Key enzyme in folate metabolism. Contributes to the de novo mitochondrial thymidylate biosynthesis pathway. 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 kcat is 35.9 sec(-1) for dihydrofolate. kcat is 130 sec(-1) for NADPH. Cofactor biosynthesis; tetrahydrofolate biosynthesis; 5,6,7,8-tetrahydrofolate from 7,8-dihydrofolate: step 1/1. Monomer. Belongs to the dihydrofolate reductase family.