Galactose, “milk sugar”, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose.

It is an aldohexose and 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.

D-Galactose is also known as brain sugar: it is a component of glycoproteins found in nerve tissue.

Galactose is a monosaccharide: When combined with glucose through a condensation reaction, the result is a disaccharide called lactose. 

The hydrolysis of lactose to glucose and galactose is catalyzed by the enzymes lactase and β-galactosidase. 

β-galactosidase  is produced by Escherichia coli.

In nature, lactose is found primarily in milk and milk products. 

Various food products made with dairy-derived ingredients contain lactose.

Galactose metabolism converts galactose into glucose, is carried out by the three principal enzymes: galactokinase (GALK), galactose-1-phosphate uridyltransferase (GALT), and UDP-galactose-4’-epimerase (GALE).

In human lactation, galactose is required in a 1 to 1 ratio with glucose enabling the mammary glands to synthesize and secrete lactose, suggesting that the synthesis of galactose is supplemented by direct uptake and of use of plasma galactose.

Glucose is more stable than galactose and is less susceptible to the formation of nonspecific glycoconjugates.

The main pathway of galactose metabolism is the Leloir pathway, and other pathways are necessary because the human body cannot directly convert galactose into energy, and must first go through one of these processes in order to utilize the sugar.

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.

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

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

It is a by-product from the third-generation ethanol production process from macroalgae.

Galactose is a component of the antigens present on blood cells that distinguish 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.

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