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Structural and mechanistic analysis of the arsenate respiratory reductase provides insight into environmental arsenic transformations

  • Tom Osborne
    ,
  • Nathaniel R. Glasser
    ,
  • Paul H. Oyala
    ,
  • Thomas H. Osborne
    ,
  • Joanne M. Santini
    ,
  • Dianne K. Newman
  • California Institute of Technology
    ,
  • University College London
Research Output: Contribution to journal Article Peer-review

Abstract

Arsenate respiration by bacteria was discovered over two decades ago and is catalyzed by diverse organisms using the well-conserved Arr enzyme complex. Until now, the mechanisms underpinning this metabolism have been relatively opaque. Here, we report the structure of an Arr complex (solved by X-ray crystallography to 1.6-Å resolution), which was enabled by an improved Arr expression method in the genetically tractable arsenate respirer Shewanella sp. ANA-3. We also obtained structures bound with the substrate arsenate (1.8 Å), the product arsenite (1.8 Å), and the natural inhibitor phosphate (1.7 Å). The structures reveal a conserved active-site motif that distinguishes Arr [(R/K)GRY] from the closely related arsenite respiratory oxidase (Arx) complex (XGRGWG). Arr activity assays using methyl viologen as the electron donor and arsenate as the electron acceptor display two-site ping-pong kinetics. A Mo(V) species was detected with EPR spectroscopy, which is typical for proteins with a pyranopterin guanine dinucleotide cofactor. Arr is an extraordinarily fast enzyme that approaches the diffusion limit (Km = 44.6 ± 1.6 μM, kcat = 9,810 ± 220 seconds-1), and phosphate is a competitive inhibitor of arsenate reduction (Ki = 325 ± 12 μM). These observations, combined with knowledge of typical sedimentary arsenate and phosphate concentrations and known rates of arsenate desorption from minerals in the presence of phosphate, suggest that (i) arsenate desorption limits microbiologically induced arsenate reductive mobilization and (ii) phosphate enhances arsenic mobility by stimulating arsenate desorption rather than by inhibiting it at the enzymatic level.

Publication Information

Output type

Research Output: Contribution to journal Article Peer-review

Original language

English

Pages from-to (Number of pages)

Pages E8614-E8623

Journal (Volume, Issue Number)

Proceedings of the National Academy of Sciences of the United States of America (Volume 115, Issue 37)

Publication milestones

  • Accepted/In press - 13/07/2018
  • Published - 13/08/2018

Publication status

Published - 13/08/2018

ISSN

0027-8424

External Publication IDs

  • handle.net: 10547/623432
  • Scopus: 85052984565

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