Intersections of B₁₂ and folate metabolism, the methionine cycle, folate cycle, and DNA synthesis showing the methyl folate “trap.” The key intersection of B₁₂ and folate occurs at the methionine synthase (MS) reaction in which the one-carbon methyl group of methyltetrahydrofolate (MethylTHF) is transferred to Hcy to form methionine. The cofactor for this reaction is B₁₂ in the form of methylcobalamin. The folate product tetrahydrofolate regains a one-carbon methylene group through the serine hydroxymethyltransferase (SHMT) reaction, and the resulting methylenetetrahydrofolate is essential for conversion of deoxyuridine to thymidine in the thymidylate synthase (TS) reaction. This reaction is rate limiting for DNA synthesis. In B₁₂ deficiency, folate becomes trapped as methylTHF. Administration of folic acid can temporarily overcome this block through dihydrofolate reductase (DHFR) reduction to tetrahydrofolate. The other product of the MS reaction, the essential amino acid methionine, after adenosylation to S-adenosyl-methionine (SAM), serves as a universal methyl donor in numerous methyltransferase reactions. The product, S-adenosyl-homocysteine (SAH), undergoes reversible hydrolysis by the enzyme adenosyl-homocysteine hydrolase (AHCY hydrolase), yielding Hcy and thus completing the methionine or remethylation cycle. Not shown in this figure is the alternative transsulfuration pathway for Hcy disposal, which requires vitamin B₆.⁸  ATP, adenosine triphosphate; DHFR, dihydrofolate reductase; H⁺, proton; MTHFR, methylene tetrahydrofolate reductase; NADP⁺, NAD phosphate; NADPH⁺, reduced NAD phosphate. Professional illustration by Patrick Lane, ScEYEnce Studios.