Redox-dependent CO2 reduction activity of CO dehydrogenase from Rhodospirillum rubrum

Carbon monoxide dehydrogenase (CODH) from Rhodospirillum rubrum catalyzes b oth the oxidation of CO and the reduction of CO2. Studies of the redox depe ndence of CO2 reduction by R. rubrum CODH show that (1) CODH is unable to c atalyze CO2 reduction at potentials greater than -300 mV; (2) the maximum a ctivity is observed at potentials less than -480 mV; and (3) the midpoint p otential (E-m) of the transition from minimum to maximum CO2 reduction acti vity occurs at similar to -339 mV. These results indicate that the C-red1 s tate of R. rubrum CODH (E-m -110 mV; g(zyx) = 2.03, 1.88, 1.71) is not comp etent to reduce CO2. Nernst analyses suggest that the reduction of CODH fro m the C-red1 State to the CO2-reducing form (C-unc,g(zyx) = 2.04, 1.93, 1.8 9; E < similar to -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 [FeS4](B) during catalysis.