Biosynthesis of Amino Sugars

Amino sugars are essential for making glycoconjugates - macromolecules containing covalently bound oligosaccharide chains. Glycoconjugates include glycoproteins and glycolipids.

General - Glucose is a metabolic precursor for all sugars not in the diet. The glycolysis intermediate, fructose-6-phosphate (F6P) is the precursor of the amino sugars. The nitrogen comes from the amide group of glutamine in the following reaction, which is catalyzed by glutamine:fructose-6-phosphate amidotransferase, is as follows:

> F6P + Glutamine <=> Glucosamine-6-phosphate + Glutamate

In this reaction, oxidation of carbon 1 is coupled to reduction of carbon 2. Next, glucosamine-6-phosphate is converted to UDP-N-acetylglucosamine, in the three steps shown in Figure 16.13.

Sialic Acid - Figure 16.14 depicts the synthesis of N-acetylneuraminic acid (sialic acid) from UDP-N-acetylglucosamine. Sialic acid is an important constituent of glycoproteins. Activated sialic acid is made by linking, not to a nucleoside diphosphate (as in other sugar derivative activations), but to a nucleoside monophosphate, CMP, in the following reaction:

> CTP + Sialic Acid <=> CMP-Sialic acid + PP i

N-acetyl--lactosamine - The reaction is catalyzed by galactosyltransferase as follows:

> UDP-Galactose + N-Acetylglucosamine <=> UDP + N-Acetyl-Lactosamine

In lactating females, the mammary gland protein, lactalbumin (which is activated hormonally in mothers shortly after birth) binds to galactosyltransferase and changes its specificity so that it catalyzes the synthesis of lactose (the major sugar of milk), as follows:

> UDP-Galactose + Glucose <=> UDP + Lactose

See also: Biosynthesis of Glycoconjugates