Multiple pathways of electron transfer in dimethyl sulfoxide reductase of Escherichia coli

Abstract

The catalytic subunit of dimethyl sulfoxide (Me2SO) reductase, DmsA, contains six blocks of sequence that are homologous to other members of the superfamily of prokaryotic molybdoenzymes. The amino-terminal block contains 5 conserved residues (Cys38, Cys42, Cys75, Lys28, and Arg77). Site-directed mutagenesis of these residues did not alter membrane localization but in some cases less enzyme accumulated. The activity of Me2SO reductase was monitored by measuring Me2SO-dependent anaerobic growth, benzyl viologen, or dimethylnaphthoquinol oxidase activity, and using a quinol pool-coupling assay. Only Cys75 and Lys28 mutant enzymes were able to support anaerobic growth with Me2SO suggesting a critical role for Cys38, Cys42, and Arg77. Benzyl viologen oxidase activity was retained in the mutants although with reduced efficiency in Cys42-Ser. Electron transport with dimethylnaphthoquinol was reduced in Cys38-Ser, Cys42-Ser, and Cys75-Ser and almost totally eliminated in the Arg77-Ser mutant. Cys38-Ser, Cys42-Ser, and Arg77-Ser were unable to support quinol oxidation although electron transfer from the quinol pool to the [Fe-S] centers in DmsB was normal. These results indicate that the amino-terminal region is involved in functional electron transfer from the quinol pool to Me2SO and that electrons from benzyl viologen, dimethylnaphthoquinol, and menaquinol may follow different paths within the catalytic subunit.