Redox-Dependent CO2 Reduction Activity of CO Dehydrogenase from Rhodospirillum rubrum†

Carbon monoxide dehydrogenase (CODH) from Rhodospirillum rubrum catalyzes both the oxidation of CO and the reduction of CO2. Studies of the redox dependence of CO2 reduction by R. rubrum CODH show that (1) CODH is unable to catalyze CO2 reduction at potentials greater than −300 mV; (2) the maximum activity is observed at potentials less than −480 mV; and (3) the midpoint potential (Em) of the transition from minimum to maximum CO2 reduction activity occurs at ∼−339 mV. These results indicate that the Cred1 state of R. rubrum CODH (Em = −110 mV; gzyx = 2.03, 1.88, 1.71) is not competent to reduce CO2. Nernst analyses suggest that the reduction of CODH from the Cred1 state to the CO2-reducing form (Cunc, gzyx = 2.04, 1.93, 1.89; E < ∼−300 mV) of the enzyme is a one-electron process. For the entire redox range, viologens stimulate CO2 reduction by CODH more than 50-fold, and it is proposed that viologens accelerate the redox equilibration of redox buffers and [Fe4S4]B during catalysis.