Mechanism of CO oxidation by carbon monoxide dehydrogenase from Clostridium thermoaceticum and its inhibition by anions.

Abstract

Carbon monoxide dehydrogenase (CODH) performs two distinct reactions at two different metal centers. The synthesis of acetyl-CoA from a methyl group, CO, and coenzyme A occurs at center A and the oxidation of CO to CO2 occurs at center C. In the work reported here, we have studied the mechanism of CO oxidation by CODH and its inhibition by thiocyanate. Our data are consistent with a ping-pong mechanism. A scheme to explain the first half-reaction was developed that includes binding of water and CO to the oxidized form of center C, deprotonation of coordinated water to yield enzyme-bound hydroxyl, nucleophilic attack on coordinated CO by OH- to form enzyme-bound carboxyl, and deprotonation and decarboxylation to form CO2 and the reduced form of center C. In the second half-reaction, the reduced enzyme is reoxidized by an electron acceptor. CO oxidation was pH dependent. The pH dependence of kcat/Km for CO gave a single pKa of 7.7 and a maximum value at 55 degrees C and high pH of 9.1 x 10(6) M-1 s-1. The pH dependence of kcat followed a two-phase titration curve with pKa values of 7.1 and 9.5 and maximum value of kcat at 55 degrees C and high pH of 3250 s-1 (1310 mumol of CO oxidized min-1 mg-1). The pH dependencies of kcat/Km and kcat are interpreted to reflect the ionization of enzyme-bound water from binary and ternary complexes with center C. Reaction with thiocyanate, azide, or cyanate was found to cause a striking shift in the EPR spectrum of center C from gav = 1.82 (g = 2.01, 1.81, 1.65) to a two-component spectrum with gav = 2.15 (g = 2.34, 2.067, 2.03) and gav = 2.17 (g = 2.34, 2.115, 2.047). Thiocyanate acted as a mixed partial inhibitor with respect to CO. The inhibition constants were pH and temperature dependent. The pH dependencies of the inhibition constants gave pKa values of approximately 7.7. Binding of thiocyanate to the oxidized form of center C appears to be favored by a negative enthalpy that is offset by a decrease in entropy yielding a slightly unfavorable free energy of association.