Abstract
Asn-linked oligosaccharides on glycoproteins are divided into four subgroups: mannan-, high-mannose-, hybrid- and complex-type. Complex-type oligosaccharides have SA-Gal-G1cNAc-branches on terminal mannosides of the core structure (Man3GlcNAc2), and they are identified as a bi-, tri-, or tetra antennary structure depending on how many branches they have. A bisect structure that has a GlcNAc linked to the β1-4 mannose of the core structure has also been observed in many species. The branched portions of oligosaccharides affect 1) the interaction of the outer parts of oligosaccharides with other molecules such as lectins by increasing the multivalency of ligands, and 2) the turnover of proteins in circulation mainly because of the bulky structure of branched oligosaccharides. Several factors determine oligosaccharide branchings. The enzymes that catalyze branchings are N-acetylglucosaminyltransferases, some of which require products of other enzymes or inhibit actions of other enzymes. Furthermore, the timing when a-mannosidase II and galactosyltransferase react on the substrate oligosaccharides may also influence the number of branches. The activities and distribution in the Golgi apparatus of these enzymes are enzymatic factors that determine the branchings. A higher structure of peptide backbones may also influence the size of sugar chains. All the mammalian enzymes regulating branched structures are now available because GnT-IV, the missing link of GnTs, has been purified and cloned by our group. Several attempts to alter sugar-chain structures by using some of those enzymes have already been reported. In the future, a method to freely control the structures of Asn-linked oligosaccharides will be developed by regulating the expressions of these enzymes.
