Dihydrofolate reductase and folic acid. Computer model showing the structure of the active site of a bacterial dihydrofolate reductase molecule (white ribbons) bound to a folic acid molecule (spheres). Dihydrofolate reductase is an enzyme that converts folic acid into a coenzyme. Enzymes and coenzymes control chemical reactions inside living cells. This enzyme is from the human gut bacterium Escherichia coli (E. coli).