Determination of an optimal potential window for catalysis by E-coli dimethyl sulfoxide reductase and hypothesis on the role of Mo(V) in the reactionpathway

Protein film voltammetry (PFV) of Escherichia coli dimethyl sulfoxide (DMSO ) reductase (DmsABC) adsorbed at a graphite electrode reveals that the cata lytic activity of this complex Mo-pterin/Fe-S enzyme is optimized within a narrow window of electrode potential. The upper and lower limits of this wi ndow are determined from the potential dependences of catalytic activity in reducing and oxidizing directions; i.e., for reduction of DMSO (or trimeth ylamine-N-oxide) and oxidation of trimethylphosphine (PMe3). At either limi t, the catalytic activity drops despite the increase in driving force: as t he potential is lowered below -200 mV (pH 7.0-8.9), the rate of reduction o f DMSO decreases abruptly, while for PMe3. an oxidative current is observed that vanishes as the potential is raised above +20 mV (pH 9.0), Analysis o f the waveshapes reveals that both activity thresholds result from one-elec tron redox reactions that arise, most likely, from groups within the enzyme ; if so, they represent "switches" that reflect the catalytic mechanism and may be of physiological relevance. The potential window of activity coinci des approximately with the appearance of the Mo(V) EPR signal observed in p otentiometric titrations, suggesting that crucial stages of catalysis are f acilitated while the active site is in the intermediate Mo(V) oxidation sta te.