The Role of Se, Mo and Fe in the Structure and Function of Carbon Monoxide Dehydrogenase

Abstract

CO dehydrogenase (EC 1.2.99.2) catalyzes the oxidation of CO according to the following equation: CO + H₂O → CO₂ + 2 e⁻ + 2 H⁺. It is a selenium-containing molybdo-iron-sulfur-flavoenzyme, which has been crystallized and structurally characterized in its oxidized state from the aerobic CO utilizing bacteria Oligotropha carboxidovorans and Hydrogenophaga pseudoflava. Both CO dehydrogenase structures show only minor differences, and the enzymes are dimers of two heterotrimers. Each heterotrimer is composed of a molybdoprotein, a flavoprotein, and an iron-sulfur protein. CO oxidation takes place at the molybdoprotein which contains a 1:1 mononuclear complex of molybdopterin-cytosine dinucleotide and a Mo-ion, along with a catalytically essential S-selanylcysteine. The latter is appropriately positioned in the SeMo-active site by a unique VAYRCSFR active site loop. In H. pseudoflava the arginine preceeding the cysteine in the active site loop is modified to a Cγ-hydroxy arginine residue which has no obvious function. The substituents in the first coordination sphere of the Mo-ion are the enedithiolate sulfur atoms of the molybdopterin-cytosine dinucleotide, two oxo- and a sulfido-group. Extended X-ray absorption fine structure spectroscopy (EXAFS), along with the crystal structure of CO dehydrogenase (23.2 U mg⁻¹) at 1.85 Å resolution, have identified a sulfur atom at 2.3 Å from the Mo-ion. The sulfur reacts with cyanide yielding thiocyanate. The corresponding inactive desulfo-CO dehydrogenase shows a typical desulfo inhibited-type of Mo-electron paramagnetic resonance (EPR) spectrum. Structural changes at the SeMo-site during catalysis are suggested by the Mo to Se distance of 3.7 Å and the Mo-S-Se angle of 113° in the oxidized enzyme which increase to 4.1 Å, and 121°, respectively, in the reduced enzyme. The intramolecular electron transport chain in CO dehydrogenase involves the following prosthetic groups and minimal distances: CO → [Mo of the molybdenum cofactor]−14.6Å−[2Fe-2S] I−12.4 Å−[2Fe-2S] II−8.7 Å−[FAD].