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) values for 22 different substrates. The enzyme has a very broad substrate specificity. The K-m values varied 470-fold, while k(cat) values varied 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 values, followed by aliphatic N-oxides. The k(cat) values for these compounds also followed the same pattern. Substitution at the 2 or 3 position of the pyridine N-oxide ring had little effect on K-m while substitution at the 4 position had a greater effect, and increased K-m. Negatively 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 conditions, and E. coli was able to use many of these compounds anaerobically as terminal electron accepters in the presence of glycerol. Anaerobic growth on sulfoxides is solely due to DmsABC expression. However, there appears to be another as yet unidentified terminal reductase capable of using pyridine N-oxides as terminal electron acceptors.