KINETIC-ANALYSIS AND SUBSTRATE-SPECIFICITY OF ESCHERICHIA-COLI DIMETHYL-SULFOXIDE REDUCTASE

We have characterized the substrate specificity of dimethyl sulfoxide reductase (DmsABC) of Escherichia coli by determining K-m and k(cat) v alues for 22 different substrates. The enzyme has a very broad substra te specificity. The K-m values varied 470-fold, while k(cat) values va ried only 20-fold, implicating K-m as the major determinant of k(cat)/ K-m values. Sulfoxides and pyridine N-oxide exhibited the lowest K-m v alues, followed by aliphatic N-oxides. The k(cat) values for these com pounds also followed the same pattern. Substitution at the 2 or 3 posi tion of the pyridine N-oxide ring had little effect on K-m while subst itution at the 4 position had a greater effect, and increased K-m. Neg atively charged substrates were poorly accepted. A few compounds that are not S- or N-oxides were also reduced by the enzyme. Most compounds reduced by DmsABC were not toxic to E. coli under anaerobic growth co nditions, and E. coli was able to use many of these compounds anaerobi cally as terminal electron accepters in the presence of glycerol. Anae robic growth on sulfoxides is solely due to DmsABC expression. However , there appears to be another as yet unidentified terminal reductase c apable of using pyridine N-oxides as terminal electron acceptors.