Enzymatic properties and biological functions of β1,4-N-acetylglucosaminyltransferase III

beta1,4-N-Acetylglucosaminyltransferase III (GnT-III) is known to be a key glycosyltransferase which plays an important role in regulating the biosynthesis of Asn-linked oligosaccharides on glycoproteins. The regulatory role of the enzyme is based on effects of a reaction product, namely the bisecting GlcNAc structure, on the biosynthetic process. This unique structure is not tolerated by other enzymes involved in the formation of the core structures, and, as a result, prevents further reactions which are catalyzed by these enzymes. This inhibitory regulation is the result of the broad specificity of GnT-III, as well as the properties of the bisecting GlcNAc. The overexpression and ectopic expression of GnT-III lead to a variety of significant alterations in the cellular functions. Although it is not known, except for a few cases, whether the direct involvement of the bisecting GlcNAc residue or the inhibition of the synthesis of a biologically important structure of the sugar chain results in these alterations, it seems certain that marked structural changes by GnT-III catalysis result in the biological alterations in the cells. These findings suggest that GnT-III and the bisecting GlcNAc play an important role in cellular functions and that N-glycans are associated with a variety of biological events.