Key enzyme in folate metabolism. Catalyzes an essential reaction for de novo glycine and purine synthesis, and for DNA precursor synthesis. (6S)-5,6,7,8-tetrahydrofolate + NADP(+) = 7,8-dihydrofolate + H(+) + NADPH Maximum activity at KCl concentration of 0.5 M and activity decreases with increasing concentration of KCl. Optimum pH is 6.0. Cofactor biosynthesis; tetrahydrofolate biosynthesis; 5,6,7,8-tetrahydrofolate from 7,8-dihydrofolate: step 1/1. Thymidine auxotrophy. Belongs to the dihydrofolate reductase family.