Figure 4.

Proposed mechanism of the C-cluster, consistent with current structural and biochemical knowledge that supports the lack of a bridging sulfide. Our structures presented here represent states I and II of this mechanism; the putative CO-bound MbCODH structure represents state II; and the CO₂-bound ChCODH structure represents state IV or V, which differ only in the protonation state of the intermediate and would thus be difficult to distinguish using X-ray crystallography. States I and VI are structurally identical and differ only in the redox state of the cluster. “B” refers to a basic residue involved in deprotonation. The identity of the base has not been resolved, and candidates include His113 and Lys587.