Galactose (from Greek γάλακτος galaktos "milk"), sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is considered a nutritive sweetener because it has food energy. It is an epimer of glucose.
Galactan is a polymer of the sugar galactose found in hemicellulose. Galactan can be converted to galactose by hydrolysis.
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. 
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 and β-galactosidase. This enzyme is produced by the lac operon in Escherichia coli.
In the human body, glucose is changed into galactose via hexoneogenesis 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.  Glycerol also contributes some to the mammary galactose production.
Galactose 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.
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