Figure 1.

Synthetic overview of C1 metabolism in plant cells. The synthesis of THF is shown on a gray background. The enzymes that are specifically inhibited by sulfanilamide and MTX are dihydropteroate synthase (1) and dihydrofolate reductase (2), respectively. The enzymes that utilize folate coenzymes are Ser hydroxymethyltransferase (3), Gly decarboxylase (4), and Met synthase (5). CGS (6) and cystathionine β-lyase (7) are involved in de novo synthesis of Met. AdoHcy hydrolase (8) catalyzes the reversible conversion of AdoHcy to Hcy and adenosine. Thr synthase (9) competes with CGS for the common substrate OPH to synthesize Thr. C1 units transported by THF are 10-formyl (10-CHO), 5-formyl (5-CHO), 5,10-methenyl (5,10-CH⁺), 5,10-methylene (5,10-CH₂), and 5-methyl (5-CH₃). cysta, Cystathionine.