Antimicrobial Peptides for Detection and Diagnostic Assays - Designing Receptors for the Next Generation of Biosensors - page 87

human neutrophils and typically possess six cysteines that form three disulfide
bonds [ 19 ], in a 1-6, 2-4, 3-5 pattern. The -defensins can inhibit nucleic acid and

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protein synthesis of bacteria. The -defensins include six cysteines that form three

disulfide bonds in a 1-5, 2-4, 3-6 pattern.
An additional group are those that are produced non-ribosomally by bacteria and
fungi. Synthesized by multienzyme complexes, peptides in this group often contain
unusual or modified amino acids. Cyclization, unusual linkages, and mixtures of
D - and L -amino acids are also frequently encountered. This group includes the
polymyxins, bacitracin, gramicidin, and alamethicin.
AMPs carry out their various antimicrobial actions via many different target
molecules; however, the mode of action is largely dependent on interaction with the
bacterial membrane. Peptides are attracted through electrostatic interactions
between their cationic side chains and anionic components of the outer bacterial
membrane, such as phospholipid head groups, LPS, and LTA. At low peptide
concentrations, parallel-oriented adsorption of AMPs to the membrane occurs,
resulting in thinning of the membrane and disruption of the natural distribution of
membrane phospholipids in a concentration-dependent manner. Peptides assume an
increasingly perpendicular orientation as their concentrations increase, allowing for
insertion into the lipid bilayer, forming a transmembrane pore. This intermediate
event — pore formation — has led to the development of several models for
explanation of membrane permeabilization. These models are not necessarily
mutually exclusive, nor competing; they are suggested as being related methods
of action [ 15 ] on a spectrum of membrane disruption where any one model may be
unable to explain all of the events that occur [ 16 ].
After a peptide-specific threshold concentration is reached, membrane
permeabilization may commence through the mechanism termed the “barrel
stave” model. Peptides self-aggregate and insert deeper into the hydrophobic
membrane core, with the hydrophobic interface of the amphipathic pointed outward
[ 22 ]. The noncharged alamethicin peptide has been shown to form a consistent
barrel stave pore with an aggregate of eight monomers [ 11 ] where translocation of
the peptides to the inner leaflet of the lipid bilayer causes severe membrane
disruption and leakage of cytoplasmic contents.
In the “toroidal pore” model, a similar sequence of events occurs whereby
aggregates of peptides associate with the membrane in a perpendicular orientation;
upon reaching a necessary threshold concentration, they then insert into the cyto-
plasmic membrane. Inducing local positive curvature strain, the peptides form a
pore lined with peptides and phospholipid head groups in an alternating fashion.
With polar side chains interfacing with the polar lipid head groups, a rounding of
the monolayer joins the leaflets of the membrane resulting in pore formation
without lipid tails exposed to peptide [ 15 , 22 ]. This mechanism allows for cationic
peptides such as LL-37, melittin, protegrin-1, and magainin-2 to reach intracellular
targets through a relatively large pore.
In a third prominent model of action, the “carpet mechanism” proposes peptide
accumulation on the lipid bilayer in a parallel orientation. After reaching a thresh-
old concentration, cationic peptides disintegrate the membrane and form micelles.

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