intermediates from one active site to another. Many NiCODH exhibit this homodimeric structure, however, there also examples of functional monomeric NiCODH, such the one found in Rhodospirillum rubrum. Reaction mechanisms Oxidative The CODH catalytic site, referred to as the C-cluster, is a [3Fe-4S] cluster bonded to a Ni-Fe moiety. Two basic amino acids (Lys587 and His 113 in M.thermoacetica'') reside in proximity to the C-cluster and facilitate acid-base chemistry required for enzyme activity. Based on IR spectra suggesting the presence of an Ni-CO complex, the proposed first step in the oxidative catalysis of CO to CO2 involves the binding of CO to Ni2+ and corresponding complexing of Fe2+ to a water molecule. The binding of CO molecule causes a shift in the coordination of the Ni atom from a square-planar to square pyramidal geometry. Dobbek et al. further propose that movement of the nickel atom's cysteine ligand brings the CO into close proximity to the hydroxyl group, and facilitate a base-catalyzed, nucleophillic attack by the iron-bound hydroxy group. A carboxy bridge between the Ni and Fe atom has been proposed as an intermediate. A decarboxylation leads to the release of CO2 and the reduction of the cluster. Although the resulting intermediate oxidation state of the Ni and the degree to which electrons are distributed throughout the Ni-[3Fe-4S] cluster is subject of some debate, the electrons in the reduced C-cluster are transferred to nearby B and D [4Fe-4S] clusters, returning the Ni-[3Fe-4S] C-cluster to an oxidized state and reducing the single electron carrier ferredoxin. Reductive Given CODH's role in CO2 fixation, it is common in the biochemistry literature for the reductive mechanism to be inferred as the “direct reverse” of the oxidative mechanism by the ”principal of microreversibility.” In the process of reducing carbon dioxide, the enzyme's C-cluster must first be activated from an oxidized to a reduced state before the Ni-CO2 bond is formed. Environmental relevance Carbon monoxide dehydrogenase is closely associated with the regulation of atmospheric CO and CO2 levels, maintaining optimal CO levels suitable for other forms of life. Microbial organisms rely on these enzymes for both energy conservation along with CO2 fixation. Often encoding for and synthesizing multiple unique forms of CODH for designated use. Further research into specific types of CODH show CO being used and condensed with CH3 (Methyl groups) to form Acetyl-CoA. Anaerobic micro-organisms like Acetogens undergo the Wood-Ljungdahl Pathway, relying on CODH to