Cofactor-dependent pathways of formaldehyde oxidation in methylotrophic bacteria
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Methylotrophic bacteria can grow on a number of substrates as energy source with only one carbon atom, such as methanol, methane, methylamine, and dichloromethane. These compounds are metabolized via the cytotoxin formaldehyde. The formaldehyde consumption pathways, especially the pathways for the oxidation of formaldehyde to CO2 for energy metabolism, are a central and critical part of the metabolism of these aerobic bacteria. Principally, two main types of pathways for the conversion of formaldehyde to CO2 have been described: (1) a cyclic pathway initiated by the condensation of formaldehyde with ribulose monophosphate, and (2) distinct linear pathways that involve a dye-linked formaldehyde dehydrogenase or C1 unit conversion bound to the cofactors tetrahydrofolate (H4F), tetrahydromethanopterin (H4MPT), glutathione (GSH), or mycothiol (MySH). The pathways involving the four cofactors have in common the following sequence of events: the spontaneous or enzyme-catalyzed condensation of formaldehyde and the respective C1 carrier, the oxidation of the cofactor-bound C1 unit and its conversion to formate, and the oxidation of formate to CO2. However, the H4MPT pathway is more complex and involves intermediates that were previously known solely from the energy metabolism of methanogenic archaea. The occurrence of the different formaldehyde oxidation pathways is not uniform among different methylotrophic bacteria. The pathways are in part also used by other organisms to provide C1 units for biosynthetic reactions (e.g., H4F-dependent enzymes) or detoxification of formaldehyde (e.g., GSH-dependent enzymes).
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