X-ray Structure of a Hg2+ Complex of Mercuric Reductase (MerA) and Quantum Mechanical/Molecular Mechanical Study of Hg2+ Transfer between the C-Terminal and Buried Catalytic Site

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• 作者：Peng Lian ; Hao-Bo Guo ; Demian Riccardi ; Aiping Dong ; Jerry M. Parks ; Qin Xu ; Emil F. Pai ; Susan M. Miller ; Dong-Qing Wei ; Jeremy C. Smith ; Hong Guo
• 刊名：Biochemistry
• 出版年：2014
• 出版时间：November 25, 2014
• 年：2014
• 卷：53
• 期：46
• 页码：7211-7222
• 全文大小：545K
• ISSN：1520-4995

文摘

Mercuric reductase, MerA, is a key enzyme in bacterial mercury resistance. This homodimeric enzyme captures and reduces toxic Hg²⁺ to Hg⁰, which is relatively unreactive and can exit the cell passively. Prior to reduction, the Hg²⁺ is transferred from a pair of cysteines (C558鈥?and C559鈥?using Tn501 numbering) at the C-terminus of one monomer to another pair of cysteines (C136 and C141) in the catalytic site of the other monomer. Here, we present the X-ray structure of the C-terminal Hg²⁺ complex of the C136A/C141A double mutant of the Tn501 MerA catalytic core and explore the molecular mechanism of this Hg transfer with quantum mechanical/molecular mechanical (QM/MM) calculations. The transfer is found to be nearly thermoneutral and to pass through a stable tricoordinated intermediate that is marginally less stable than the two end states. For the overall process, Hg²⁺ is always paired with at least two thiolates and thus is present at both the C-terminal and catalytic binding sites as a neutral complex. Prior to Hg²⁺ transfer, C141 is negatively charged. As Hg²⁺ is transferred into the catalytic site, a proton is transferred from C136 to C559鈥?while C558鈥?becomes negatively charged, resulting in the net transfer of a negative charge over a distance of 鈭?.5 脜. Thus, the transport of this soft divalent cation is made energetically feasible by pairing a competition between multiple Cys thiols and/or thiolates for Hg²⁺ with a competition between the Hg²⁺ and protons for the thiolates.