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 CO(2). Studies of the redox dependence of CO(2) reduction by R. rubrum CODH show that (1) CODH is unable to catalyze CO(2) reduction at potentials greater than -300 mV; (2) the maximum activity is observed at potentials less than -480 mV; and (3) the midpoint potential (E(m)) of the transition from minimum to maximum CO(2) reduction activity occurs at approximately -339 mV. These results indicate that the C(red1) state of R. rubrum CODH (E(m) = -110 mV; g(zyx)() = 2.03, 1.88, 1.71) is not competent to reduce CO(2). Nernst analyses suggest that the reduction of CODH from the C(red1) state to the CO(2)-reducing form (C(unc), g(zyx)() = 2.04, 1.93, 1.89; E < approximately -300 mV) of the enzyme is a one-electron process. For the entire redox range, viologens stimulate CO(2) reduction by CODH more than 50-fold, and it is proposed that viologens accelerate the redox equilibration of redox buffers and [Fe(4)S(4)](B) during catalysis.