葉酸
All of the biological functions of folic acid are performed by THF and its methylated derivatives. Hence folic acid must first be reduced to THF. This four electron reduction proceeds in two chemical steps both catalyzed by the same enzyme, dihydrofolate reductase. Folic acid is first reduced to dihydrofolate and then to tetrahydrofolate. Each step consumes one molecule of NADPH (biosynthetically derived from vitamin B3) and produces one molecule of NADP. Mechanistically, hydride is transferred from NADPH to the C6 position of the pteridine ring.