Carbon monoxide dehydrogenase

Aerobic carboxydotrophic bacteria utilize copper-molybdenum flavoenzymes. Anaerobic bacteria utilize nickel-iron based CODHs due to their oxygen sensitive natur. CODH containing a Mo-[2Fe-2S]-FAD active site have been found in aerobic bacteria, while a distinct class of Ni-[3Fe-4S] CODH enzymes have been purified from anaerobic bacteria.[5][6][7] Both classes of CODH catalyze the reversible conversion between carbon dioxide (CO₂) and carbonmonoxide (CO). CODH exists in both monofuctional and bifunctional forms. In the latter case, CODH forms a bifunctional cluster with acetyl-CoA synthase, as has been well characterized in the anaerobic bacteria Moorella thermoacetica.[8][9]

Structure of CODH/ACS in M.thermoacetica." Alpha (ACS) and beta (CODH) subunits are shown. (1)The A-cluster Ni-[4Fe-4S]. (2)C-cluster Ni-[3Fe-4S]. (3) B-Cluster [4Fe-4S]. (4) D-cluster [4Fe-4S]. Designed from 3I01

Homodimeric Ni-CODH consists of five metal complexes referred to as clusters. Each differing in individual coordination geometry, presence of nickel, and location of the active site in either sub-unit α or β.[10] Multiple research groups have proposed crystal structures for the α₂β₂ tetrameric enzyme CODH/ACS from the acetogenic bacteria M. thermoacetica, including two recent examples since 2009: 3I01 2Z8Y. The two β units are the site of CODH activity and form the central core of the enzyme. In total, the 310 kDa enzyme contains seven iron-sulfur [4Fe-4S] clusters. Each α unit contains a single metal cluster. Together, the two β units contains five clusters of three types. CODH catalytic activity occurs at the Ni-[3Fe-4S] C-clusters while the interior [4Fe-4S] B and D clusters transfer electrons away from the C-cluster to external electron carriers such as ferredoxin. The ACS activity occurs in A-cluster located in the outer two α units.[6][7]

A noteworthy feature of the M. thermoacetica CODH/ACS is an internal gas tunnel connecting the multiple active sites.[11] The full role of the gas channel in regulating the rate catalytic activity is still a subject of investigation, but several studies support the notion that molecules of CO do in fact travel directly from the C-cluster to the ACS active site without leaving the enzyme. For instance, the rate of acetyl-CoA synthase activity in the bifunctional enzyme is not affected by the addition of hemoglobin, which would compete for CO in bulk solution,[12] and isotopic labeling studies show that carbon monoxide derived from the C-cluster is preferentially used at the A-cluster over unlabeled CO in solution.[13] Protein engineering of the CODH/ACS in M.thermoacetica revealed that mutating residues, so as to functionally block the tunnel, stopped acety-CoA synthesis when only CO₂ was present.[14] The discovery of a functional CO tunnel places CODH on a growing list of enzymes that independently evolved this strategy to transfer reactive intermediates from one active site to another.[15]

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.[24] Anaerobic micro-organisms like Acetogens undergo the Wood-Ljungdahl Pathway, relying on CODH to produce CO by reduction of CO₂ needed for the synthesis of Acetyl-CoA from a methyl, coenzyme a (CoA) and corrinoid iron-sulfur protein.[25] Other types show CODH being utilized to generate a proton motive force for the purposes of energy generation. CODH is used for the CO oxidation, producing two protons which are subsequently reduced to form dihydrogen (H₂, known colloquially as molecular hydrogen), providing the necessary free energy to drive ATP generation.[26]