Q.34 Which is not the antimicrobial peptide?
Immunoglobulin is Not an Antimicrobial Peptide
Immunoglobulins (antibodies) are large glycoproteins (~150 kDa) mediating adaptive immunity, not the small cationic peptides (10-50 amino acids) defining antimicrobial peptides (AMPs). This makes option (3) correct.
Introduction
Antimicrobial peptides represent the innate immune system’s first line defense—small, amphipathic peptides disrupting microbial membranes. Immunoglobulins belong to adaptive immunity via B-cell receptors. This distinction is crucial for immunology exam preparation.
Option Analysis
AMPs criteria: 10-50 aa, +2 to +9 net charge, 30-60% hydrophobic, membrane-lytic via barrel-stave/toroidal pore formation.
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(1) Cathelicidin: AMP. LL-37 (human): 37-residue α-helical peptide from hCAP18 precursor. Salt-sensitive, synergizes with defensins, vitamin D-induced.
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(2) Defensin: AMP. α-defensins (HNP1-4, cryptdins), β-defensins (hBD1-4), θ-defensins (circular). 6-cysteine disulfide motifs form β-sheet amphipathicity.
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(3) Immunoglobulin: Not AMP. Quaternary glycoprotein (2 heavy + 2 light chains); Fab binds specific antigens, Fc triggers complement/phagocytosis. Adaptive, not membrane-lytic.
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(4) Histatin: AMP. Salivary histidine-rich peptides (12-24 aa); candidacidal via intracellular ROS, transcends membranes. Non-cysteine, α-helical.
AMP Classification
| Family | Structure | Location | Targets |
|---|---|---|---|
| Cathelicidins | α-helix | Skin, neutrophils | Bacteria, fungi |
| Defensins | β-sheet (disulfides) | Epithelia, granules | Gram+/-, viruses |
| Histatins | α-helix (His-rich) | Saliva | Candida, S. mutans |
| Immunoglobulins | Glycoprotein | Serum, mucosa | Specific antigens |
Mechanism Distinction
AMPs: Electrostatic → Membrane insertion → Pore formation → Cell lysis
Ig: Antigen binding → Opsonization → Phagocytosis/complement
Exam trap: Both combat microbes, but AMPs = innate/direct vs. Ig = adaptive/specific.
Biological Roles
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AMPs: Bridge innate-adaptive (chemoattractants), modulate inflammation
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Ig deficiency: Recurrent infections ( Bruton’s, SCID)
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AMPs mutated: Susceptibility to skin/lung infections (psoriasis, cystic fibrosis)
Exam Relevance
GATE Life Sciences: “IMMUNOglobulins = IMMUNOlogical specificity; AMPs = ANTIMicrobial Peptides.” Visualize: AMPs = tiny swords piercing membranes; Ig = huge nets tagging specific targets.
