Role of Amino Acid Residues for Dioxygen Activation in the Second Coordination Sphere of the Dicopper Site of pMMO

Mechanisms for Methane and Ammonia Oxidation by Particulate Methane Monooxygenase

Particulate MMO (pMMO) catalyzes the oxidation of methane to methanol and also ammonia to hydroxylamine. Experimental characterization of the active site has been very difficult partly because the enzyme is membrane-bound. However, recently, there has been major progress mainly through the use of cryogenic electron microscopy (cryoEM). Electron paramagnetic resonance (EPR) and X-ray spectroscopy have also been employed. Surprisingly, the active site has only one copper. There are two histidine ligands and one asparagine ligand, and the active site is surrounded by phenyl alanines but no charged amino acids in the close surrounding. The present study is the first quantum chemical study using a model of that active site (CuD). Low barrier mechanisms have been found, where an important part is that there are two initial proton-coupled electron transfer steps to a bound O2 ligand before the substrate enters. Surprisingly, this leads to large radical character for the oxygens even though they are protonated. That result is very important for the ability to accept a proton from the substrates. Methods have been used which have been thoroughly tested for redox enzyme mechanisms.

Direct Methane Oxidation by Copper- and Iron-Dependent Methane Monooxygenases

Methane is a potent greenhouse gas that contributes significantly to climate change and is primarily regulated in Nature by methanotrophic bacteria, which consume methane gas as their source of energy and carbon, first by oxidizing it to methanol. The direct oxidation of methane to methanol is a chemically difficult transformation, accomplished in methanotrophs by complex methane monooxygenase (MMO) enzyme systems. These enzymes use iron or copper metallocofactors and have been the subject of detailed investigation. While the structure, function, and active site architecture of the copper-dependent particulate methane monooxygenase (pMMO) have been investigated extensively, its putative quaternary interactions, regulation, requisite cofactors, and mechanism remain enigmatic. The iron-dependent soluble methane monooxygenase (sMMO) has been characterized biochemically, structurally, spectroscopically, and, for the most part, mechanistically. Here, we review the history of MMO research, focusing on recent developments and providing an outlook for future directions of the field. Engineered biological catalysis systems and bioinspired synthetic catalysts may continue to emerge along with a deeper understanding of the molecular mechanisms of biological methane oxidation. Harnessing the power of these enzymes will necessitate combined efforts in biochemistry, structural biology, inorganic chemistry, microbiology, computational biology, and engineering.

Mechanistic Study of the Activation and the Electrocatalytic Reduction of Hydrogen Peroxide by Cu-tmpa in Neutral Aqueous Solution

Hydrogen peroxide plays an important role as an intermediate and product in the reduction of dioxygen by copper enzymes and mononuclear copper complexes. The copper(II) tris(2-pyridylmethyl)amine complex (Cu-tmpa) has been shown to produce H2O2 as an intermediate during the electrochemical 4-electron reduction of O2. We investigated the electrochemical hydrogen peroxide reduction reaction (HPRR) by Cu-tmpa in a neutral aqueous solution. The catalytic rate constant of the reaction was shown to be one order of magnitude lower than the reduction of dioxygen. A significant solvent kinetic isotope effect (KIE) of 1.4 to 1.7 was determined for the reduction of H2O2, pointing to a Fenton-like reaction pathway as the likely catalytic mechanism, involving a single copper site that produces an intermediate copper(II) hydroxo species and a free hydroxyl radical anion in the process.

Synthesis of Carbonato- and Doubly Oxido-Bridged Diruthenium(III,IV) Complex and Reactions with Cations

Doubly oxido-bridged transition metal moieties, {M₂(μ-O)₂}, play important roles as oxidation reaction centers in nature. This work features a diruthenium(III,IV) complex with a doubly oxido-bridged core {Ru₂^III,IV(μ-O)₂}³⁺ with a carbonato bridged between the two ruthenium centers, M[{Ru^III,IV(ebpma)}₂(μ-O)₂(μ-O₂CO)]₂(PF₆)₃ (M[1_CO3]₂(PF₆)₃; Carbonato complex, ebpma; ethylbis(2-pyridymethyl)amine), and explores the interactions of this complex with cations (H⁺ and M⁺). M[1_CO3]₂(PF₆)₃ was formed via reactions of a singly oxido-bridged complex, [{Ru^III,IVCl₂(ebpma)}₂(μ-O)]PF₆·(CH₃)₂CO, with M₂CO₃ (M = K, Na) or with CO₂(g), adjusted to around pH 12 with NaOH(aq.), in a water-acetone mixed solvent. The Carbonato complex was isolated as a powder in the form of M[1_CO3]₂(PF₆)₃ (M = K, Na), because of the interactions between the carbonato moiety and K⁺ or Na⁺ in the solid structure. In acidic aqueous solutions, unexpectedly, the carbonato ligand remained bound to the doubly bridged core, {Ru₂^III,IV(μ-O)₂}³⁺ or {Ru₂^III,IV(μ-O)(μ-OH)}⁴⁺, without decarboxylation even under pH 1.0. Two-step one-protonation/deprotonation occurred reversibly between pH 1.0 and 13.2 to the bridging oxido and carbonato ligands. The structures of the corresponding one- and two-protonated complexes ([1_CO3H]²⁺ and [1_CO32H]³⁺) were successfully characterized.

Catalytic Oxygenation of Hydrocarbons by Mono-μ-oxo Dicopper(II) Species Resulting from O-O Cleavage of Tetranuclear CuI /CuII Peroxo Complexes

One of the challenges of catalysis is the transformation of inert C-H bonds to useful products. Copper-containing monooxygenases play an important role in this regard. Here we show that low-temperature oxygenation of dinuclear copper(I) complexes leads to unusual tetranuclear, mixed-valent μ₄ -peroxo [Cu^I /Cu^II ]₂ complexes. These Cu₄ O₂ intermediates promote irreversible and thermally activated O-O bond homolysis, generating Cu₂ O complexes that catalyze strongly exergonic H-atom abstraction from hydrocarbons, coupled to O-transfer. The Cu₂ O species can also be produced with N₂ O, demonstrating their capability for small-molecule activation. The binding and cleavage of O₂ leading to the primary Cu₄ O₂ intermediate and the Cu₂ O complexes, respectively, is elucidated with a range of solution spectroscopic methods and mass spectrometry. The unique reactivities of these species establish an unprecedented, 100 % atom-economic scenario for the catalytic, copper-mediated monooxygenation of organic substrates, employing both O-atoms of O₂ .