Glycoproteins - Hughes, R. 1985

Biosynthesis
Control of glycan biosynthesis
Glycosyltransferase modulators

Another type of control over glycosylation reactions can be illustrated by galactosyltransferase, which is responsible for lactose synthesis (Fig. 3.3). In Mammary Glands, this activity is exhibited by a complex consisting of a catalytic glycoprotein subunit with a Molecular Weight of 50,000 and α-lactalbumin with a molecular weight of 14,000. These subunits form a 1 : 1 molecular complex that catalyzes The transfer of galactose from UDP-galactose to glucose (Fig. 3.3,B). However, in Tissues that do not produce lactose, only the catalytic subunit is present. In this case, it transfers galactose from UDP-galactose to the terminal N-acetylglucosamine residues in Glycoproteins or to free N-acetylglucosamine, forming the ga1β1→4glcNAc sequence found in many N-glycans. The α-lactalbumin regulatory subunit promotes the binding of glucose to galactosyltransferase through a series of synergistic equilibria, lowering the K_m value from 2 M to 2 mM or lower. Since mammary tissue is the only tissue containing α-lactalbumin, Lactose synthesis is catalyzed at physiological glucose concentrations. Thus, α-lactalbumin acts as a non-catalytic modulator of galactosyltransferases.

It would be extremely interesting to discover whether other Examples of this type of genetic Introduction/15.html">Regulation of enzyme Activity exist, particularly among Glycosyltransferases capable of attaching the same monosaccharide to highly similar substrates.