Q.35
Valinomycin, a cyclic peptide antibiotic, facilitates the transport of which one
of the following ions?
Options:
(A) K+
(B) Ca2+
(C) Na+
(D) H+
Valinomycin, a 12-membered cyclic depsipeptide antibiotic, functions as the classic potassium-specific ionophore, disrupting bacterial membrane potential (Δψ) by shuttling K⁺ across lipid bilayers. Its selectivity (K⁺ >> Na⁺) arises from perfect cavity size matching K⁺ ionic radius (1.33 Å).
The correct answer is (A) K+, forming a stable 1:1 valinomycin-K⁺ complex that diffuses through hydrophobic membrane cores, collapsing electrochemical gradients essential for ATP synthesis.
Why (A) K+ is Valinomycin’s Specific Target
Six carbonyl oxygens from valinomycin’s depsipeptide backbone coordinate K⁺ in a rigid bracelet-like structure. Hydrophobic exterior (valine, lactate side chains) enables membrane solubility. Complex stability: K⁺ (log K = 6.5) >> Na⁺ (log K = 1.5), preventing non-specific transport.
Explanation of All Options
Ionophore selectivity determined by cavity size and coordination chemistry:
-
(A) K+
Correct. Perfect fit (1.33 Å ionic radius). Six O atoms form octahedral complex. Used experimentally to clamp K⁺ gradients. -
(B) Ca2+
Wrong. Too large (0.99 Å, high charge density). Calcimycin (A23187) is Ca²⁺ ionophore; valinomycin rejects divalent cations. -
(C) Na+
Incorrect. Too small (0.95 Å). Loose complex destabilizes; selectivity ratio K⁺/Na⁺ ≈ 10,000:1. -
(D) H+
No. Proton transport via uncouplers (CCCP, FCCP). Valinomycin neutral, can’t solvate H⁺ or carry charge across membrane.
Quick Ionophore Selectivity Table
| Option | Ion | Ionic Radius | Coordination Fit | Transported by Valinomycin? |
|---|---|---|---|---|
| A | K+ | 1.33 Å | Perfect | Yes |
| B | Ca2+ | 0.99 Å | Too small, +2 charge | No |
| C | Na+ | 0.95 Å | Too loose | No |
| D | H+ | ~0.87 Å | Proton uncoupler needed | No |
Biotech relevance: Valinomycin used in K⁺ electrode sensors, mitochondrial studies, patch-clamp electrophysiology. Disrupts bacterial PMF, killing Gram-positive bacteria. Memory trick: “Valinomycin = Very Accurate for Large K⁺.” Cavity visual: bracelet clasps single K⁺ ion perfectly.
