The C-cluster of the enzyme carbon monoxide dehydrogenase (CODH) is a structurally distinctive Ni-Fe-S cluster employed to catalyze the reduction of CO2 to CO as part of the Wood-Ljungdahl carbon fixation pathway. Using X-ray crystallography, we have observed unprecedented conformational dynamics in the C-cluster of the CODH from Desulfovibrio vulgaris, providing the first view of an oxidized state of the cluster. Combined with supporting spectroscopic data, our structures reveal that this novel, oxidized cluster arrangement plays a role in avoiding irreversible oxidative degradation at the C-cluster. Furthermore, mutagenesis of a conserved cysteine residue that binds the C-cluster in the oxidized state but not in the reduced state suggests that the oxidized conformation could be important for proper cluster assembly, in particular Ni incorporation. Together, these results lay a foundation for future investigations of C-cluster activation and assembly, and contribute to an emerging paradigm of metallocluster plasticity.

CO dehydrogenases (CODHs) catalyse the reversible conversion between CO and CO . Genomic analysis indicated that the metabolic functions of CODHs vary. The genome of Carboxydothermus hydrogenoformans encodes five CODHs (CODH-I-V), of which CODH-IV is found in a gene cluster near a peroxide-reducing enzyme. Our kinetic and crystallographic experiments reveal that CODH-IV differs from other CODHs in several characteristic properties: it has a very high affinity for CO, oxidizes CO at diffusion-limited rate over a wide range of temperatures, and is more tolerant to oxygen than CODH-II. Thus, our observations support the idea that CODH-IV is a CO scavenger in defence against oxidative stress and highlight that CODHs are more diverse in terms of reactivity than expected.

Ni-containing CO dehydrogenases (CODHs) are very efficient metalloenzymes that catalyze the conversion between CO2 and CO. They are a source of inspiration for designing CO2-reduction catalysts and can also find direct use in biotechnology. They are deemed extremely sensitive to O2, but very little is known about this aspect of their reactivity. We investigated the reaction with O2 of Carboxydothermus hydrogenoformans (Ch) CODH II and the homologous, recently characterized CODH from Desulfovibrio vulgaris (Dv) through protein film voltammetry and solution assays (in the oxidative direction). We found that O2 reacts very quickly with the active site of CODHs, generating species that reactivate upon reduction--this was unexpected. We observed that distinct CODHs exhibit different behaviors: Dv CODH reacts half as fast with O2 than Ch CODH, and only the former fully recovers the activity upon reduction. The results raise hope that fast CO/CO2 biological conversion may be feasible under aerobic conditions.