Mechanism of the Methane → Methanol Conversion Reaction Catalyzed by Methane Monooxygenase:  A Density Functional Study

Abstract

The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH 2 )(H 2 O)Fe(µ-O) 2 (η 2 -HCOO) 2 Fe(NH 2 )(H 2 O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe IV atoms, L 4 Fe IV (µ-O) 2 Fe IV L 4 , the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L 4 Fe IV (µ-O)(µ-OH(‚‚‚CH 3 ))Fe III L 4 , which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe III -Fe IV mixed valence state. This short-lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L 4 Fe III (OHCH 3 )-(µ-O)Fe III L 4 , which has an Fe III -Fe III core. The barrier of the rate-determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH 4 oxidation reaction to form the methanol complex, I + CH 4 f VI, was found to be exothermic by 39 kcal/mol.

Review of NAD(P)H-dependent oxidoreductases: Properties, engineering and application

“…In MMO, activation of molecular oxygen is carried out by a metallic cofactor consisting of two Fe ions each coordinated by six ligands (a combination of His and Glu residues and water molecules) [141]. After one oxygen atom is transferred to the substrate, the reduced form of the metallic centre is regenerated by the transfer of two electrons from NAD(P)H [142]. There are two types of MMO, soluble MMO (sMMO) and membrane-bound particulate MMO (pMMO).…”

Ab initioquantum chemistry: Methodology and applications

“…Ab initio calculations provide a wealth of detail that is not available from experiment and a degree of confidence in the results that is not available from more empirical approaches. In systems such as methane monooxygenase, in which a substantial number of careful calculations have been performed by several groups (81,(91)(92)(93)(94)(95)(96) and close contact with experiment has been achieved for many aspects of the catalytic process, a comprehensive picture of the functioning of the enzyme is beginning to be assembled by means of the interaction of theory and experiment. Last, the coupling of ab initio quantum QM͞MM methods with simulation and protein structure prediction techniques permits investigation of events in which reactive chemistry is coupled to substantial conformational changes, such as the catalytic loop motion in triose phosphate isomerase (82).…”

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

“…Extended x-ray absorption fine structure (EXAFS) analysis of Q has found a short FeOFe distance of 2.5 Å, which has led us to propose an [Fe IV 2 ( -O) 2 ] diamond core structure (6). Density functional theory (DFT) calculations from a number of groups also support the viability of such an intermediate (7)(8)(9)(10)(11). Although several mononuclear oxoiron(IV) complexes have been structurally characterized (12), thus providing valuable information regarding the catalytic mechanism of mononuclear nonheme iron enzymes (13) 3 [L c , tris(5-ethyl-2-pyridylmethyl)amine] (1c, Scheme 1) (14).…”