<p>This family consists of formaldehyde-activating enzyme, or the corresponding domain of longer, bifunctional proteins. It links formaldehyde to the C1 carrier tetrahydromethanopterin (H4MPT), an analog of tetrahydrofolate, and is common among species with H4MPT [<cite idref="PUB00035396"/>]. The ribulose monophosphate (RuMP) pathway, which removes the toxic metabolite formaldehyde by assimilation, runs in the opposite direction in some species to produce ribulose 5-phosphate for nucleotide biosynthesis, leaving formaldehyde as an additional metabolite. In these species, formaldehyde activating enzyme may occur as a fusion protein with D-arabino 3-hexulose 6-phosphate formaldehyde lyase from the RuMP pathway.</p>