Members of this protein family are 3-hexulose-6-phosphate synthase (HPS), or the HPS domain of a fusion protein. This enzyme is part of the ribulose monophosphate (RuMP) pathway, which in one direction removes the toxic metabolite formaldehyde by assimilation into fructose-6-phosphate. In the other direction, in species lacking a complete pentose phosphate pathway, the RuMP pathway yields ribulose-5-phosphate, necessary for nucleotide biosynthesis, at the cost of also yielding formaldehyde. These latter species tend usually have a formaldehyde-activating enzyme to attach formaldehyde to the C1 carrier tetrahydromethanopterin. In these species, the enzyme is viewed as a lyase rather than a synthase and is called D-arabino 3-hexulose 6-phosphate formaldehyde lyase. Note that there is some overlap in specificity with the Escherichia coli enzyme 3-keto-L-gulonate 6-phosphate decarboxylase.