终於找到了Orz http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.4816 The different structures of N- and O-linked oligosaccharides reflect differences in their biosynthesis. O-linked sugars are added one at a time, and each sugar transfer is catalyzed by a different glycosyltransferase enzyme. In contrast, biosynthesis of N-linked oligosaccharides begins with the addition of a large preformed oligosaccharide, containing 14 sugar residues; subsequently certain sugar residues are removed and others are added, one at a time, in a defined order with each reaction catalyzed by a different enzyme. As described below, the various steps in the formation of both O- and N-linked oligosaccharides occur in specific organelles. O-linked Enzyme是一个一个把糖接上去的 N-linked Enzyme是接完一串之後再修饰小部分 每个过程都有不同的酵素参与(换句话说就是酵素的构型决定Sequence) All known glycosyltransferases that act on secretory proteins are integral membrane proteins with active sites facing the lumen of the organelle. Each glycosyltransferase is specific for both the donor sugar nucleotide and the acceptor molecule. The galactosyltransferase depicted in Figure 17-32, for instance, only transfers a galactose residue (from UDP-galactose), and only to the 3 carbon atom of an acceptor N-acetylgalactosamine residue. A different enzyme transfers galactose to the 4 carbon of N-acetylglucosamine, and yet another transfers galactose to the 3 carbon of galactose. 目前已知作用在外泌蛋白的醣转移酵素都是胞器膜上的镶膜蛋白 每个醣转移酵素只针对特定的糖和受体有作用 举个例子 某些galactosyltransferase只转移UDP-gal.的gal.到N-acetylgalactosamine的3号碳上 而其他种类的galactosyltransferase却可以转到N-acetylglucosamine的4号碳等等 --

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