Responsible for the initial oxygenation of methane to methanol in methanotrophs. It also catalyzes the monohydroxylation of a variety of unactivated alkenes, alicyclic, aromatic and heterocyclic compounds. H(+) + methane + NADH + O2 = H2O + methanol + NAD(+) H(+) + methane + NADPH + O2 = H2O + methanol + NADP(+) Binds 2 iron ions. M.capsulatus has two forms of methane monooxygenase, a soluble and a membrane-bound type. The soluble type consists of four components (A to D): protein A, comprising three chains, in an alpha-2, beta-2, gamma-2 configuration, is a nonheme iron protein containing an unusual mu-hydroxo bridge structure at its active site and interacts with both oxygen and methane. Belongs to the TmoA/XamoA family.