Q.76 Folic acid synthesis in bacteria is competitively inhibited by sulfonamides. Which of the
following essential components is replaced by sulfonamides during the process of folic
acid synthesis?
(A) Pteridine                            (B) Glutamic acid
(C) Para–amino benzoic acid (D) Sulfamethaxazole

Sulfonamides competitively inhibit folic acid synthesis in bacteria by mimicking para-aminobenzoic acid (PABA), blocking the enzyme dihydropteroate synthase. The correct answer is (C) Para-amino benzoic acid.​

Folic Acid Pathway Overview

Bacteria synthesize folic acid de novo using three key precursors: pteridine (from GTP), PABA, and glutamic acid. The critical step inhibited by sulfonamides occurs when dihydropteroate synthase (DHPS) condenses activated pteridine pyrophosphate (pterin-PP) with PABA to form dihydropteroate, which then adds glutamate tails to become tetrahydrofolate. Humans obtain folate from diet, making bacteria selectively vulnerable.​

Option Analysis

• (A) Pteridine: Forms the ring structure from GTP via enzymes like FolE; sulfonamides do not mimic or replace it.​

• (B) Glutamic acid: Added later by folylpolyglutamate synthase for solubility and function; not targeted by sulfonamides.​

• (C) Para-amino benzoic acid: Correct; sulfonamides structurally resemble PABA, competing for DHPS active site and preventing incorporation.​

• (D) Sulfamethoxazole: A specific sulfonamide drug (in co-trimoxazole), not a natural component replaced—it’s the inhibitor itself.​

Folic acid synthesis in bacteria sulfonamides PABA plays a pivotal role in antimicrobial action, especially for competitive exams like CSIR NET Life Sciences. Bacteria rely on de novo folic acid production for DNA/RNA synthesis, unlike humans who acquire it preformed.​

Bacterial Folic Acid Pathway

The pathway starts with GTP converting to 7,8-dihydropteroate pyrophosphate (pteridine moiety), which DHPS links to PABA amidated with glutamate to form dihydropteroate, then dihydrofolate via DHFR, and finally tetrahydrofolate. Sulfonamides target DHPS, halting this at the PABA step.​

Sulfonamides Mechanism

Sulfonamides, structural PABA analogs, bind DHPS competitively, forming non-functional dihydropteroate analogs and starving bacteria of folate for nucleotide synthesis—bacteriostatic effect. Excess PABA overcomes inhibition, confirming competition.​

Exam-Relevant Insights

• Pteridine unaffected upstream.​

• Glutamate added downstream.​

• Sulfamethoxazole exemplifies sulfonamides, not replaced component.​
  Ideal for CSIR NET questions on antibiotic mechanisms in microbiology/biochemistry.​