Insight into the mechanism of biological methanol activation based on the crystal structure of the methanol-cobalamin methyltransferase complex
Insight into the mechanism of biological methanol activation based on the crystal structure of the methanol-cobalamin methyltransferase complex
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DOI：
10.1073/pnas.0603650103
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发表时间：
2006-12-12
影响因子：
11.1
通讯作者：
Ermler, Ulrich
中科院分区：
文献类型：
--
作者：
Hagemeier, Christoph H.;Krueer, Markus;Ermler, Ulrich
一些产甲烷和产乙酸微生物具有异解裂解甲醇的C-O键的催化能力。为了深入了解这一具有挑战性的化学反应的难以捉摸的酶促机制，我们研究了来自巴克甲烷八叠球菌的甲醇活化 MtaBC 复合物，该复合物由含锌 MtaB 和携带 5-羟基苯并咪唑基钴酰胺的 MtaC 亚基组成。在这里，我们报告了该复合物的 2.5 埃晶体结构，组织为 (MtaBC)(2)。异四聚体。 MtaB 折叠成一个 TIM 桶，并包含一个新颖的锌结合基序。锌 (II) 位于 C 末端 β 桶末端形成的漏斗底部，并与两个半胱氨酰硫（Cys-220 和 Cys-269）和一个羧酸氧（Glu-164）连接。 MtaC 在结构上与甲硫氨酸合酶的钴胺素结合域相关。位于 Rossmann 结构域顶部的类咕啉辅助因子深入到 MtaB 的漏斗中，定义了锌 (II) 和类咕啉钴之间的区域，该区域必定是甲醇的结合位点。活性位点几何结构支持 S(N)2 反应机制，其中甲醇中的 C-O 键被强亲电子试剂锌 (II) 激活，并因超亲核试剂钴 (I) 酰胺的攻击而裂解。锌(II) 环境的特点是酸性簇，增加了锌(II) 上的电荷密度，极化甲醇，不利于甲醇羟基的去质子化。讨论了 MtaBC 结构对反应第二步（其中甲基基团转移至辅酶 M）的影响。
Some methanogenic and acetogenic microorganisms have the catalytic capability to cleave heterolytically the C-O bond of methanol. To obtain insight into the elusive enzymatic mechanism of this challenging chemical reaction we have investigated the methanol-activating MtaBC complex from Methanosarcina barkeri composed of the zinc-containing MtaB and the 5-hydroxybenzimidazolylcobamide-carrying MtaC subunits. Here we report the 2.5-angstrom crystal structure of this complex organized as a (MtaBC)(2). heterotetramer. MtaB folds as a TIM barrel and contains a novel zinc-binding motif. Zinc(II) lies at the bottom of a funnel formed at the C-terminal beta-barrel end and ligates to two cysteinyl sulfurs (Cys-220 and Cys-269) and one carboxylate oxygen (Glu-164). MtaC is structurally related to the cobalamin-binding domain of methionine synthase. Its corrinoid cofactor at the top of the Rossmann domain reaches deeply into the funnel of MtaB, defining a region between zinc(II) and the corrinoid cobalt that must be the binding site for methanol. The active site geometry supports a S(N)2 reaction mechanism, in which the C-O bond in methanol is activated by the strong electrophile zinc(II) and cleaved because of an attack of the supernucleophile cob(I)amide. The environment of zinc(II) is characterized by an acidic cluster that increases the charge density on the zinc(II), polarizes methanol, and disfavors deprotonation of the methanol hydroxyl group. Implications of the MtaBC structure for the second step of the reaction, in which the methyl group is transferred to coenzyme M, are discussed.