A mechanistic and electrochemical study of the interaction between dimethyl sulfide dehydrogenase and its electron transfer partner cytochrome c2

Abstract

Dimethyl sulfide dehydrogenase isolated from the photosynthetic bacterium Rhodovulum sulfidophilum is a heterotrimeric enzyme containing a molybdenum cofactor at its catalytic site, as well as five iron-sulfur clusters and a heme b cofactor. It oxidizes dimethyl sulfide (DMS) to dimethyl sulfoxide in its native role and transfers electrons to the photochemical reaction center. There is genetic evidence that cytochrome c 2 mediates this process, and the steady state kinetics experiments reported here demonstrated that cytochrome c 2 accepts electrons from DMS dehydrogenase. At saturating concentrations of both substrate (DMS) and cosubstrate (cytochrome c 2), Michaelis constants, K M,DMS and K M,cyt of 53 and 21 卬 respectively, were determined at pH 8. Further kinetic analysis revealed a "ping-pong" enzyme reaction mechanism for DMS dehydrogenase with its two reactants. Direct cyclic voltammetry of cytochrome c 2 immobilized within a polymer film cast on a glassy carbon electrode revealed a reversible FeIII/II couple at +328 mV versus the normal hydrogen electrode at pH 8. The FeIII/II redox potential exhibited only minor pH dependence. In the presence of DMS dehydrogenase and DMS, the peak-shaped voltammogram of cytochrome c 2 is transformed into a sigmoidal curve consistent with a steady-state (catalytic) reaction. The cytochrome c 2 effectively mediates electron transfer between the electrode and DMS dehydrogenase during turnover and a significantly lower apparent electrochemical Michaelis constant of13(ᱩ 占was obtained. The pH optimum for catalytic DMS oxidation by DMS dehydrogenase with cytochrome c 2 as the electron acceptor was found to be approximately 8.3.