title: 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.
abstract: 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' 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 ∼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 Hg2+ with a competition between the Hg2+ and protons for the thiolates.
authors.affiliation: The State Key Laboratory of Microbial Metabolism and College of Life Sciences and Biotechnology, Shanghai Jiao Tong University , Shanghai 200240, China.
authors.forename: Peng
authors.initials: P
authors.lastname: Lian
authors.forename: Hao-Bo
authors.initials: HB
authors.lastname: Guo
authors.forename: Demian
authors.initials: D
authors.lastname: Riccardi
authors.forename: Aiping
authors.initials: A
authors.lastname: Dong
authors.forename: Jerry M
authors.initials: JM
authors.lastname: Parks
authors.forename: Qin
authors.initials: Q
authors.lastname: Xu
authors.forename: Emil F
authors.initials: EF
authors.lastname: Pai
authors.forename: Susan M
authors.initials: SM
authors.lastname: Miller
authors.forename: Dong-Qing
authors.initials: DQ
authors.lastname: Wei
authors.forename: Jeremy C
authors.initials: JC
authors.lastname: Smith
authors.forename: Hong
authors.initials: H
authors.lastname: Guo
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000737:chemistry / Q000378:metabolism; D008628:Mercury / Q000378:metabolism*; D008958:Models, Molecular; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D055503:Protein Multimerization; D011550:Pseudomonas aeruginosa / Q000737:chemistry* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D010088:Oxidoreductases; C021551:mercuric reductase; D008628:Mercury; D003545:Cysteine