The 5-formyltetrahydrofolate futile cycle reduces pathway stochasticity in an extended hybrid-stochastic model of folate-mediated one-carbon metabolism

In folate-mediated one-carbon metabolism (FOCM), 5-formyltetrahydrofolate (5fTHF), a one-carbon substituted tetrahydrofolate (THF) vitamer, acts as an intracellular storage form of folate and as an inhibitor of the folate-dependent enzymes phosphoribosylaminoimidazolecarboxamide formyltransferase (AICARFT) and serine hydroxymethyltransferase (SHMT). Cellular levels of 5fTHF are regulated by a futile cycle comprising the enzymes SHMT and 5,10-methenyltetrahydrofolate synthetase (MTHFS). MTHFS is an essential gene in mice; however, the roles of both 5fTHF and MTHFS in mammalian FOCM remain to be fully elucidated. We present an extension of our previously published hybrid-stochastic model of FOCM by including the 5fTHF futile-cycle to explore its effect on the FOCM network. Model simulations indicate that MTHFS plays an essential role in preventing 5fTHF accumulation, which consequently averts inhibition of all other reactions in the metabolic network. Moreover, in silico experiments show that 10-formylTHF inhibition of MTHFS is critical for regulating purine synthesis. Model simulations also provide evidence that 5-methylTHF (and not 5fTHF) is the predominant physiological binder/inhibitor of SHMT. Finally, the model simulations indicate that the 5fTHF futile cycle dampens the stochastic noise in FOCM that results from both folate deficiency and a common variant in the methylenetetrahydrofolate reductase (MTHFR) gene.