Selective Production of Methanol from Methane and Molecular Oxygen at Atmospheric Temperature and Pressure Using Methane Monooxygenases

Abstract

Methane monooxygenase (MMO) is a metalloenzyme that is biosynthesized by methane-oxidizing bacteria (methane-utilizing bacteria). Two types of MMO have been isolated from the cells of methane-oxidizing bacteria: iron-containing, soluble MMO (sMMO) and copper-containing, membrane-bound MMO (particulate MMO, pMMO). Methane conversion via MMOs is attractive as a sustainable methane-utilization technique because the enzymatic methane conversion proceeds at ambient temperature and pressure and with 100% selectivity toward methanol. However, the catalytic cycle involves tremendously complex molecular mechanisms. The two MMOs undergo changes in their protein scaffolds in order to interact with four substrates (methane, molecular oxygen, electrons, and protons), deliver the substrates to the catalytic site, and control each stage of the catalytic cycle. Therefore, both the properties of the catalytic site and the active oxygen species involved in the catalysis and the overall structure of the protein and its dynamics have been investigated. sMMO has been extensively studied, as it is relatively stable under non-biological conditions. In this chapter, the coordination structure of the iron site, catalytic cycle at the iron site, structure of the protein complex, dynamics of each of the components of the protein complex, and interaction between the components of the protein complex are summarized. In contrast, the methane oxidation mechanism of pMMO remains less well understood than that of sMMO. However, the progress in the investigation of pMMO in the twenty-first century has been remarkable. In this chapter, recent studies regarding the protein structure, copper site properties, and the catalytic cycle at the copper site of pMMO are described.