Deciphering the oxygen activation mechanism at the CuC site of particulate methane monooxygenase

The enzymatic oxidation of methane to methanol was discovered in methanotrophs over 110 years ago. Nevertheless, the mechanism of action of particulate methane monooxygenase (pMMO) remains elusive, especially regarding O2 activation and the nature of the active species of the enzyme. Here we decipher the catalytic cycle of pMMO in the presence of the physiological reductant duroquinol (DQH2). We demonstrate that O2 activation is in fact initiated by a CuC(ii)–DQH− species generated by deprotonation of DQH2. Our simulations capture the exclusive pathway for the sequential formation of the intermediates, CuC(ii)−O2•−, CuC(ii)−OOH− and H2O2, along the O2 reduction pathway. Furthermore, H2O2 activation by CuC(ii)−DQH− is initiated by dissociation of DQH• to yield CuC(i), followed by CuC(i)-catalysed O−O homolysis, en route to the formation of the CuC(ii)−O•− species, which is responsible for C−H oxidations. These findings uncover the important roles of the phenol co-substrate for O2 activation and help resolve the enigmatic mechanism of pMMO. The catalytic mechanism of oxygen activation employed by particulate methane monooxygenase for the oxidation of methane has remained elusive. Now, computational simulations suggest an important role of the phenol co-substrate and a catalytic cycle is proposed.