Dihydropteroate synthase action

The answer is c. (Hardman, pp 1058-1059. Katzung, pp 793-795.) Trimethoprim inhibits dihydro folic acid reductase. Sulfamethoxazole inhibits p-aminobenzoic acid (PABA) from being incorporated into folic acid by competitive inhibition of dihydropteroate synthase. Either action inhibits the synthesis of tetrahydrofolic acid. 

Mammals must obtain their tetrahydrofolate requirements from their diet, but microorganisms are able to synthesize this material. This offers scope for selective action and led to the use of sulfanilamide and other antibacterial sulfa drugs, compounds that competitively inhibit the biosynthetic enzyme (dihydropteroate synthase) that incorporates p-aminobenzoic acid into the structure (see Box 7.23). 

The classification system is describing not only the chemical family belonging to a specific mode of action but all compounds via their common names counted to each family, as shown in Table 1.2 for the modes of action such as Inhibition of DHP (dihydropteroate) synthase , Microtubule assembly inhibition , Inhibition of mitosis/microtubule organization , Inhibition of VLCFAs (Inhibition of cell division) and Inhibition of cell wall (cellulose) synthesis as examples (not mentioned in other chapters of this book) (for a detailed table see www. plantprotection.org/HRAC/). The scheme The World of Herbicides available under this internet address also shows all chemical structures of the different herbicides belonging to the different chemical families. 

---