The biosynthesis of folic acid compounds in plants. V. Reaction mechanism of the dihydropteroate-synthesizing enzyme from pea seedlings.

Cell-free extracts of pea seedlings contain the enzyme system which catalyzes the formation of folate compounds from 2-amino-4-hydroxy-6-substituted pteridines and p-aminobenzoic acid or p-aminobenzoylglutamic acid. Active pteridines were 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine and its pyrophosphate ester, i.e., 2-amino-4-hydroxy-6-pyrophosphorylmethyldihydropteridine, and a considerable amount of folate compound was formed enzymatically from the latter compound in the presence of Mg2+ as a cofactor, whereas the less amount was formed from the former in the presence of both ATP and Mg2+. The products were characterized, by a bioautographic technique, as dihydropteroic acid and dihydrofolic acid from p-aminobenzoic acid and p-aminobenzoylglutamic acid, respectively, in the presence of pyrophosphorylmethyldihydropteridine. In the enzymatic reaction, p-aminobenzoic acid was more active as substrate than p-aminobenzoylglutamic acid. When p-aminobenzoic acid and L-glutamic acid were used in place of p-aminobenzoic acid in the enzyme system, dihydropteroic acid and dihydrofolic acid were formed in the presence of the pteridine, ATP and Mg2+. From these results presented, the following pathway for the biosynthesis of folate compounds in plants was proposed: