Methylotrophy

Methylotrophs are organisms that can use reduced C1 compounds such as methane, methanol, and methylamine as a source of carbon and energy. This is an oxidative form of growth that usually has methanol, formaldehyde and formate as intermediates in Figure1. Methylotrophs are ubiquitous in our environment but specialists such as the methanotrophs are found in the aerobic parts of marshes or hot springs near where methanogenesis takes place.

Compounds such as methane are first oxidized at the expense of NADH or ATP to methanol, and the energy is recovered through subsequent oxidations all the way to carbon dioxide and water. Although organisms such as Paracoccus can use the CO2 generated this way for autotrophic assimilation of carbon via the Calvin cycle nonautotrophs such as Methylobacterium extorquens can assimilate carbon at the level of formaldehyde. These heterotrophic methylotrophs must balance the dissimilatory energy generation steps with assimilation. The Heterotrophs use complex pathways to transfer the methyl group in formaldehyde onto molecules compatible with central anabolic pathways.

                 Figure  1.  Microbial  transformations during growth on C1 compounds. RuMP, ribulose monophosphate.

The key enzyme of methylotrophy methanol dehydrogenase oxidizes methanol to form- aldehyde using an unusual prosthetic group. Pyrroloquinoline quinone interacts with the electron transport system during its reduction and concomitant production of formaldehyde. The Formaldehyde is toxic to most organisms and it is unusual to see it as a cytoplasmic intermediate. Methylotrophs keep formaldehyde levels very low using a tetrahydromethanopterin transferase to move the aldehyde group to a less toxic compound for further metabolism.