Organization of clusters and internal electron pathways in CO dehydrogenase from Clostridium thermoaceticum: relevance to the mechanism of catalysis and cyanide inhibition.

Cyanide inhibits the CO oxidation activity of carbon monoxide dehydrogenase from Clostridium thermoaceticum by binding tightly to the form of the C-cluster yielding the gav = 1.82 signal (the C1.82 form). CN- dissociates and the enzyme reactivates upon addition of CO, CO2 plus dithionite, or CS2 plus dithionite. Dithionite slows the inhibition of the enzyme by CN-, but it cannot reactivate the enzyme. This behavior is explained by assuming that binding of CO, CO2, or CS2 at a modulator site accelerates the dissociation of CN- from the C-cluster. With CN- bound at the C-cluster, dithionite, but not CO, can reduce those Fe-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 follows: C-cluster-->Fe-S Clusters-->external electron acceptors. The electron used to reduce the NiFe complex originates predominantly from the 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 reduction is not part of the catalytic mechanism for CO oxidation. The form of the C-cluster yielding the g(av) = 1.86 signal (C1.86) is proposed to be two electrons more reduced than C1.82 and able to bind and reduce CO2. CO is proposed to be oxidized by C1.82. Neither CO or CN- appears to bind C1.86.