Schachter H, Narasimhan S, Gleeson P, Vella G
Can J Biochem Cell Biol. 1983 Sep;61(9):1049-66. doi: 10.1139/o83-134.
Many mammalian and avian complex carbohydrates (glycoproteins and glycolipids) have highly branched oligosaccharides. Although the function of complex carbohydrates is not known, there is evidence to suggest that oligosaccharide branching may be an important factor in the process by which cells recognize one another and their environment. Asparagine-linked (N-glycosyl) oligosaccharides can be subdivided into at least 12 classes according to their branching patterns. It is presently believed that these classes all stem from a common precursor oligosaccharide containing three D-glucose, nine D-mannose, and two N-acetyl-D-glucosamine residues. This precursor is incorporated into the protein backbone in the rough endoplasmic reticulum and is then processed within the endoplasmic reticulum and Golgi apparatus by a series of highly specific glycosidases and glycosyltransferases to yield the various classes of N-glycosyl oligosaccharides. The branches that occur in N-glycosyl oligosaccharides are usually initiated by the incorporation of a N-acetylglucosamine (GlcNAc) residue. Our laboratory has studied four of the N-acetylglucosaminyltransferases (GlcNAc-transferases) involved in this initiation process. We have defined various factors which determine the synthetic pathway. There are at least three types of control that are commonly found. (i) Tissues differ in the relative activities of the different glycosyltransferases and glycosidases and, therefore, competition between two or more enzymes for a common intermediate often determines the synthetic route. (ii) The incorporation of a key glycosyl residue into an oligosaccharide may convert a nonsubstrate to a substrate for either a glycosyltransferase or a glycosidase. (iii) Conversely, the incorporation of a key residue may convert a substrate into a nonsubstrate. Other controls are undoubtedly operative during glycoprotein synthesis: e.g., the effect of the polypeptide sequence on transferase specificity, the distribution of transferases along the endomembrane system, and compartmentation and the availability of substrates and cofactors. These factors have not been studied in our laboratory. However, the oligosaccharides made by the hen oviduct correlate quite well with the control factors elucidated by our approach; other tissues are presently under investigation. Recent studies on the three-dimensional structures of N-glycosyl oligosaccharides have enabled us to explain certain features of glycosyltransferase substrate specificity on the basis of steric factors.
许多哺乳动物和鸟类的复合碳水化合物（糖蛋白和糖脂）都有高度分支的寡糖。虽然复合碳水化合物的功能尚不清楚，但有证据表明寡糖分支可能是细胞相互识别及其环境过程中的一个重要因素。天冬酰胺连接的（N-糖基化）寡糖根据其分支模式可至少分为12类。目前认为，这些类别都起源于一种含有三个D-葡萄糖、九个D-甘露糖和两个N-乙酰-D-葡萄糖胺残基的共同前体寡糖。这种前体在内质网中被整合到蛋白质主链中，然后在内质网和高尔基体中通过一系列高度特异性的糖苷酶和糖基转移酶进行加工，以产生各种类别的N-糖基寡糖。N-糖基寡糖中出现的分支通常由N-乙酰葡萄糖胺（GlcNAc）残基的掺入引发。我们实验室研究了参与这一引发过程的四种N-乙酰葡萄糖胺转移酶（GlcNAc转移酶）。我们已经确定了决定合成途径的各种因素。通常发现至少有三种类型的调控。（i）不同组织中不同糖基转移酶和糖苷酶的相对活性不同，因此，两种或更多种酶对共同中间体的竞争常常决定合成途径。（ii）关键糖基残基掺入寡糖中可能会将非底物转化为糖基转移酶或糖苷酶的底物。（iii）相反，关键残基的掺入可能会将底物转化为非底物。在糖蛋白合成过程中无疑还有其他调控机制在起作用：例如，多肽序列对转移酶特异性的影响、转移酶沿内膜系统的分布、区室化以及底物和辅因子的可用性。我们实验室尚未研究这些因素。然而，母鸡输卵管产生的寡糖与我们方法所阐明的调控因素相当吻合；目前正在对其他组织进行研究。最近对N-糖基寡糖三维结构的研究使我们能够根据空间因素解释糖基转移酶底物特异性的某些特征。
