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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

Here we report that 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 netmore » 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.« less
Authors:
 [1] ;  [2] ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ;  [7] ;  [5] ;  [2] ;  [2]
  1. Shanghai Jiao Tong Univ. (China). State Key Lab. of Microbial Metabolism and College of Life Sciences and Biotechnology; Univ. of Tennessee, Knoxville, TN (United States). Dept. of Biochemistry and Cellular and Molecular Biology; Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Biosciences Division
  2. Univ. of Tennessee, Knoxville, TN (United States). Dept. of Biochemistry and Cellular and Molecular Biology; Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Biosciences Division
  3. Univ. of Toronto, ON (Canada). Dept. of Biochemistry
  4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Biosciences Division
  5. Shanghai Jiao Tong Univ. (China). State Key Lab. of Microbial Metabolism and College of Life Sciences and Biotechnology
  6. Univ. of Toronto, ON (Canada). Dept. of Biochemistry; Univ. of Toronto, ON (Canada). Dept. of of Medical Biophysics and Molecular Genetics; Univ. Health Network, Toronto, ON (Canada). Campbell Family Inst. for Cancer Research
  7. Univ. of California, San Francisco, CA (United States). Dept. of Pharmaceutical Chemistry
Publication Date:
OSTI Identifier:
1164539
Grant/Contract Number:
FG03- 01ER63087; RR07707; W-31-109-Eng-38; SC0004895; AC02-05CH11231; TGMCA08X032; 2012CB721000; 11JC1406400; 20120073110057; FG03-01ER63087
Type:
Published Article
Journal Name:
Biochemistry
Additional Journal Information:
Journal Volume: 53; Journal Issue: 46; Journal ID: ISSN 0006-2960
Publisher:
American Chemical Society (ACS)
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); Canada Research Chairs Program; National Basic Research Program of China
Country of Publication:
United States
Language:
ENGLISH
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 59 BASIC BIOLOGICAL SCIENCES