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首页> 外文期刊>Biochemistry >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 Cysteine Pairs
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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 Cysteine Pairs

机译：汞还原酶（MerA）的Hg2 +配合物的X射线结构以及C末端和埋藏的催化位点半胱氨酸对之间Hg2 +转移的量子力学/分子力学研究

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摘要

Mercuric reductase, MerA, is a key enzyme in bacterial mercury resistance. This homodimeric enzyme captures and reduces toxic Hg2+ to Hg-0, which is relatively unreactive and can exit the cell passively. Prior to reduction, the Hg2+ is transferred from a pair of cysteines (C558' and C559' 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' while C558' becomes negatively charged, resulting in the net transfer of a negative charge over a distance of similar to 7.5 angstrom. 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.
机译：汞还原酶MerA是细菌耐汞性的关键酶。这种同二聚体酶捕获并降低有毒的Hg2 +到Hg-0的水平，Hg-0相对没有反应性，可以被动地离开细胞。在还原之前，Hg2 +在一个单体的C端从一对半胱氨酸（使用Tn501编号的C558'和C559'）转移到另一对单体的催化位点的另一对半胱氨酸（C136和C141）。在这里，我们介绍了Tn501 MerA催化核的C136A / C141A双突变体C末端Hg2 +配合物的X射线结构，并通过量子力学/分子力学（QM / MM）计算探索了这种Hg转移的分子机理。 。发现该转移几乎是热中性的，并通过了稳定的三配位中间体，该中间体比两个末端态的稳定性稍差。对于整个过程，Hg2 +始终与至少两个硫醇盐配对，因此在C末端和催化结合位点均以中性复合物形式存在。在传输Hg2 +之前，C141带负电。当Hg2 +转移到催化位点时，质子从C136转移到C559'，而C558'变成带负电，导致负电荷在大约7.5埃的距离上净转移。因此，通过使多个Cys硫醇和/或硫醇盐之间的竞争对Hg2 +与Hg2 +和质子硫醇盐之间的竞争配对，使这种柔软的二价阳离子的运输在能量上可行。

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