Regulation of de novo purine biosynthesis by methenyltetrahydrofolate synthetase in neuroblastoma.
5-Formyltetrahydrofolate (5-formylTHF) is the only folate derivative that does not serve as a cofactor in folate-dependent one-carbon metabolism. Two metabolic roles have been ascribed to this folate derivative. It has been proposed to 1) serve as a storage form of folate because it is chemically stable and accumulates in seeds and spores and 2) regulate folate-dependent one-carbon metabolism by inhibiting folate-dependent enzymes, specifically targeting folate-dependent de novo purine biosynthesis. Methenyltetrahydrofolate synthetase (MTHFS) is the only enzyme that metabolizes 5-formylTHF and catalyzes its ATP-dependent conversion to 5,10-methenylTHF. This reaction determines intracellular 5-formylTHF concentrations and converts 5-formylTHF into an enzyme cofactor. The regulation and metabolic role of MTHFS in one-carbon metabolism was investigated in vitro and in human neuroblastoma cells. Steady-state kinetic studies revealed that 10-formylTHF, which exists in chemical equilibrium with 5,10-methenylTHF, acts as a tight binding inhibitor of mouse MTHFS. [6R]-10-formylTHF inhibited MTHFS with a K(i) of 150 nM, and [6R,S]-10-formylTHF triglutamate inhibited MTHFS with a K(i) of 30 nm. MTHFS is the first identified 10-formylTHF tight-binding protein. Isotope tracer studies in neuroblastoma demonstrate that MTHFS enhances de novo purine biosynthesis, indicating that MTHFS-bound 10-formylTHF facilitates de novo purine biosynthesis. Feedback metabolic regulation of MTHFS by 10-formylTHF indicates that 5-formylTHF can only accumulate in the presence of 10-formylTHF, providing the first evidence that 5-formylTHF is a storage form of excess formylated folates in mammalian cells. The sequestration of 10-formylTHF by MTHFS may explain why de novo purine biosynthesis is protected from common disruptions in the folate-dependent one-carbon network.
Field
Martha S
MS
Division of Nutritional Sciences, Graduate Field of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853, USA.
Szebenyi
Doletha M E
DM
Stover
Patrick J
PJ
D000818:Animals; D001665:Binding Sites; D019731:Carbon-Nitrogen Ligases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000502:physiology*; D045744:Cell Line, Tumor; D006801:Humans; D051379:Mice; D008958:Models, Molecular; D009447:Neuroblastoma / Q000201:enzymology*; D011687:Purines / Q000096:biosynthesis*; D013379:Substrate Specificity
D011687:Purines; D019731:Carbon-Nitrogen Ligases; C021557:5,10-methenyltetrahydrofolate synthetase; C030985:purine