Crystal structure and mechanism of CO dehydrogenase, a molybdo iron-sulfur flavoprotein containing S-selanylcysteine

CO dehydrogenase from the aerobic bacterium Oligotropha carboxidovorans catalyzes the oxidation of CO with H2O, yielding CO2, two electrons, and two H+. Its crystal structure in the air-oxidized form has been determined to 2.2 A. The active site of the enzyme, which contains molybdenum with three oxygen ligands, molybdopterincytosine dinucleotide and S-selanylcysteine, delivers the electrons to an intramolecular electron transport chain composed of two types of [2Fe-2S] clusters and flavin-adenine dinucleotide, CO dehydrogenase is composed of an 88.7-kDa molybdoprotein (L), a 30.2-kDa flavoprotein (M), and a 17.8-kDa iron-sulfur protein (S). It is organized as a dimer of LMS heterotrimers and resembles xanthine dehydrogenase/oxidase in many, but not all, aspects. A mechanism based on a structure with the bound suicide-substrate cyanide is suggested and displays the necessity of S-selanylcysteine for the catalyzed reaction.