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Volume 271, Number 44, Issue of November 1, 1996 pp. 27339-27345
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Consequences of Removal of a Molybdenum Ligand (DmsA-Ser-176) of Escherichia coli Dimethyl Sulfoxide Reductase

(Received for publication, February 5, 1996, and in revised form, August 7, 1996)

From the Department of Biochemistry and the Medical Research Council Group in the Molecular Biology of Membranes, University of Alberta, Edmonton, Alberta T6G 2H7, Canada

We have used site-directed mutagenesis and EPR spectroscopy to examine the consequences of altering the molybdenum ligand in Escherichia coli dimethyl sulfoxide (Me₂SO) reductase (DmsABC). Mutagenesis of DmsA-Ser-176 to Ala, Cys, or His abolishes both respiratory growth on Me₂SO and in vitro benzyl viologen:Me₂SO oxidoreductase activity. EPR spectroscopy reveals changes in the line shape and the g_av of the Mo(V) signals of the S176A and S176C enzymes. The midpoint potentials (E_m,7) of the Mo(VI)/Mo(V) and Mo(V)/Mo(IV) couples in DmsABC are 15 and 175 mV. The E_m,7 of the Mo(V)/Mo(IV) couple in the S176A mutant is 35 mV; however, the Mo(V) species could not be further oxidized with ferricyanide. Titration of the S176C mutant produced several overlapping Mo(V) species occurring at E_h > 150 mV, suggesting heterogeneity in the molybdenum environment. A Mo(V) spectrum was not visible in S176H membranes poised between 435 to 350 mV or oxidized with 200 µM ferricyanide. No differences were detected in the EPR spectra of the reduced [4Fe-4S] clusters of DmsABC and the S176A and S176H mutant enzymes; however, the S176C mutation altered the EPR line shape of one of the reduced [4Fe-4S] clusters.

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