Methylotrophs are a diverse group of microorganisms that can use reduced one-carbon compounds, such as methanol or methane, as the carbon source for their growth; and multi-carbon compounds that contain no carbon-carbon bonds, such as dimethyl ether and dimethylamine. This group of microorganisms also includes those capable of assimilating reduced one-carbon compounds by way of carbon dioxide using the ribulose bisphosphate pathway. These organisms should not be confused with methanogens which on the contrary produce methane as a by-product from various one-carbon compounds such as carbon dioxide. Some methylotrophs can degrade the greenhouse gas methane, and in this case they are called methanotrophs. The abundance, purity, and low price of methanol compared to commonly used sugars make methylotrophs competent organisms for production of amino acids, vitamins, recombinant proteins, single-cell proteins, co-enzymes and cytochromes. Metabolism The key intermediate in methylotrophic metabolism is formaldehyde, which can be diverted to either assimilatory or dissimilatory pathways. Methylotrophs produce formaldehyde through oxidation of methanol and/or methane. Methane oxidation requires the enzyme methane monooxygenase (MMO). Methylotrophs with this enzyme are given the name methanotrophs. The oxidation of methane (or methanol) can be assimilatory or dissimilatory in nature (see figure). If dissimilatory, the formaldehyde intermediate is oxidized completely into CO2 to produce reductant and energy. If assimilatory, the formaldehyde intermediate is used to synthesize a 3-Carbon (C3) compound for the production of biomass. Many methylotrophs use multi-carbon compounds for anabolism, thus limiting their use of formaldehyde to dissimilatory processes, however methanotrophs are generally limited to only C1metabolism. Catabolism Methylotrophs use the electron transport chain to conserve energy produced from the oxidation of C1 compounds. An additional activation step is required in methanotrophic metabolism to allow degradation of chemically-stable methane. This oxidation to methanol is catalyzed by MMO, which incorporates one oxygen atom from O2 into methane and reduces the other oxygen atom to water, requiring two equivalents of reducing power. Methanol is then oxidized to formaldehyde through the action of methanol dehydrogenase (MDH) in bacteria, or a non-specific alcohol oxidase in yeast. Electrons from methanol oxidation are passed to a membrane-associated quinone of the electron transport chain to produce ATP. In dissimilatory processes, formaldehyde is completely oxidized to CO2 and excreted. Formaldehyde is oxidized to formate via the action of Formaldehyde dehydrogenase (FALDH), which provides electrons directly to a membrane associated quinone of the electron transport chain, usually cytochrome b or c. In the case of NAD+