Koutmos Markos, Pejchal Robert, Bomer Theresa M, Matthews Rowena G, Smith Janet L, Ludwig Martha L
Biophysics Research Division, Life Sciences Institute, and Department of Biological Chemistry, University of Michigan, Ann Arbor, MI 48109, USA.
Proc Natl Acad Sci U S A. 2008 Mar 4;105(9):3286-91. doi: 10.1073/pnas.0709960105. Epub 2008 Feb 22.
Enzymes possessing catalytic zinc centers perform a variety of fundamental processes in nature, including methyl transfer to thiols. Cobalamin-independent (MetE) and cobalamin-dependent (MetH) methionine synthases are two such enzyme families. Although they perform the same net reaction, transfer of a methyl group from methyltetrahydrofolate to homocysteine (Hcy) to form methionine, they display markedly different catalytic strategies, modular organization, and active site zinc centers. Here we report crystal structures of zinc-replete MetE and MetH, both in the presence and absence of Hcy. Structural investigation of the catalytic zinc sites of these two methyltransferases reveals an unexpected inversion of zinc geometry upon binding of Hcy and displacement of an endogenous ligand in both enzymes. In both cases a significant movement of the zinc relative to the protein scaffold accompanies inversion. These structures provide new information on the activation of thiols by zinc-containing enzymes and have led us to propose a paradigm for the mechanism of action of the catalytic zinc sites in these and related methyltransferases. Specifically, zinc is mobile in the active sites of MetE and MetH, and its dynamic nature helps facilitate the active site conformational changes necessary for thiol activation and methyl transfer.
拥有催化锌中心的酶在自然界中执行各种基本过程，包括将甲基转移至硫醇。不依赖钴胺素的（MetE）和依赖钴胺素的（MetH）甲硫氨酸合酶就是这样的两个酶家族。尽管它们进行相同的净反应，即将甲基从甲基四氢叶酸转移至同型半胱氨酸（Hcy）以形成甲硫氨酸，但它们展现出明显不同的催化策略、模块化组织和活性位点锌中心。在此，我们报告了在有和没有Hcy存在的情况下，锌饱和的MetE和MetH的晶体结构。对这两种甲基转移酶催化锌位点的结构研究揭示，在两种酶中，Hcy结合以及内源性配体被取代后，锌的几何结构出现意外反转。在这两种情况下，锌相对于蛋白质支架的显著移动伴随着反转。这些结构为含锌酶激活硫醇提供了新信息，并使我们提出了这些及相关甲基转移酶催化锌位点作用机制的范例。具体而言，锌在MetE和MetH的活性位点是可移动的，其动态特性有助于促进硫醇激活和甲基转移所需的活性位点构象变化。
