The chemical structure of folic acid (Fig. 28.42) was found to be composed of a pteridine heterocyclic system (rings A and B), p-aminobenzoic acid (PABA) and Glu; hence the name pteroylglutamate (PteGlu). In plants and bacteria, folic acid is synthesized by converting guanosine triphosphate (GTP) through a multistep sequence into the pteridine heterocycle. This is followed by dihydropteroate synthase (EC 220.127.116.11), which adds the PABA moiety and finally the glutamate residue is added by dihydrofolate synthase (EC 18.104.22.168). It is of note that the antibacterial sulfonamides compete with PABA for the bacterial form of dihydropteroate synthase and, thereby, interfere with bacterial folic acid synthesis. This
Figure 28.41 • Biotin-mediated carboxylation reaction.
pteridine p-aminobenzoic acid glutamate
OH 5 HN 10
OH 5 HN 10
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