FIG. 2.

One-carbon unit metabolism for provision of methyl donors in cells. Folate, histidine, methionine, glycine and serine participate in the transfer of one-carbon units for the synthesis of purines and DNA as well as methylation reactions in animals. The enzymes that catalyzed the indicated reactions are: 1) folate reductase; 2) N⁵-N¹⁰-methylene H₄-folate dehydrogenase (a trifunctional enzyme possessing N¹⁰-formyl H₄-folate synthetase, N⁵-N¹⁰-methylene H₄-folate dehydrogenase, and N⁵-N¹⁰-methenyl H₄-folate cyclohydrolase activities); 3) N¹⁰-formyl H₄-folate dehydrogenase; 4) serine hydroxymethyl transferase; 5) N⁵-N¹⁰-methylene H₄-folate reductase; 6) methionine synthase; 7) S-adenosylmethionine synthase; 8) S-adenosylmethionine as a major methyl group donor in methyltransferase reactions; 9) formiminotransferase-cyclodeaminase (a bifunctional enzyme possessing glutamate formiminotransferase and formimidoyl H₄-folate cyclodeaminase activities); 10) serine hydroxymethyl transferase, N⁵-N¹⁰-methenyl H₄-folate cyclohydrolase, and spontaneous reaction at pH 4 to 7.0; 11) N⁵-N¹⁰-methenyl H₄-folate synthetase; 12) a sequential series of enzymes: histidase, urocanase, and imidazolone propionate hydrolase; 13) thymidylate synthase; 14) formyltransferase; 15) an oxidant for cells; 16) betaine:homocysteine methyltransferase; 17) choline dehydrogenase; and 18) S-adenosylhomocysteine hydrolase. dTMP, deoxythymidine 5'-monophosphate; dUMP, deoxyuridine 5'-monophosphate; Gly, glycine; H₄-folate, tetrahydrofolate; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine; Ser, serine; Vit, vitamin.