Some characteristics of the dihydrofolate synthetase from Serratia indica

Dihydrofolate synthetase (EC 6.3.2.12) from Serratia indica IFO 3759 require a divalent cation and a univalent cation for its activity. The divalent cation requirement was satisified by magnesium ion, manganese ion or ferrous ion. High activity was obtained with 5 mM of magnesium ion. The effect of manganese ion was weak. The univalent cation requirement was satisfied by potassium ion, ammonium ion or rubidium ion, and high activity was obtained with 100 mM of each univalent cation. Increase in the potassium concentration lowered Km values for dihydropteroate and L-glutamate, and raised V max for ATP and dihydropteroate. Potassium ion had little effect on Km value for ATP. These results suggest that potassium ion may function on the affinities of dihydropteroate and L-glutamate to the enzyme. Dihydrofolate synthetase was inhibited by the addition of reduced forms of homopteroic acid. Stronger inhibition was observed by dihydrohomopteroate than by tetrahydrohomopteroate.