A-Level Amines, amides and amino acids: worked solution

Question

Sulfonamides (sulfa drugs) are bacteriostatic: rather than killing bacteria, they simply prevent them from growing and multiplying. Their general structure is H2N-C6H4-SO2NHR, in which the R group may be replaced by a wide range of heterocyclic or aromatic groups. Altering R changes properties such as the solubility of the sulfonamide, or how strongly it binds to plasma proteins, but it does not change the way the drug acts. Sulfapyridine, the sulfonamide in which R is a 2-pyridyl group, is an effective antibacterial agent, though its water solubility depends strongly on pH. Outline how sulfapyridine works (its mode of action).

Sulfonamides are close structural analogues of p-aminobenzoic acid (PABA), which bacteria need to synthesise folic acid. Because the drug resembles PABA in shape and size, it competes with PABA for the active site of the bacterial enzyme (dihydropteroate synthase) that makes folate. By competitively inhibiting this enzyme the drug blocks folic acid synthesis, so the bacteria cannot make the nucleic acid precursors needed to grow and multiply (bacteriostatic).