Methane monooxygenases; physiology, biochemistry and structure

Methane-utilizing bacteria (methanotrophs), which inhabit various environments such as wetlands, rice paddies and the surface of aquatic plants are known to convert methane to methanol using methane monooxygenase enzymes (MMOs). There are two distinct types of MMOs: the copper-containing membrane-bound enzyme (pMMO) and the iron-containing soluble enzyme (sMMO). Since MMOs catalyze methane oxidation at ambient temperature and pressure, they are potential biocatalysts for industrial methanol production from methane. Understanding the mechanism of the MMO-catalyzed reaction is crucial to develop a new biocatalyst. The catalytic mechanism of MMO has been extensively studied in aspects of enzyme kinetics, protein structure, spectroscopy, biomimetic chemistry and computation. Based on these studies, the catalytic mechanism of sMMO is relatively well understood, and a rigid catalytic mechanism is proposed. On the other hand, the studies of pMMO are still at their early stages and there are debates as to which of several copper sites are the true active sites. In this review, we describe our current knowledge of MMOs including the metabolism, regulation of expression, their enzymatic properties, and the structural and biochemical features. We summarize recent structural and biochemical studies of pMMO and discuss the future directions to develop efficient and robust biocatalysts.