18 - Antimicrobial Peptides Flashcards
General structure of AMPs
- signal peptide
- propeptide
- mature peptide
Signal peptide
- conserved
- prompts cell to translocate/ secrete peptide
Propeptide
- conserved (whole/partially)
- anionic
Mature peptide
- variable
- cationic
- becomes active following cleavage
What are the structures of conventional peptide
Post-translational modifications of AMPs important
1) cysteine-rich, disulphide bonds e.g., defensin (1- disulphide bond eg bactenecin, 2- disulphide bonds eg protegrin, 3- disulphide bonds eg defensin)
2) α-helical eg cecropin
3) Extended structure, peptide-enriched e.g. indolicidin
4) Loop structure e.g. θ-defensin
What are the structures of ‘unconventional’/antibacterial fragments of larger proteins?
- histones
- e.g., lactoferrin - kaliocin, lactoerricin, lactoferrampin
- apolipoproteins, neuropeptides, pore forming toxins, fatty acids, respiratory/other pigments, lectins
Features of defensives in mammalian AMPs
- Small, 3-6 kDa peptides
- Active against Gram + & Gram – bacteria (some variation with individual peptides)
- Structural motif: cysteines (6-8 residues), disulphide bonds (3-4) Arginine-rich
- Bonds essential for maintaining hydrophobic sheet
- Active against Gram +ve & Gram -ve bacteria, fungi, some enveloped viruses and eukaryotic cells
-Deficiencies associated with Crohn’s disease
What are the types of mammalian AMPs?
alpha, beta, and theta defensins, and mammalian cathelicidins
What is the structure of alpha-defensins?
- signal - 19aa
- propiece - 40-45aa
- active peptide - 30aa
What are some features of alpha-defensins?
- First reported 1960s from rabbit neutrophils
- 6, α-defensins found in humans
- 4 α-defensins isolated from azurophilic granules of neutrophils (HNP1-4) Also found in NK cells, B cells
- Enteric defensins (α defensins HNP5 & 6) found in Paneth cells, uro-genital epithelial cells
- Constitutive expression
Structure of beta-defensins (hBD)
- signal & propiece - 20-30 aa
- active peptide - 35-42aa
What are some features of beta defensins?
- Predominantly found in epithelial tissues
- Some β defensins have short alpha-helix
- hBD1 – epithelial cells of urinary, respiratory system; Constitutive
- hBD-2 & 3 also present in GI epithelia, psoriatic skin, upregulated
by LPS or cytokines - hBD4-6 discovered in human epididymis
Where have beta defensins been found?
- pig, rat, sheep, chimp, cow, goat, human, macaque, mouse
- highest proportion found in cows
What are some features of theta defensins
- 1999: circular θ mini-defensins identified in Rhesus monkeys
- Heterodimeric peptide, expressed in monkey leukocytes
- Broad spectrum antimicrobial activity, inhibits bacterial toxins, active against HIV
- ‘Human’ θ defensins synthesized – christened ‘retrocyclins’
- Of interest re development for topical microbicides: application to prevent HIV-1 and HSV infections
- Human ancestral genes: possible re-awakening?
Structure of theta defensins?
18 a-a, 3 disulphide bonds
Structure of mammalian cathelicidins
- three parts: signal (20-30aa), cathelin domain (100-120aa) and peptide (19-94aa)
- and the conserved prepare region and the variable active domain
Features of mammalian cathelicidins
- Primarily in mammals, ca. 40 members
- N-terminal highly conserved, identifies cathelicidins
- Expressed in bone marrow, stored in peroxidase-negative neutrophil granules, then exocytosed from activated cells
- Most are without antibacterial activity until C-terminal proteolytically removed
- One cathelicidin found in humans - LL-37 (α-helical)
How do AMPs work?
- clustering of cationic and hydrophobic domains
- AMPs may be expressed constitutively and stored before release at site or time of need
- in some cells/tissues, AMP expression induced by microorganisms
Types of interactions of AMPs
- hydrophobic interactions (weak) on plasma membrane of a multicellular animal
- electrostatic and hydrophobic interactions (strong) on bacterial cytoplasmic membrane
Name the models related to AMPs
- The Shai-Matsuzaki-Huang Model
- Barrel Stave Killing
- Toroidal Pore Killing
- Carpet Model
What does the Shai Matsuzaki Huang Model propose?
- it involves the association of AMPs with the membrane surface, parallel to it, with the hydrophilic face of the helix oriented towards the water phase
- a ccording to this model, membrane perturbation is caused by the strain produced in the outer layer of the membrane by peptide insertion
- when a threshold membrane-bound peptide/lipid ratio is reached, this strain is released by the formation of membrane defects
What does the Barrel Stave Killing Model propose?
- One of the first hypotheses put forward was the “barrel-stave” mechanism
- peptides insert into the membrane in a transbilayer orientation, and aggregate to form a pore, with their hydrophilic faces lining the water-filled lumen of the channel and their apolar residues pointing towards the membrane
- even though this model was proposed more than 30 years ago, a conclusive demonstration of its validity was obtained only for the peptaibol alamethicin
What does the Toroidal Pore Killing Mechanism propose?
- AMPs bind to lipids, at critical concentration, bacterial membrane is caused to curve inwards through the pore
- Some association of the AMP-pore components with the lipid head groups of the bacterial membrane retained (e.g. the fish AMP piscidin from the striped bass, Morone)
What does the Carpet Model propose?
- AMPs aggregate on bacterial membrane
- Disruption of bilayer occurs in a detergent-like fashion
- This leads to the formation of micelles (e.g. dermaseptin from the frog genus Phyllomedusa)
What does the Carpet Model propose?
- AMPs aggregate on bacterial membrane
- Disruption of bilayer occurs in a detergent-like fashion
- This leads to the formation of micelles (e.g. dermaseptin from the frog genus Phyllomedusa)
Summary of AMP modes of action
- AMPs can cause direct neutralisation with membrane disruption (i.e., carpet, barrel stave & toroidal pore models) and targeting internal structures
- AMPs can cause immunomodulation of PMNs & monocytes resulting in increased bacterial clearance and inflammation control
what anti-cancer activity do AMPs have?
- Not all AMPs kill cancer cells
- Examples of naturally-occurring anti-cancer AMPs include LL-37, HNP-1, lactoferricin, magainin-2
- Killing believed to be linked to different membrane properties
AMPs: cancer vs. normal
CANCER:
- net negative charge: phosphatidylserine (PS), O-glycosylated mucins
- greater membrane fluidity
- greater surface area, more microvilli
NORMAL:
- neutral: sphingomyelin, phosphatidylethanolamine (PE), phosphatidylcholine (PC)
- cholesterol - may interfere with AMP insertion into membrane
AMPs: cancer vs. normal
CANCER:
- net negative charge: phosphatidylserine (PS), O-glycosylated mucins
- greater membrane fluidity
- greater surface area, more microvilli
NORMAL:
- neutral: sphingomyelin, phosphatidylethanolamine (PE), phosphatidylcholine (PC)
- cholesterol - may interfere with AMP insertion into membrane
