Molecular Biology of Particulate Methane Monooxygenase

Abstract

Methane oxidizing bacteria (methanotrophs) grow on methane as their sole source of carbon and energy by oxidizing this one-carbon (CI) compound, via methanol, formaldehyde and formate, to carbon dioxide. Carbon is assimilated into biomass at the level of formaldehyde by either the serine or the ribulose monophosphate pathway. The first step in the methane oxidation pathway is catalysed by the enzyme methane monooxygenase (MMO). The MMO can exist in two forms, a soluble, cytoplasmic enzyme complex (sMMO) or a membrane-bound, particulate enzyme (pMMO). The sMMO is only found in methanotrophs such as Methylococcus spp., Methylosinus spp., and some Methylocystis and Methylomonas spp. The sMMO has been purified from several methanotrophs and the genes encoding this enzyme complex have been cloned and sequenced from Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b. The biochemistry and molecular biology of sMMO has recently been reviewed extensively (Lipscomb 1994; Dalton et al. 1993; Murrell 1992, 1994) and will not be covered here. The pMMO appears to be present in all methanotrophs but is less-well characterised than sMMO. The enzyme has proved difficult to purify in active form, presumably because of instability in the pMMO polypeptides once they are removed from the membranes. In those methanotrophs that possess both sMMO and pMMO, eg Mc. capsulatus (Bath) and Ms. Trichosporium OB3b, the copper-to-biomass ratio of cultures seems to regulate the switch between expression of sMMO or pMMO (Stanley et al. 1983). A high copper-to-biomass ratio favours expression of the pMMO and sMMO is repressed by copper ions (copper also appears to inhibit sMMO activity). Cells grown at low copper-to-biomass ratios express sMMO and there is now good evidence for transcriptional control of sMMO (and pMMO) expression by copper ions (A. Nielsen, J.C. Murrell, unpublished).