Fig. 1. Principal Components of the Folate Biochemical Cycle. Abbreviations: DHFR = dihydrofolate reductase; MTHFR = methylenetetrahydrofolate reductase. Reactions: Biosynthesis of nucleotides for incorporation into DNA and RNA; Remethylation of homocysteine to form methionine (vitamin B₁₂ serves as a coenzyme in this reaction); Methylation of substrates, including DNA, RNA, phospholipids, and proteins; MTHFR, which catalyzes the formation of 5-methyltetrahydrofolate needed for methylation reactions; Dihydrofolate reductase enzyme.