Figure 2.

Pathways in the aerobic methanotrophic bacterium Methylococcus capsulatus involved in the metabolism of single-carbon compounds, as determined by genome sequencing and proteome analysis. Formaldehyde produced from methane can be metabolized in the following alternative ways: (a) through the ribulose monophosphate (RuMP) cycle, which can either generate biomass (via the assimilatory (A) RuMP cycle) or CO₂ (via the dissimilatory (D) RuMP cycle); (b) by conversion to formate via intermediates containing tetrahydromethanopterin (H₄MPT); (c) via methylene-tetrahydrofolate (methylene-H₄F) to the serine cycle and from there into biomass. Under certain conditions, there can be an excess of formaldehyde and formate; the former can be used up through pathway (c) and the latter by reduction to methylene-H₄F (d) and thus directed into the serine cycle. CO₂ produced in any of these reactions can be converted to biomass by either (e) the serine cycle or (f) the Calvin-Benson-Bassham (CBB) cycle.