The mechanism of Mo-/Cu-dependent CO dehydrogenase

Abstract

Density functional theory computations at the B3LYP/SDDp//B3LYP/Lanl2DZ level were performed on model complexes derived from [(Me(2)C(2)S(2))Mo(O)(2)-S-CuSMe](2-) or its oxo protonated form to gain insight into the reaction steps involved in substrate oxidation of a Mo-/Cu-dependent CO dehydrogenase. Only the bisoxo but not the hydroxo oxo complex was found to oxidize CO exothermically. A thiocarbamate complex structurally characterized as the reaction product of the enzyme with the inhibitor n-butylisonitrile corresponds to a thermodynamic well on the potential energy surface. For the formation of the analogous thiocarbonate complex from CO oxidation, however, we do not find a significant thermodynamic driving force. In the protein matrix of the enzyme this species should be further destabilized, as it requires the metal centers to move apart considerably from each other.

The Hydrogenase Activity of the Molybdenum/Copper-containing Carbon Monoxide Dehydrogenase of Oligotropha carboxidovorans

“…reduction of protons to H 2 ), as seen with the Ni-Fe CO dehydrogenases, suggests that deprotonation of the copper hydride intermediate is strongly downhill thermodynamically and functionally irreversible (15). Nevertheless, there are common elements of the reaction mechanism proposed here for reduction by H 2 and that proposed previously for reduction by CO (11,12). These include initial binding of substrate to the copper of the binuclear center in a manner that activates substrate (by back-bonding from copper into a * orbit in the case of CO, by polarization of the H-H bond in the case of H 2 ) and utilization of the highly delocalized nature of the redox-active orbital of the binuclear center to bring about its reduction via the copper rather than the molybdenum.…”

“…We consider it unlikely that bicarbonate displaces only the equatorial MoϭO of oxidized enzyme (to give a Mo(V)/Cu(I) species analogous to a presumed Mo(VI)/Cu(I) intermediate proposed on the basis of computational studies (11,12), as this does not account for the experimentally observed loss of proton coupling in the EPR signal in the presence of bicarbonate. Finally, with the H 2 -reduced native enzyme, given the effectiveness with which CO dehydrogenase oxidizes H 2 , we consider it very likely that that, as has been proposed for CO oxidation where CO initially binds to the copper of the binuclear center, H 2 also first binds at the copper of the binuclear center (11,12), displacing the H 2 O coordinated to the copper prior to catalysis. We believe that the observed EPR signal (as with CO as substrate) arises from enzyme that has already become partially reduced by reaction with prior turnover under the reaction conditions, such that H 2 is bound to a partially reduced Mo(V)/Cu(I) binuclear center.…”

“…The molybdenum has a square pyramidal coordination geometry with an apical MoϭO; the equatorial plane consists of the two sulfurs from the pyranopterin cofactor, an equatorial MoϭO and a -sulfido sulfur that bridges to the copper, which is also coordinated by Cys 388 of the protein (9,10). Although not commonly referred to in mechanistic discussions of CO dehydrogenase, the copper is also coordinated by a HO Ϫ /H 2 O ligand at a distance of 2.4 Å. Computational studies of the reaction mechanism suggest that this ligand is displaced by CO at the outset of catalysis (11,12).…”

The Effect of Backbone Constraints: The Case of Water Oxidation by the Oxygen‐Evolving Complex in PSII

“…There have been two previous studies on the mechanism of Mo,Cu-dependent CO dehydrogenase. [25,44] An important finding in these studies was the presence of intermediates with SÀC bonds, one of them shown to the right in Figure 5. The structure to the left is the one without substrate, which has a MoÀCu bond length of 2.97 .…”

Kinetic and Spectroscopic Studies of the Molybdenum-Copper CO Dehydrogenase from Oligotropha carboxidovorans

“…A reaction mechanism has been proposed involving the formation of an SCO 2 intermediate, on the basis of the crystal structure of CODH in complex with the inhibitor n-butylisocyanide (7). However, computational studies suggest that such a thiocarbonate intermediate actually leads to a deep minimum on the potential energy surface, making CO 2 release extremely difficult (16,17). In addition, model compound studies suggest that CO oxidation could occur directly at the Cu(I) site, followed by electron transfer to Mo via the electronic delocalization of the Mo(-S)Cu moiety (18).…”