State Key Laboratory of Protein and Plant Gene Research, School of Life Sciences, Peking University, Beijing 100871, PR China.
J Mol Biol. 2011 Sep 30;412(4):688-97. doi: 10.1016/j.jmb.2011.08.005. Epub 2011 Aug 5.
Cobalamin-independent methionine synthase (MetE) catalyzes the direct transfer of a methyl group from methyltetrahydrofolate to l-homocysteine to form methionine. Previous studies have shown that the MetE active site coordinates a zinc atom, which is thought to act as a Lewis acid and plays a role in the activation of thiol. Extended X-ray absorption fine structure studies and mutagenesis experiments identified the zinc-binding site in MetE from Escherichia coli. Further structural investigations of MetE from Thermotoga maritima lead to the proposition of two models: "induced fit" and "dynamic equilibrium", to account for the catalytic mechanisms of MetE. Here, we present crystal structures of oxidized and zinc-replete MetE from Streptococcus mutans at the physiological pH. The structures reveal that zinc is mobile in the active center and has the possibility to invert even in the absence of homocysteine. These structures provide evidence for the dynamic equilibrium model.
钴胺素非依赖型蛋氨酸合成酶（MetE）催化将甲基从甲基四氢叶酸直接转移到 l-高半胱氨酸上，形成蛋氨酸。先前的研究表明，MetE 的活性位点与一个锌原子配位，该锌原子被认为是一种路易斯酸，在巯基的活化中起作用。扩展的 X 射线吸收精细结构研究和突变实验确定了来自大肠杆菌的 MetE 的锌结合位点。进一步对来自海洋栖热菌的 MetE 的结构研究提出了两种模型：“诱导契合”和“动态平衡”，以解释 MetE 的催化机制。在这里，我们展示了在生理 pH 值下变形链球菌氧化态和锌充足态 MetE 的晶体结构。这些结构表明锌在活性中心中是可移动的，即使在没有高半胱氨酸的情况下也有可能反转。这些结构为动态平衡模型提供了证据。
