Potentiometric CO titrations of carbon monoxide dehydrogenase and CO-inhibition of the NI-removing reaction with 1,10--phenanthroline

Abstract

Carbon monoxide dehydrogenase from Clostridium thermoaceticum catalyzes two reactions involving redox chemistry (the reversible oxidation Of CO to C02, and the synthesis of acetyl-CoA) using three types of Ni and Fe-S structures called the A-, B-, and C-clusters. In this study, the enzyme was incubated under various partial pressures of the reductant and substrate CO, under Ar and C02 atmospheres, and then monitored by electron paramagnetic resonance spectroscopy (EPR). The resulting spectral changes were simulated assuming a one-electron reduction of the B-cluster (B.,, + e-= Bed) and a one-electron reduction and CO-binding of the A-cluster (A.,, + e-+ CO = A[ ]-CO). The C-cluster was simulated using a two-electron reduction(Cedl + 2e-=C,,d2)(model 1) and a model that assumed two one-electron reductions of the C-cluster(C[ ] + e-= Cint and Cit e-= C[ ])(model 2). The development of EPR signals corresponding to the three clusters and the redox potential of these clusters were found to be quite different under Ar and CO2 atmospheres. The A, B, and C clusters EPR signals developed at potentials-250, 150, and 250 mV more Positive, respectively, under a C02 atmosphere relative to an Ar atmosphere. The EPR signals of the A and C clusters also developed at much lower concentrations of CO in a C02 atmosphere. The saturation properties of the B cluster EPR signal (g,,, = 1.94)changed significantly under C02. These C02 effects confirm and strengthen a previous proposal that C02 binding effects a conformational change that alters the reduction potentials and electron transfer properties of the enzyme's redox clusters. The possible catalytic significance of the effect Of C02 is discussed. CO was found to inhibit the reaction of the enzyme with 1, I 0-phenanthroline, with Ki = 840 atm-1. Since this reaction selectively removes the Ni from the A-cluster, inhibition by CO implies that CO binds this Ni and blocks phen from chelating it. However, other studies suggest that CO binds to an Fe of the A-cluster, and the possibility that CO blocks phen indirectly should be considered.