Physiology and Biochemistry of the Aerobic Methanotrophs

Methanotrophs are a widely distributed group of aerobic bacteria that use methane as their source of carbon and energy. They play key roles in the global carbon cycle, including controlling anthropogenic and natural emissions of the greenhouse gas methane. Methanotrophs oxidize methane using the unique enzyme methane monooxygenase which exists in two structurally and biochemically distinct forms. One form, the membrane-associated or particulate methane monooxygenase (pMMO), is found in most known methanotrophs and is located in the cytoplasmic membrane. Another form, the soluble methane monooxygenase (sMMO), is found in some methanotrophs and is located in the cytoplasm. Both forms of MMO can co-oxidize a range of hydrocarbons and chlorinated pollutants and hence are interesting with respect to the biotechnological potential of methanotrophs. Methanol is further oxidized to formaldehyde, formate, and CO2, by specific methylotrophic enzymes, while biomass is built from formaldehyde, formate, CO2, or a combination thereof via three cyclic biochemical pathways: the ribulose monophosphate (RuMP) cycle, the serine pathway, and the Calvin-Benson-Bassham (CBB) cycle. The availability of genome sequences of methanotrophs enables postgenomic studies to investigate the regulation of methane oxidation in the laboratory and in the environment by natural methanotrophs and in laboratory or industrial conditions by platform organisms. Recent studies have included synthetic biology approaches and in future may incorporate the design of new pathways