Figure 4.

Possible pathways for methane oxidation in verrucomicrobial methanotrophs. pMMO oxidises methane to methanol (CH₃OH), while an unknown electron donor is oxidised. The lanthanide-dependent XoxF methanol dehydrogenase (MDH) could subsequently oxidise methanol to either formate (HCOOH) or formaldehyde (CH₂O), while donating electrons to its redox partner XoxGJ. If formate is produced it could diffuse into the cytoplasm and be converted to CO₂ by the NAD⁺-dependent formate dehydrogenase (FDH). CO₂ is fixed into biomass via the Calvin-Benson-Bassham (CBB) cycle. Alternatively, if formaldehyde is produced, it could bind to tetrahydrofolate (H₄F) spontaneously or enzymatically by an unidentified enzyme, to form methylene-tetrahydrofolate (CH₂-H₄F). The enzyme FolD converts CH₂-H₄F to methenyl-tetrahydrofolate (CH-H₄F), which is subsequently converted to formyl-tetrahydrofolate (CHO-H₄F). This product is then converted to H₄F and formate, while producing ATP. P: periplasm; C: cytoplasm.