Antimicrobial Peptides
Learn about antimicrobial peptides — their structures, membrane-disruption mechanisms, role in innate immunity, and low potential for resistance development.
What Are Antimicrobial Peptides?
Antimicrobial peptides (AMPs) are short cationic and amphipathic peptides produced by virtually all multicellular organisms. They are a critical component of innate immunity, providing a rapid first line of defense against bacteria, fungi, and some viruses.
Major AMP Families
Defensins Defensins are cysteine-rich peptides (29-35 amino acids) stabilized by three disulfide bonds. They are classified as alpha-defensins (found in neutrophils and Paneth cells) and beta-defensins (produced by epithelial cells). Alpha-defensins constitute a major component of neutrophil granules.
Cathelicidins Cathelicidins are synthesized as preproproteins with a conserved cathelin prodomain. The active C-terminal peptide is released by proteolytic cleavage during infection or tissue damage. The only human cathelicidin is LL-37, a 37-residue peptide with broad-spectrum activity.
Magainins Magainins were first isolated from frog skin (Xenopus laevis). They are 21-27 amino acid peptides with net positive charge. Frog skin produces magainins in response to microbial colonization, suggesting a rapid deployable defense system.
Membrane Disruption Mechanism
AMPs are cationic, so they are electrostatically attracted to the negatively charged bacterial membrane (rich in phosphatidylglycerol and lipopolysaccharide). In contrast, mammalian membranes have a net neutral surface charge due to cholesterol and zwitterionic phospholipids.
Once bound, AMPs insert into the lipid bilayer and disrupt membrane integrity through several models:
- Barrel-stave model: Peptides line up to form a transmembrane pore
- Toroidal pore model: Peptides induce lipid headgroups to curve through the pore
- Carpet model: Peptides coat the membrane surface and solubilize lipids like a detergent
The result is leakage of cellular contents, loss of membrane potential, and cell death.
Role in Innate Immunity
AMPs are produced constitutively or induced rapidly upon infection. They operate in tissues where adaptive immunity is slow to respond — skin, gut epithelium, respiratory tract. Beyond direct killing, many AMPs also act as immunomodulators, recruiting immune cells and modulating inflammatory responses.
Resistance Potential
Unlike conventional antibiotics, AMPs target the physical structure of the lipid bilayer. Bacteria cannot easily mutate their membrane composition without compromising viability. This makes resistance development far less likely, although not impossible. Some bacteria modify their lipopolysaccharide or upregulate proteases to degrade AMPs.
Summary of AMPs
| AMP | Source | Organism | Key Feature |
|---|---|---|---|
| Human beta-defensin 1 | Epithelium | Human | Constitutive expression |
| Alpha-defensin (HNP1-4) | Neutrophils | Human | Stored in granules |
| LL-37 | Neutrophils, epithelium | Human | Only human cathelicidin |
| Magainin 2 | Skin | Frog | Broad-spectrum, non-hemolytic |
| Cecropin | Fat body | Insect | First AMP discovered |
| Nisin | Lactococcus lactis | Bacterium | Lantibiotic, used in food preservation |
Learning Tip
When memorizing AMP mechanisms, remember: cationic charge targets the membrane, amphipathicity enables insertion, and the lipid bilayer itself is the target. This is why resistance is hard — the target is a physical structure, not a single enzyme.