Q.77 Which one of the following is TRUE about siderophores in bacteria?
(A) Siderophores are secreted only when soluble iron is available in the medium
(B) Siderophores form complex with ferrous ions in the medium
(C) Siderophores are the only route of iron uptake
(D) Siderophores form complex with ferric ions in the medium
Siderophores in bacteria form complexes with ferric ions (Fe³⁺) to scavenge iron under low-availability conditions.
The correct answer is (D).
Siderophore Function Details
Siderophores are low-molecular-weight chelators secreted by bacteria when iron is scarce. They have extremely high affinity for Fe³⁺ (K_d ~10⁻²²–10⁻⁵² M), forming stable hexadentate complexes via catechol, hydroxamate, or carboxylate groups. The ferri-siderophore is then recognized by outer membrane receptors (e.g., FepA in E. coli) and transported via TonB-dependent systems.
Option Breakdown
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(A) Secreted only when soluble iron available: False—opposite true. Iron-regulated via Fur repressor; apo-siderophores produced under Fe-limitation (nutritional immunity).
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(B) Complex with ferrous ions (Fe²⁺): Wrong—siderophores bind Fe³⁺ specifically; Fe²⁺ enters via FeoB or via reduction post-uptake.
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(C) Only route of iron uptake: False—bacteria use heme uptake, ferrous transporters (Feo), or xenosiderophores too.
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(D) Complex with ferric ions: Correct—definitional property; e.g., enterobactin chelates Fe³⁺ for pathogenicity.
Introduction to Siderophores Bacteria Ferric Ions
Siderophores bacteria ferric ions chelation enables iron scavenging during infection—vital GATE Life Sciences microbiology. Secreted under Fe-starvation, bind Fe³⁺ (not Fe²⁺), transported via TonB. Q.77 tests specificity over common misconceptions.
Mechanism Highlights
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Induction: Fur-Fe²⁺ represses; low Fe → apo-siderophore secretion.
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Binding: Fe³⁺ > 10³⁰-fold preference vs. Fe²⁺/other metals.
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Uptake: Ferri-siderophore → periplasm → reduction → Fe²⁺ release.
Option Comparison Table
| Option | Status | Reason |
|---|---|---|
| (A) With soluble Fe | ❌ False | Secreted when Fe ABSENT |
| (B) Ferrous Fe²⁺ | ❌ Wrong | Bind Fe³⁺ only |
| (C) Sole route | ❌ False | Multiple systems exist |
| (D) Ferric Fe³⁺ | ✅ Correct | High-affinity chelation |
GATE Prep Tip
Siderophores bacteria ferric ions = Fe³⁺ chelation under limitation. Distinguish Fur regulation, Fe³⁺ vs Fe²⁺, multiple uptake routes for PYQs. Answer: (D).
