Molecular Characterization and Gene Expression Profiling ... - CUSAT

1.7.10.2 Attachment
Chapter 1
Once close to the microbial surface, peptides must traverse capsular
polysaccharides before they can interact with the outer membrane, which
contains LPS in gram-negative bacteria, and traverse capsular
polysaccharides, teichoic acids and lipoteichoic acids before they can interact
with the cytoplasmic membrane in gram-positive bacteria. This concept is
important but is rarely addressed in mechanistic studies. Once peptides have
gained access to the cytoplasmic membrane they can interact with lipid
bilayers. In vitro studies of AMPs incubated with single or mixed lipids in
membranes or vesicles show that peptides bind in two physically distinct
states (Huang, 2000). At low peptide/lipid ratios, α-helical peptides, β-sheet
peptides and defensins adsorb and embed into the lipid head group region
in a functionally inactive state (referred to as the surface or S state) that
stretches the membrane (Chen et al., 2003). The extent of membrane thinning
is specific to the peptide and directly proportional to the peptide
concentration.
1.7.10.3 Peptide insertion and membrane permeability
At low peptide/lipid ratios, peptides are bound parallel to a lipid
bilayer (Yang et al., 2001). As the peptide/lipid ratio increases, peptides
begin to orientate perpendicular to the membrane. At high peptide/lipid
ratios, peptide molecules are orientated perpendicularly and insert into the
bilayer, forming transmembrane pores (referred to as the I state). The I state
peptide/lipid ratio varies with both the peptide and target lipid composition
(Lee et al., 2004), and a number of models have been proposed to explain
membrane permeabilization.
Barrel-stave model
In the ‘barrel-stave model’ (Fig. 1.8), peptide helices form a bundle in
the membrane with a central lumen, much like a barrel composed of helical
peptides as the staves (Ehrenstein and Lecar, 1997; Yang et al., 2001). This
Molecular Characterization and Gene Expression Profiling of Antimicrobial Peptides in Penaeid Shrimps
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