Mannosidases are a diverse group of enzymes that are important in the biological processing of mannose-containing polysaccharides and complex glycoconjugates. They are found in 12 of the >160 sequence-based glycosidase families. We discuss evidence that nature has evolved a small set of common mechanisms that unite almost all of these mannosidase families. Broadly, mannosidases (and the closely related rhamnosidases) perform catalysis through just two conformations of the oxocarbenium ion-like transition state: a B2,5 (or enantiomeric 2,5B) boat and a 3H4 half-chair. This extends to a new family (GT108) of GDPMan-dependent β-1,2-mannosyltransferases/phosphorylases that perform mannosyl transfer through a boat conformation as well as some mannosidases that are metalloenzymes and require divalent cations for catalysis. Yet, among this commonality lies diversity. New evidence shows that one unique family (GH99) of mannosidases use an unusual mechanism involving anchimeric assistance via a 1,2-anhydro sugar (epoxide) intermediate.

中文翻译：

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甘露糖苷酶机制：在构象和催化作用的交点处。

甘露糖苷酶是多种酶，它们在含甘露糖的多糖和复合糖缀合物的生物加工中很重要。它们在> 160个基于序列的糖苷酶家族中的12个中发现。我们讨论的证据表明，自然界已经进化出一小部分共同的机制，将几乎所有这些甘露糖苷酶家族联合在一起。广义上讲，甘露糖苷酶（和紧密相关的鼠李糖苷酶）仅通过氧羰基离子样过渡态的两种构象进行催化：B2,5（或对映异构体2,5B）船和3H4半椅子。这延伸到GDPMan依赖的β-1,2-甘露糖基转移酶/磷酸酶的新家族（GT108），它们通过船构象进行甘露糖基转移，以及一些甘露糖苷酶（金属酶，需要二价阳离子进行催化）。然而，这种共性中包括多样性。新证据表明，甘露糖苷酶的一个独特家族（GH99）使用一种不寻常的机制，涉及通过1，2-脱水糖（环氧）中间体的邻氨基苯甲酸协助。