Lobsanov Yuri D, Vallée Francois, Imberty Anne, Yoshida Takashi, Yip Patrick, Herscovics Annette, Howell P Lynne
Program in Structural Biology and Biochemistry, Research Institute, The Hospital for Sick Children, 555 University Ave., Toronto, Ontario M5G 1X8, Canada.
J Biol Chem. 2002 Feb 15;277(7):5620-30. doi: 10.1074/jbc.M110243200. Epub 2001 Nov 19.
Class I alpha1,2-mannosidases (glycosylhydrolase family 47) are key enzymes in the maturation of N-glycans. This protein family includes two distinct enzymatically active subgroups. Subgroup 1 includes the yeast and human endoplasmic reticulum (ER) alpha1,2-mannosidases that primarily trim Man(9)GlcNAc(2) to Man(8)GlcNAc(2) isomer B whereas subgroup 2 includes mammalian Golgi alpha1,2-mannosidases IA, IB, and IC that trim Man(9)GlcNAc(2) to Man(5)GlcNAc(2) via Man(8)GlcNAc(2) isomers A and C. The structure of the catalytic domain of the subgroup 2 alpha1,2-mannosidase from Penicillium citrinum has been determined by molecular replacement at 2.2-A resolution. The fungal alpha1,2-mannosidase is an (alphaalpha)(7)-helix barrel, very similar to the subgroup 1 yeast (Vallée, F., Lipari, F., Yip, P., Sleno, B., Herscovics, A., and Howell, P. L. (2000) EMBO J. 19, 581-588) and human (Vallée, F., Karaveg, K., Herscovics, A., Moremen, K. W., and Howell, P. L. (2000) J. Biol. Chem. 275, 41287-41298) ER enzymes. The location of the conserved acidic residues of the catalytic site and the binding of the inhibitors, kifunensine and 1-deoxymannojirimycin, to the essential calcium ion are conserved in the fungal enzyme. However, there are major structural differences in the oligosaccharide binding site between the two alpha1,2-mannosidase subgroups. In the subgroup 1 enzymes, an arginine residue plays a critical role in stabilizing the oligosaccharide substrate. In the fungal alpha1,2-mannosidase this arginine is replaced by glycine. This replacement and other sequence variations result in a more spacious carbohydrate binding site. Modeling studies of interactions between the yeast, human and fungal enzymes with different Man(8)GlcNAc(2) isomers indicate that there is a greater degree of freedom to bind the oligosaccharide in the active site of the fungal enzyme than in the yeast and human ER alpha1,2-mannosidases.
I类α1,2-甘露糖苷酶（糖基水解酶家族47）是N-聚糖成熟过程中的关键酶。该蛋白家族包括两个不同的具有酶活性的亚组。亚组1包括酵母和人类内质网（ER）α1,2-甘露糖苷酶，它们主要将Man(9)GlcNAc(2)修剪为Man(8)GlcNAc(2)异构体B，而亚组2包括哺乳动物高尔基体α1,2-甘露糖苷酶IA、IB和IC，它们通过Man(8)GlcNAc(2)异构体A和C将Man(9)GlcNAc(2)修剪为Man(5)GlcNAc(2)。桔青霉亚组2α1,2-甘露糖苷酶催化结构域的结构已通过分子置换在2.2埃分辨率下确定。真菌α1,2-甘露糖苷酶是一个（αα）(7)-螺旋桶，与亚组1酵母（瓦莱，F.，利帕里，F.，叶普，P.，斯莱诺，B.，赫斯科维茨，A.，和豪厄尔，P.L.（2000年）《欧洲分子生物学组织杂志》19，581 - 588）和人类（瓦莱，F.，卡拉韦格，K.，赫斯科维茨，A.，莫雷门，K.W.，和豪厄尔，P.L.（2000年）《生物化学杂志》275，41287 - 41298）内质网酶非常相似。催化位点保守酸性残基的位置以及抑制剂基夫内新和1-脱氧甘露基野尻霉素与必需钙离子的结合在真菌酶中是保守的。然而，两个α1,2-甘露糖苷酶亚组之间的寡糖结合位点存在主要结构差异。在亚组1酶中，一个精氨酸残基在稳定寡糖底物方面起关键作用。在真菌α1,2-甘露糖苷酶中，这个精氨酸被甘氨酸取代。这种取代和其他序列变异导致碳水化合物结合位点更宽敞。对酵母、人类和真菌酶与不同Man(8)GlcNAc(2)异构体之间相互作用的建模研究表明，与酵母和人类内质网α1,2-甘露糖苷酶相比，真菌酶活性位点中结合寡糖的自由度更大。
