associated dehydrogenases, NADH is produced. Finally, formate is oxidized to CO2 by cytoplasmic or membrane-bound Formate dehydrogenase (FDH), producing NADH and CO2. Anabolism The main metabolic challenge for methylotrophs is the assimilation of single carbon units into biomass. Through de novo synthesis, methylotrophs must form carbon-carbon bonds between 1-Carbon (C1) molecules. This is an energy intensive process, which facultative methylotrophs avoid by using a range of larger organic compounds. However, obligate methylotrophs must assimilate C1 molecules. There are four distinct assimilation pathways with the common theme of generating one C3 molecule. Bacteria use three of these pathways while Fungi use one. All four pathways incorporate 3 C1 molecules into multi-carbon intermediates, then cleave one intermediate into a new C3 molecule. The remaining intermediates are rearranged to regenerate the original multi-carbon intermediates. Bacteria Each species of methylotrophic bacteria has a single dominant assimilation pathway. The three characterized pathways for carbon assimilation are the ribulose monophosphate (RuMP) and serine pathways of formaldehyde assimilation as well as the ribulose bisphosphate (RuBP) pathway of assimilation. Ribulose bisphosphate (RuBP) cycle Unlike the other assimilatory pathways, bacteria using the RuBP pathway derive all of their organic carbon from CO2 assimilation. This pathway was first elucidated in photosynthetic autotrophs and is better known as the Calvin Cycle. Shortly thereafter, methylotrophic bacteria who could grow on reduced C1 compounds were found using this pathway. First, 3 molecules of ribulose 5-phosphate are phosphorylated to ribulose 1,5-bisphosphate (RuBP). The enzyme ribulose bisphosphate carboxylase (RuBisCO) carboxylates these RuBP molecules which produces 6 molecules of 3-phosphoglycerate (PGA). The enzyme phosphoglycerate kinase phosphorylates PGA into 1,3-diphosphoglycerate (DPGA). Reduction of 6 DPGA by the enzyme glyceraldehyde phosphate dehydrogenase generates 6 molecules of the C3 compound glyceraldehyde-3-phosphate (GAP). One GAP molecule is diverted towards biomass while the other 5 molecules regenerate the 3 molecules of ribulose 5-phosphate. Ribulose monophosphate (RuMP) cycle A new pathway was suspected when RuBisCO was not found in the methanotroph Methylmonas methanica. Through radio-labelling experiments, it was shown that M. methanica used the Ribulose monophate (RuMP) pathway. This has led researchers to propose that the RuMP cycle may have preceded the RuBP cycle. Like the RuBP cycle, this cycle begins with 3 molecules of ribulose-5-phosphate. However, instead of phosphorylating ribulose-5-phosphate, 3 molecules of formaldehyde form a C-C bond through an aldol condensation, producing 3 C6 molecules of 3-hexulose 6-phosphate (hexulose phosphate). One of these molecules of hexulose phosphate is converted into