ORGANIZATION OF CLUSTERS AND INTERNAL ELECTRON PATHWAYS IN CO DEHYDROGENASE FROM CLOSTRIDIUM-THERMOACETICUM - RELEVANCE TO THE MECHANISM OFCATALYSIS AND CYANIDE INHIBITION

Cyanide inhibits the CO oxidation activity of carbon monoxide dehydrog enase from Clostridium thermoaceticum by binding tightly to the form o f the C-cluster yielding the g(av) = 1.82 signal (the C-1.82 form). CN - dissociates and the enzyme reactivates upon addition of CO, CO2 plus dithionite, or CS2 plus dithionite. Dithionite slows the inhibition o f the enzyme by CN-, but it cannot reactivate the enzyme. This behavio r is explained by assuming that binding of CO, CO2, or CS2 at a modula tor site accelerates the dissociation of CN- from the C-cluster. With CN- bound at the C-cluster, dithionite, but not CO, can reduce those F e-S clusters in the enzyme whose redox status can be monitored at 420 nm. The electron pathway used for CO oxidation appears to be as follow s: C-cluster --> Fe-S Clusters --> external electron acceptors. The el ectron used to reduce the NiFe-complex originates predominantly from t he C-cluster, and this reduction is inhibited when CN- is bound at the C-cluster. The NiFe complex is reduced more slowly (in the absence of CN-) than CO is catalytically oxidized, indicating that this reductio n is not part of the catalytic mechanism for CO oxidation. The form of the C-cluster yielding the g(av) = 1.86 signal (C-1.86) is proposed t o be two electrons more reduced than C-1.82 and able to bind and reduc e CO2. CO is proposed to be oxidized by C-1.82. Neither CO or CN- appe ars to bind C-1.86.