Control of glycoprotein synthesis. Bovine milk UDPgalactose:N-acetylglucosamine 4-.beta.-galactosyltransferase catalyzes the preferential transfer of galactose to the GlcNAc.beta.l1,2Man.alpha.1,3- branch of both bisected and nonbisected complex biantennary asparagine-linked oligosaccharides

Abstract

Bovine milk UDP-galactose:N-acetylglucosamine .beta.-4-galactosyltransferase has been used to investigate the effect of a bisecting GlcNAc [N-acetyl-D-glucosamine] residue (linked .beta.1,4 to the .beta.-linked mannose of the trimannosyl core of aparagine-linked complex oligosaccharides). Columns of immobilized lectins (concanavalin A, erythroagglutinating phytohemagglutinin, and Ricinus communis agglutinin 12) were used to separate the various products of the reactions. Preferential galactosylation of the GlcNAc.beta.1,2Man.alpha.1,3 [GlcNAc Beta 1,2-mannose alpha-1] arm occurred both in the absence and in the presence of bisecting GlcNAc residue; the ratio of the rates of galactosylation of the Man.alpha.1,3 arm to the Man.alpha.1,6 arm was 6.5 in the absence of bisecting GlcNAc and 2.8 in its presence. The bisecting GlcNAc residue reduced galactosylation ofthe Man.alpha.1,3 arm by .apprx. 78% probably due to steric hindrance of the GlcNAc.beta.1,2Man.alpha.1,3Man.beta.1,4 region of the substrate by the bisecting GlcNAc. This steric hindrance prevents the action of 4 other enzymes involved in assembly of complex asparagine-linked oligosaccharides and indicates the importance of the bisecting GlcNAc residue in the control of glycoprotein biosynthesis.The Man.alpha.1,3 arm of biantennary oligosaccharides is believed to be freely accessible to enzyme action whereas the Man.alpha.1,6 arm is believed to be folded back toward the core. This may explain the preferential action of Gal-transferase on the Man.alpha.1,3 arm of both bisected and nonbisected oligosaccharides.