X-ray Structure of a Hg(2+) Complex of Mercuric Reductase (MerA) and Quantum Mechanical/Molecular Mechanical Study of Hg(2+) Transfer between the C-Terminal and Buried Catalytic Site Cysteine Pairs

[Image: see text] Mercuric reductase, MerA, is a key enzyme in bacterial mercury resistance. This homodimeric enzyme captures and reduces toxic Hg(2+) to Hg(0), which is relatively unreactive and can exit the cell passively. Prior to reduction, the Hg(2+) 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 Hg(2+) 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, Hg(2+) 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 Hg(2+) transfer, C141 is negatively charged. As Hg(2+) 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 ∼7.5 Å. Thus, the transport of this soft divalent cation is made energetically feasible by pairing a competition between multiple Cys thiols and/or thiolates for Hg(2+) with a competition between the Hg(2+) and protons for the thiolates.