Final answer:
Sulfonamides act as competitive inhibitors, blocking the bacterial enzyme responsible for folic acid synthesis, and are selectively toxic against bacteria because humans do not synthesize folic acid.
Step-by-step explanation:
Sulfonamides function as competitive inhibitors. These antibiotics are structural analogs of para-aminobenzoic acid (PABA), allowing them to competitively inhibit the enzyme sulfihydropteroate synthase. This enzyme is crucial in the bacterial synthesis of dihydrofolic acid, which is the precursor of folic acid, needed for DNA, RNA, and protein synthesis. By blocking the enzymatic pathway necessary for folic acid synthesis, sulfonamides exert a bacteriostatic effect on a wide range of bacteria.
Humans are not affected by sulfonamides in the way bacteria are, because humans obtain folic acid through their diet rather than synthesizing it intracellularly. This is why sulfonamides are selectively toxic for bacteria and can be used as an antibiotic treatment, although allergic reactions to sulfa drugs are common.