Dihydroneopterin aldolase (DHNA) and 7,8-dihydroneopterin triphosphate epimerase domain (DHNTPE); these enzymes have been designated folB and folX, respectively. Folate derivatives are essential cofactors in the biosynthesis of purines, pyrimidines, and amino acids, as well as formyl-tRNA. Mammalian cells are able to utilize pre-formed folates after uptake by a carrier-mediated active transport system. Most microbes and plants lack this system and must synthesize folates de novo from guanosine triphosphate. One enzyme from this pathway is DHNA which catalyses the conversion of 7,8-dihydroneopterin to 6-hydroxymethyl-7,8-dihydropterin in the biosynthetic pathway of tetrahydrofolate. Though it is known that DHNTPE catalyzes the epimerization of dihydroneopterin triphosphate to dihydromonapterin triphosphate, the biological role of this enzyme is still unclear. It is hypothesized that it is not an essential protein since a folX knockout in E. coli has a normal phenotype and the fact that folX is not present in H. influenza. In addition both enzymes have been shown to be able to compensate for the other's activity albeit at slower reaction rates. The functional enzyme for both is an octamer of identical subunits. Mammals lack many of the enzymes in the folate pathway including, DHNA and DHNTPE.
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