MTHFS (methenyltetrahydrofolate synthetase) is an essential enzyme in folate-mediated one-carbon metabolism that catalyzes the irreversible, ATP-dependent conversion of 5-formyltetrahydrofolate (5-FTHF) to 5,10-methenyltetrahydrofolate 1. The enzyme functions in both cytoplasmic and mitochondrial compartments, with similar kinetic properties and molecular weights of approximately 25 kDa 2. MTHFS plays a critical regulatory role by preventing 5-FTHF accumulation, which would otherwise inhibit folate-dependent enzymes involved in purine biosynthesis 34. The enzyme is subject to feedback inhibition by 10-formylTHF, which exists in equilibrium with its product 5,10-methenyltetrahydrofolate, providing tight metabolic regulation 3. Mutations in MTHFS cause a rare neurodevelopmental disorder characterized by microcephaly, epilepsy, cerebral hypomyelination, and severe developmental delays 5. The enzyme's essential nature is demonstrated by its requirement for mouse viability and its role in maintaining folate homeostasis through the 5-FTHF futile cycle, which reduces metabolic stochasticity in folate metabolism 4. MTHFS also exhibits unexpected catecholamine-binding properties, suggesting additional regulatory mechanisms linking neurotransmitter metabolism to folate homeostasis 6.