Methenyltetrahydrofolate cyclohydrolase is rate limiting for the enzymatic conversion of 10-formyltetrahydrofolate to 5,10-methylenetetrahydrofolate in bifunctional dehydrogenase-cyclohydrolase enzymes.

The kinetic properties of three methylenetetrahydrofolate dehydrogenase-cyclohydrolase (D/C) enzymes (the NADP-dependent bifunctional domain of the human cytoplasmic trifunctional enzyme, the human mitochondrial NAD-dependent bifunctional enzyme, and the NAD(P)-dependent bifunctional enzyme from Photobacterium phosphoreum) were determined in both forward and reverse directions. In the forward direction, the enzymes possess widely different ratios of kcat C/Kcat D, but all channel methenylH4folate produced by the D activity to the C activity with approximately the same efficiency. A deuterium isotope effect is observed with the human NADP-dependent enzyme in both forward and reverse dehydrogenase assays, consistent with hydride transfer being rate limiting for the interconversion of methenyl- and methyleneH4folate. However, no kinetic isotope effect is observed for the overall reverse reaction (formylH4folate to methyleneH4folate). We devised an assay to measure the reverse cyclohydrolase activity independent of the dehydrogenase, and determined that the Kcat (overall reverse) for each enzyme is approximately equal to the Kcat for its reverse cyclohydrolase activity. Therefore, the rate-limiting step in the overall reverse reaction is not hydride transfer by the dehydrogenase, but the production of methenylH4folate catalyzed by the cyclohydrolase. The reverse cyclohydrolase activities of the NADP-dependent D/C and the P. phosphoreum enzymes, but not the mitochondrial NAD-dependent enzyme, can be stimulated 2-fold by the addition of 2',5'-ADP. The results suggest that the cyclohydrolases of the human NADP dependent and P. phosphoreum enzymes are optimized to catalyze the reverse reaction in the presence of bound coenzyme. These results imply that essentially all of the methenylH4folate produced by the cyclohydrolase in the reverse reaction is channeled to the dehydrogenase.