Methane oxidation by the copper methane monooxygenase: Before and after the cryogenic electron microscopy structure of particulate methane monooxygenase from Methylococcus capsulatus (Bath)

The particulate methane monooxygenase (pMMO) is a copper monooxygenase that converts methane into methanol in methanotrophic bacteria. As this enzyme converts methane into methanol efficiently and selectively under ambient conditions, it has become the paradigm for understanding the design of nature to facilitate this process so that we could develop a biomimetic catalyst to accomplish this difficult C1 chemistry in the laboratory. With the advent of the recent 2.5 Å cryo-electron microscopy structure of the pMMO from Methylococcus capsulatus (Bath), it is now evident that the catalytic site of hydroxylation in pMMO is a Cu^ICu^ICu^I tricopper cluster (Cu^I: copper ions) sequestered within the transmembrane domain of this protein complex. With three reducing equivalents embodied in this structural motif, the tricopper cluster can react with O₂ to irreversibly cleave the O-O bond to harness the highly reactive oxene for rapid and direct insertion into the C-H bonds of methane. Here, we review the structural, biochemical, and biophysical studies over the past three decades that have culminated in this important advance in the chemistry of methane oxidation.