Domain structure and function of 10-formyltetrahydrofolate dehydrogenase.

10-Formyltetrahydrofolate dehydrogenase catalyzes the NADP(+)-dependent oxidation of 10-formyltetrahydrofolate to CO2 and tetrahydrofolate. Previous studies have shown that the enzyme binds the physiological pentaglutamate form of tetrahydrofolate product so tightly that it remains bound during size exclusion chromatography (Cook, R. J., and Wagner, C. (1982) Biochemistry 21, 4427-4434). In addition to the dehydrogenase activity, the enzyme from rat liver has been reported to exhibit both 10-formyltetrahydrofolate hydrolase and aldehyde dehydrogenase activities (Cook, R. J., Lloyd, R. S., and Wagner, C. (1991) J. Biol. Chem. 266, 4965-4973). We have purified the enzyme from rabbit liver and found that it catalyzes the same three reactions with similar kinetic constants and that it is a 99-kDa homotetramer, as reported previously for the rat and pig enzymes. Previous studies have suggested that the enzyme is composed of three domains and has separate folate binding sites for the dehydrogenase and hydrolase activities. We have investigated the domain structure of the rabbit enzyme. Differential scanning calorimetry reveals two thermal transitions, indicating the presence of two independently folded domains. The pentaglutamate form of tetrahydrofolate and NADP+ each stabilize one of the thermal transitions, showing that these ligands bind to separate domains. Limited proteolytic digestions by several proteases cleave the enzyme in a linker region between the two domains. After proteolytic cleavage, the domains no longer remain associated and do not catalyze the 10-formyltetrahydrofolate dehydrogenase reaction. Isolation and characterization of the intact domains revealed that the N-terminal domain only catalyzes the NADP(+)-independent 10-formyltetrahydrofolate hydrolase activity and the C-terminal domain only catalyzes the NADP(+)-dependent aldehyde dehydrogenase activity. The kinetic constants of these isolated domains are similar to those of the intact enzyme. Binding studies on the native enzyme using fluorescence and isothermal titration calorimetry indicated that the enzyme binds one molecule of tetrahydrofolate and two molecules of NADP+ per tetramer. Dissociation constants for both ligands were also determined by these methods.