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

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    Authors
    Glasser, Nathaniel R.
    Oyala, Paul H.
    Osborne, Thomas H.
    Santini, Joanne M.
    Newman, Dianne K.
    Issue Date
    2018-08-13
    Subjects
    ArrAB
    bacterial arsenate respiration
    biogeochemistry
    enzymology
    F850 Environmental Sciences
    
    Metadata
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    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.
    Citation
    Glasser NR, Oyala PH, Osborne TH, Santini JM, Newman DK (2018) 'Structural and mechanistic analysis of the arsenate respiratory reductase provides insight into environmental arsenic transformations', Proceedings of the National Academy of Sciences, 115 (37), pp.E8614-E8623.
    Publisher
    National Academy of Sciences
    Journal
    Proceedings of the National Academy of Sciences
    URI
    http://hdl.handle.net/10547/623432
    DOI
    10.1073/pnas.1807984115
    PubMed ID
    30104376
    PubMed Central ID
    PMC6140538
    Additional Links
    https://www.pnas.org/content/115/37/E8614.short
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6140538/
    Type
    Article
    Language
    en
    ISSN
    0027-8424
    ae974a485f413a2113503eed53cd6c53
    10.1073/pnas.1807984115
    Scopus Count
    Collections
    Biomedical and biological science

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