A Unified Electrocatalytic Description of the Action of Inhibitors of Nickel Carbon Monoxide Dehydrogenase

Several small molecules and ions, notably carbon monoxide, cyanide, cyanate, and hydrogen sulfide, are potent inhibitors of Ni-containing carbon monoxide dehydrogenases (Ni-CODH) that catalyze very rapid, efficient redox interconversions of CO2 and CO. Protein film electrochemistry, which probes the dependence of steady-state catalytic rate over a wide potential range, reveals how these inhibitors target particular oxidation levels of Ni-CODH relating to intermediates (C-ox, C-red1, and C-red2) that have been established for the active site. The following properties are thus established: (1) CO suppresses CO2 reduction (CO is a product inhibitor), but its binding affinity decreases as the potential becomes more negative. (2) Cyanide totally inhibits CO oxidation, but its effect on CO2 reduction is limited to a narrow potential region (between -0.5 and -0.6 V), below which CO2 reduction activity is restored. (3) Cyanate is a strong inhibitor of CO2 reduction but inhibits CO oxidation only within a narrow potential range just above the CO2/CO thermodynamic potential-EPR spectra confirm that cyanate binds selectively to C-red2. (4) Hydrogen sulfide (H2S/HS-) inhibits CO oxidation but not CO2 reduction-the complex on/off characteristics are consistent with it binding at the same oxidation level as C-ox and forming a modified version of this inactive state rather than reacting directly with C-red1. The results provide a new perspective on the properties of different catalytic intermediates of Ni-CODH-uniting and clarifying many previous investigations.