- Anaerobic Carbon Monoxide Dehydrogenase: Mechanism of CO-Oxidation at the [NiFe4S4OHx] Cluster and Nickel-Processing by its ATPase CooC (2008)
- Anaerobic CO dehydrogenases (CODH) catalyze the reversible oxidation of CO to CO2 at a complex metal center containing Ni, Fe and S called cluster C. In this work, a heterologous expression system of CODHII from Carboxydothermus hydrogenoformans in E. coli and the crystal structures of CODHII in different states are reported. CO2 bridging the Ni-Fe1 site is observed in a reduced state with exogeneously supplied CO2-source, and is reductively activated by binding to cluster C. The ligand CO2 completes the square-planar coordination of the Ni ion and replaces the H2O/OH- ligand at the Fe1 ion in the other two states. The H2O/OH- ligand is replenished by a neighboring network of water molecules. Protons produced from the reversible CO oxidation are expelled through a semi-conserved histidine channel. The mutation of His96, one of four semi-conserved histidines to Asp diminished CO-oxidation activity to 2.7% of wild-type, indicating its essential role in catalysis. The structure of H96D CODHII reveals that the abolished activity is not originated from protein misfolding and/or absence of metal centers in the protein. Another variant of CODHII devoid of cluster C, but containing clusters B and D, was structurally characterized. This cluster C-missing CODHII shows an identical protein scaffold to the structure of active enzyme, and the residues coordinating the metals of cluster C display the same location as the active wild-type enzyme. From all metals of cluster C, only the Fe1 ion displays an alternative position, which appears to be the reason for the obseved cluster C heterogeneity. The structures presented in this work define the mechanism of CO oxidation/CO2 reduction at the Ni-Fe site of cluster C and show the channels that facilitate the transport of substrate/product during catalysis. The ATPase CooC1 from C. hydrogenoformans belongs to the MinD family of the SIMIBI class NTPases containing a deviant walker A motif. The protein has been proposed to participate in the maturation of cluster C of CODH (Jeon et al, 2001). As-isolated CooC1 shows monomeric state in solution with a molecular weight of 32 kDa. The presence of Ni(II) induces dimerization of CooC1, and the protein binds one Ni(II) per dimer with nanomolar affinity. The crystal structure of Metal-bound CooC1 clearly identified the conserved CXC motif as the metal-binding site, which is responsible for the dimerization. In solution, CooC1 also undergoes nucleotide-dependent dimerization and forms a stable dimer in the presence of ATP. The K8A CooC1 mutant, where Ala replaced the signature Lys is incapable of both ATP hydrolysis and ATP-dependent stable dimer formation. Compared to other structural homologues of the MinD family, the ATP-induced dimer structure of CooC1 shows a different conformation than the Ni-induced dimeric state. On the basis of biochemical data and structures of CooC1 in combination with a model of ATP-driven dimerization, a reaction cycle of CooC1 in Ni-processing for the maturation of cluster C is presented.