Van Petegem F, Contreras H, Contreras R, Van Beeumen J
Department of Biochemistry, Physiology and Microbiology, University of Ghent, K.L. Ledeganckstraat 35, 9000 Ghent, Belgium.
J Mol Biol. 2001 Sep 7;312(1):157-65. doi: 10.1006/jmbi.2001.4946.
The process of N-glycosylation of eukaryotic proteins involves a range of host enzymes that delete or add saccharide monomers. While endoplasmic reticulum (E.R.) mannosidases cleave only one mannose to produce the Man8B isomer, an alpha-1,2-mannosidase from Trichoderma reesei can sequentially cleave all four 1,2-linked mannose sugars from a Man(9)GlcNAc(2) oligosaccharide, a feature reminiscent of the activity of Golgi mannosidases. We now report the structure of the T. reesei enzyme at 2.37 A resolution. The enzyme folds as an (alpha alpha)(7) barrel. The substrate-binding site of the T. reesei mannosidase differs appreciably from the Saccharomyces cerevisiae enzyme. In the former, shorter loops at the surface allow substrate protein to come closer to the catalytic site. There is more internal space available, so that different oligosaccharide conformations are sterically allowed in the T. reesei alpha-1,2-mannosidase.
真核生物蛋白质的N-糖基化过程涉及一系列宿主酶，这些酶会去除或添加糖类单体。虽然内质网（E.R.）甘露糖苷酶仅切割一个甘露糖以产生Man8B异构体，但来自里氏木霉的α-1,2-甘露糖苷酶可以从Man(9)GlcNAc(2)寡糖中依次切割所有四个1,2-连接的甘露糖糖，这一特性让人联想到高尔基体甘露糖苷酶的活性。我们现在报告了里氏木霉该酶在2.37 Å分辨率下的结构。该酶折叠成一个(αα)(7)桶状结构。里氏木霉甘露糖苷酶的底物结合位点与酿酒酵母的酶有明显不同。在前者中，表面较短的环允许底物蛋白更接近催化位点。有更多的内部空间，因此在里氏木霉α-1,2-甘露糖苷酶中，不同的寡糖构象在空间上是允许的。
