Galactose sometimes abbreviated Gal, is a monosaccharidesugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is a C-4 epimer of glucose. A galactose molecule linked with a glucose molecule forms a lactose molecule. Galactan is a polymeric form of galactose found in hemicellulose, and forming the core of the galactans, a class of natural polymeric carbohydrates.
Etymology
The word galactose was coined by Charles Weissman in the mid 19th century and is derived from Greek galaktos and the generic chemical suffix for sugars -ose. The etymology is comparable to that of the wordlactose in that both contain roots meaning "milk sugar". Lactose is a disaccharide of galactose plus glucose.
Structure and isomerism
Galactose exists in both open-chain and cyclic form. The open-chain form has a carbonyl at the end of the chain. Four isomers are cyclic, two of them with a pyranose ring, two with a furanose ring. Galactofuranose occurs in bacteria, fungi and protozoa, and is recognized by a putative chordate immune lectin intelectin through its exocyclic 1,2-diol. In the cyclic form there are two anomers, named alpha and beta, since the transition from the open-chain form to the cyclic form involves the creation of a new stereocenter at the site of the open-chain carbonyl. In the beta form, the alcohol group is in the equatorial position, whereas in the alpha form, the alcohol group is in the axial position.
Relationship to lactose
Galactose is a monosaccharide. When combined with glucose, through a condensation reaction, the result is the disaccharide lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzymes lactase and β-galactosidase. The latter is produced by the lac operon in Escherichia coli. In nature, lactose is found primarily in milk and milk products. Consequently, various food products made with dairy-derived ingredients can contain lactose. 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, galactose-1-phosphate uridyltransferase, and UDP-galactose-4’-epimerase. In human lactation, glucose is changed into galactose via hexoneogenesis to enable the mammary glands to secrete lactose. However, most lactose in breast milk is synthesized from galactose taken up from the blood, and only 35±6% is made from galactose from de novo synthesis. Glycerol also contributes some to the mammary galactose production.
Metabolism
Glucose 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. 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. The Leloir pathway then carries out the conversion of α-D-galactose to UDP-glucose via three principal enzymes: Galactokinase phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P; Galactose-1-phosphate uridyltransferase transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose; and finally, UDP galactose-4’-epimerase interconverts UDP-galactose and UDP-glucose, thereby completing the pathway. Galactosemia is an inability to properly break down galactose due to a genetically inherited mutation in one of the enzymes in the Leloir pathway. As a result, the consumption of even small quantities is harmful to galactosemics.
Clinical significance
Chronic systemic exposure of mice, rats, and Drosophila to D-galactose causes the acceleration of senescence. It has been reported that high dose exposure of D-galactose can cause reduced sperm concentration and sperm motility in rodent and has been extensively 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/day. More research is necessary to ascertain possible risks. Some ongoing studies suggest galactose may have a role in treatment of focal segmental glomerulosclerosis. 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. In O and A antigens, there are two monomers of galactose on the antigens, whereas in the B antigens there are three monomers of galactose. A disaccharide composed of two units of galactose, galactose-alpha-1,3-galactose, has been recognized as a potential allergen present in mammal meat. Alpha-gal allergy may be triggered by lone star tick bites.
History
In 1855, E. O. Erdmann noted that hydrolysis of lactose produced a substance besides glucose. Galactose was first isolated and studied by Louis Pasteur in 1856 and he called it "lactose". In 1860, Berthelot renamed it "galactose" or "glucose lactique". In 1894, Emil Fischer and Robert Morrell determined the configuration of galactose.
