Structural and functional reconstruction in situ of the [CuSMoO₂] active site of carbon monoxide dehydrogenase from the carbon monoxide oxidizing eubacterium Oligotropha carboxidovorans

Abstract

Carbon monoxide dehydrogenase from the bacterium Oligotropha carboxidovorans catalyzes the oxidation of CO to CO₂ at a unique [CuSMoO₂] cluster. In the bacteria the cluster is assembled post-translational. The integration of S, and particularly of Cu, is rate limiting in vivo, which leads to CO dehydrogenase preparations containing the mature and fully functional enzyme along with forms of the enzyme deficient in one or both of these elements. The active sites of mature and immature forms of CO dehydrogenase were converted into a [MoO₃] centre by treatment with potassium cyanide. We have established a method, which rescues 50% of the CO dehydrogenase activity by in vitro reconstitution of the active site through the supply of sulphide first and subsequently of Cu(I) under reducing conditions. Immature forms of CO dehydrogenase isolated from the bacterium, which were deficient in S and/or Cu at the active site, were similarly activated. X-ray crystallography and electron paramagnetic resonance spectroscopy indicated that the [CuSMoO₂] cluster was properly reconstructed. However, reconstituted CO dehydrogenase contains mature along with immature forms. The chemical reactions of the reconstitution of CO dehydrogenase are summarized in a model, which assumes resulphuration of the Mo-ion at both equatorial positions at a 1:1 molar ratio. One equatorial Mo–S group reacts with Cu(I) in a productive fashion yielding a mature, functional [CuSMoO₂] cluster. The other Mo–S group reacts with Cu(I), then Cu₂S is released and an oxo group is introduced from water, yielding an inactive [MoO₃] centre.