Galactose

Galactose
Identifiers
CAS number	26566-61-0
PubChem	6036
MeSH	Galactose
Properties
Molecular formula	C6H12O6
Molar mass	180.156 g mol-1
Melting point	167°C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Galactose (Gal) is a type of sugar that is less sweet than glucose. It is considered a nutritive sweetener because it has food energy. Its name comes from the Ancient Greek word for milk, γάλακτος (galaktos). It is an epimer of glucose.

Galactan is a polymer of the sugar galactose. It is found in hemicellulose and can be converted to galactose by hydrolysis. Galactose solubility in water is 68.30 grams per 100 grams of water at 20–25°C.

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 (six-membered) ring, two with a furanose (five-membered) ring. Galactofuranose occurs in bacteria, fungi and protozoa. ^[1]

Cyclic forms of galactose

Relationship to lactose

Galactose is a monosaccharide. When combined with glucose, through a dehydration reaction, the result is the disaccharide lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzyme lactase, a β-galactosidase.

In the human body, glucose is changed into galactose via hexoneogenesis in order to enable the mammary glands 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. ^[2] Glycerol also contributes some to the mammary galactose production. ^[3]

Galactose and glucose are produced by hydrolysis of lactose by β-galactosidase. This enzyme is produced by the lac operon in Escherichia coli (E. coli).

Sources

It is found in dairy products, sugar beets, and other gums and mucilages.

It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues.

Clinical significance

Chronic systemic exposure of mice, rats, and Drosophila to D-galactose causes the acceleration of senescence and has been used as an aging model.^[4] Two studies have suggested a possible link between galactose in milk and ovarian cancer.^[5][6] Other studies show no correlation, even in the presence of defective galactose metabolism.^[7][8] 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.^[9] More research is necessary to ascertain possible risks.

There are some ongoing studies that suggest that 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.^[10]

Liver galactose metabolism

In the liver, galactose is converted to glucose 6-phosphate in the following reactions:

       galacto-                uridyl                phosphogluco-
        kinase               transferase                mutase
   gal --------> gal 1 P ------------------> glc 1 P -----------> glc 6 P
                            ^           \
                           /             v
                        UDP-glc       UDP-gal
                           ^             /
                            \___________/
                              epimerase

Metabolic disorders

There are 3 types of galactosemia or galactose deficiencies:

References