Major methylenetetrahydrofolate reductase required to generate the methyl groups necessary for methionine synthetase to convert homocysteine to methionine. Performs 15 to 20 percent of the total methylenetetrahydrofolate reductase activity of the cells. (6S)-5-methyl-5,6,7,8-tetrahydrofolate + NADP(+) = (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + H(+) + NADPH (6S)-5-methyl-5,6,7,8-tetrahydrofolate + NAD(+) = (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + H(+) + NADH One-carbon metabolism; tetrahydrofolate interconversion. Leads to partial auxotrophy for methionine. Belongs to the methylenetetrahydrofolate reductase family.