Q35. Which one of the following antibiotics can form an ion channel in the
bacterial membrane?
(A) Ampicillin
(B) Gramicidin A
(C) Gentamicin
(D) Rifampicin
Gramicidin A is the antibiotic that forms an ion channel in the bacterial membrane, making it the correct answer for this CSIR NET Life Sciences question. This peptide disrupts ion gradients by creating pores selective for monovalent cations like Na⁺ and K⁺.
Option Analysis
Ampicillin (A): This beta-lactam antibiotic inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), preventing peptidoglycan cross-linking and causing cell lysis. It does not interact with or form channels in the membrane.
Gramicidin A (B): Correct choice. This linear pentadecapeptide inserts into lipid bilayers as dimers, forming beta-helical ion channels that allow monovalent cation transport across bacterial membranes, leading to depolarization and cell death. Used against Gram-positive bacteria.
Gentamicin (C): An aminoglycoside that binds the 30S ribosomal subunit, causing mRNA misreading and inhibiting protein synthesis. It enters via porins but does not form membrane channels.
Rifampicin (D): Inhibits bacterial DNA-dependent RNA polymerase by binding its beta subunit, blocking RNA chain elongation. Targets transcription, not membrane function.
Mechanism Details
Gramicidin A self-assembles into channels within lipid bilayers, with tryptophan residues aiding membrane insertion and stability. These pores conduct ions at high rates (~10^7-10^9 s⁻¹), collapsing proton motive force essential for bacterial ATP synthesis.
Other options target intracellular processes: cell wall (Ampicillin), translation (Gentamicin), transcription (Rifampicin), none involving direct membrane permeation via channels.
Exam Relevance
For CSIR NET, recognize Gramicidin A as a classic ionophore antibiotic contrasting beta-lactams, aminoglycosides, and rifamycins. Differentiate membrane-disrupting agents from synthesis inhibitors.
