The oxidation-reduction and electrocatalytic properties of CO dehydrogenase from Oligotropha carboxidovorans

CO dehydrogenase (CODH) from the Gram-negative bacterium Oligotropha carboxidovorans is a complex metalloenzyme from the xanthine oxidase family of molybdenum-containing enzymes, bearing a unique binuclear Mo-S-Cu active site in addition to two 2Fe-2S] clusters (FeSI and FeSII) and one equivalent of FAD. CODH catalyzes the oxidation of CO to CO₂ with the concomitant introduction of reducing equivalents into the quinone pool, thus enabling the organism to utilize CO as sole source of both carbon and energy. Using a variety of EPR monitored redox titrations and spectroelectrochemistry, we report the redox potentials of CO dehydrogenase at pH 7.2 namely Mo^VI/V, Mo^V/IV, FeSI^2+/+, FeSII^2+/+, FAD/FADH and FADH/FADH^?. These potentials are systematically higher than the corresponding potentials seen for other members of the xanthine oxidase family of Mo enzymes, and are in line with CODH utilising the higher potential quinone pool as an electron acceptor instead of pyridine nucleotides. CODH is also active when immobilised on a modified Au working electrode as demonstrated by cyclic voltammetry in the presence of CO.