N-Acetylglucosaminyltransferase-III (β-1,4-mannosyl-glycoprotein β1,4-N-acetylglu- cosaminyltransferase: EC 18.104.22.168) catalyzes the formation of a unique structure, bisecting GlcNAc, and is involved in the biosynthesis of complex and hybrid types of N-glycans. The addition of the bisecting GlcNAc residue to a core β-mannose by the enzyme prevents the actions of other GlcNAc transferases involved in the biosynthesis of multiantennary sugar chains, leading to a decrease in the branch formation of N-glycans. Because of this regulatory role, the enzyme has been considered to be a key glycosyltransferase in N-glycan biosynthesis. Relatively high levels of the activity were found in kidney and brain of mammals (Nishikawa et al. 1988a). Expression of GnT-III is enhanced during hepatocarcinogenesis, while the activity is nearly undetectable in normal liver (Narasimhan et al. 1988; Nishikawa et al. 1988b). Since expression of the enzyme appears to lead to a remarkable structural alteration of the sugar chains on the cell surface, it seems that the enzyme is associated with various biological events such as differentiation and carcinogenesis.
KeywordsSugar Chain Orotic Acid GlcNAc Residue Branch Formation Glycoprotein Synthesis
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