5,10-Methenyltetrahydrofolate synthetase catalyzes the irreversible conversion of 5-formyl-tetrahydropteroylpolyglutamates (5-CHO-HPteGlu) to 5,10-methenyltetrahydropteroylpolyglutamates (5, 10-CH-HPteGlu). The equilibrium of the nonenzymatic reaction, which equilibrates slowly in the absence of enzyme, greatly favors 5-CHO-HPteGlu. The enzyme couples the reaction to the hydrolysis of ATP shifting the equilibrium to favor 5,10-CH-HPteGlu. Substrate-dependent non-equilibrium isotope exchange of ADP into ATP was observed, suggesting the formation of a phosphorylated intermediate of 5-CHO-HPteGluduring the enzyme-catalyzed reaction. The competitive inhibitor 5-formyltetrahydrohomofolate also supported the ADP to ATP exchange, suggesting that this molecule could also form a phosphorylated intermediate. The initial rates of the ADP-ATP exchange with saturating ADP were about 70 s for both compounds, while the k values for product formation were 5 s for 5-CHO-HPteGlu and 0.005 s for 5-formyltetrahydrohomofolate. Starting with 5-[O]CHO-HPteGlu, it was shown by P NMR that the formyl oxygen of the substrate was transferred to the product phosphate during the reaction. This further supports the existence of a phosphorylated intermediate. The formyl group of 5-CHO-HPteGlu is known to be an equilibrium mixture of two rotamers. Stopped-flow analysis of the enzymatic reaction showed that only one of the rotamers serves as a substrate for the enzyme.

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