containing foods and ovarian cancer, and showed statistically insignificant increases in risk for consumption of lactose at ≥30 g/d. More research is necessary to ascertain possible risks.
Some ongoing studies suggest galactose may have a role in treatment of focal segmental glomerulosclerosis
Focal segmental glomerulosclerosis
Focal segmental glomerulosclerosis is a cause of nephrotic syndrome in children and adolescents, as well as an important cause of kidney failure in adults.It is also known as "focal glomerular sclerosis" or "focal nodular glomerulosclerosis"....
(a kidney disease resulting in kidney failure and proteinuria). This effect is likely to be a result of binding of galactose to FSGS factor.
Galactose is a component of the antigens present on blood cells that determine blood type within the ABO blood group system
ABO blood group system
The ABO blood group system is the most important blood type system in human blood transfusion. The associated anti-A antibodies and anti-B antibodies are usually IgM antibodies, which are usually produced in the first years of life by sensitization to environmental substances such as food,...
.