Current chemical conversion of methane to methanol requires a step of high temperature and high pressure with low efficiency (Fischer-Tropsch). Developing catalyst that allows the reaction of methane to methanol at relative low temperature is critical to utilize natural gas as more portable and safer energy source. The benefit is also expected in potential environmental applications, such as reducing green house gas and bioremediation of land and ocean contaminated by oil spills.

Developing such a catalyst is, yet, very challenging due to the characteristic C-H bond (104.9 kcal/mol) of methane, which is extremely stable and hard to break apart. However, the Nature already knows the trick. Methanotrophic bacteria living in soil are well known for using methane as their sole carbon source and can convert methane to methanol. Characteristic feature of these bacteria is due to the enzyme called methane monooxygenase (MMO). MMO converts,

CH4 + O2 + 2H+ + 2e  ->  CH3OH + H2O

Methane monooxygenase is composed of four subunits; MMO hydroxylase (MMOH), MMO reductase (MMOR), MMO regulatory subunit (MMOB) and MMO inhibitory subunit (MMOD). The crystal structure of MMOH, which performs the actual chemical conversion of methane to methanol, revealed a heterohexameric architecture with a glutamate-bridged di-iron active site in each alpha subunit. MMOR shuttles electrons from NADH to the di-iron center of MMOH. MMOB alters reduction potentials at the di-iron center and aids in substrate access to the active site. MMOD inhibits the catalytic activity of MMOH. The catalytic cycle of MMOH is only accomplished in the presence of regulators, thus observing geometry changes of the active site upon regulator binding is critical to understand the catalytic conversion of methane to methanol.

Structures of Individual MMO components, the crystal structure of MMOH and NMR structures of MMOR and MMOB, have been determined and provided us an overview of how individual subunit might function in reaction. How MMOB and MMOR influence to the catalytic activity of MMOH, however, still remained obscure due to the lack of structural information of complexes.

Our goal is to determine the complex structure of MMO in the presence of each auxiliary subunit using both X-ray and Cryo-EM approaches (Lee et al, 2013, Nature).