**3. Models to describe MOA**

The activity of AMPs must be considered at the cytoplasmic membrane since most of the AMPs pass through the cell membrane [26]. MOA of peptides depends upon the number of properties including the amino acids sequence, net charge, secondary structure, amphipathicity, hydrophobicity, etc. [27]. There are different mechanisms by which AMPs cause membrane disruption [39]. The capability of AMP's to bind with bacterial membrane leads to their significant development [40, 41]. There are various models hypothesized by scientists, used to describe the mechanisms of binding AMPs on a membrane including the barrel-stave, toroidal pore wormhole, carpet model as depicted in **Figure 4** [6].

The models that describe structurally less-defined mechanisms are interfacial activity, segregation of lipids into domains, non-lamellar phases formation, and the transient pore mechanism. Antimicrobial peptides' mechanism of action has been described using a variety of models. Models which are significantly applied to MOA of AMPs on the membrane are barrel stave, toroidal pore wormhole, and carpet mechanism among all the various models as depicted in **Table 1** [30, 41]. The two main pore formation models are barrel-stave and toroidal pore. The non-pore formation model is the carpet mechanism. The mechanism can be divided further into two based on cellular absorption processes: ATP dependent and ATP independent uptake process. The barrel-stave model, carpet model, or toroidal model are all ATP-independent uptake mechanisms, while macropinocytosis is an energy-dependent uptake mechanism [9]. Example of AMPs that follows energy independent cell-penetration mechanism is MMG, alamethicin and gramicidin S [9]. Example of AMP that acts through energy-dependent endocytic pathway CGA-N9 [42].

#### **3.1 Barrel-Stave model**

In the barrel-stave model, AMPs bind with the membrane outer surface via electrostatic interaction following it then undergo a conformational change attaining an amphipathic structure. Peptides with a special direction are placed between the membrane and they laterally interact with each other to form an ion channel [31].

#### **Figure 4.**

*Mechanism of interaction of the antimicrobial peptide with microbial membrane. (a) ATP independent cellular uptake mechanism: barrel stave model, carpet model, toroidal pore wormhole model. (b) ATPdependent cellular uptake mechanism: macropinocytosis [9].*

When peptide concentration reaches a critical threshold, the peptide monomers form an aggregate on the surface of the membrane, then they create a structure that is made up of a huge concentration of peptides inserted inside the membrane to form a ring just like a "barrel" pore. "Stave" here indicates the spokes which are contained inside the barrel [43]. The hydrophobic residues of the aggregated peptides face outward towards the hydrophobic region of the membrane, while the hydrophilic regions of the peptides face inward, forming an aqueous transmembrane pore that triggered exudation of intracellular contents and resulting death of cells [9]. Some examples of peptides that work through the barrel stave model mechanisms are alamethicin and gramicidin S [13, 44, 45]. Bioinformatic analysis of protegrin 1 conformed that the calculated energy of peptide insertion in artificial membranes was most congruent with this model (**Figure 4(a)**) [26, 33].

#### **3.2 Toroidal pore wormhole model**

The "toroidal pore wormhole" model works similarly to the "barrel stave" mechanism. The peptides are first attracted to the membrane in parallel orientation and then go through secondary structural modifications that are equivalent to those seen in the barrel stave model. The hydrophilic head of peptides faces the hydrophilic region of lipids in this arrangement, and the aqueous phase is outside of the membrane, whereas the hydrophobic portion is located in the membrane's hydrophobic core. The hydrophobic region of the peptides attach to the phospholipid head regions and displace them. This generates a rupture in the hydrophobic part of the membrane, resulting in a strain. The strain, as well as membrane thinning, creates the surface of bilayer fragile to the AMPs by destructing the composition of the membrane [33].

When the critical threshold concentration of peptides is achieved, the peptides form a self aggregate and thus create the toroidal pore complex, directing *Antimicrobial Peptides: Mechanism of Action DOI: http://dx.doi.org/10.5772/intechopen.99190*


#### **Table 1.**

*Various Models for the interaction of AMPs with membrane [29, 30].*

themselves in a perpendicular direction to the bilayer surface with the hydrophobic residues not accessible to the phospholipid head groups. The peptides still have an interaction with the phospholipid head regions and are not localized inside the hydrophobic region of the membrane, which distinguishes this from the barrel stave pore. Since this configuration is less stable than a barrel stave pore, thus it is more transitory. Peptide charge appears to alter the stability of pores, with a significant amount of positive side chain residues inducing repulsion and resulting in transitory pores with very short half-lives [43]. The peptides interact through electrostatic attraction with the membrane and following it undergo the same conformational alterations in the same way as the barrel stave model.

In this model, the peptides can orient themselves in a perpendicular direction too in the bilayer membrane [46], also this model does not need specific peptide– peptide interactions to occur. Instead, the peptides cause pore formation within a local curvature of the membrane which is partially formed by the phospholipid head regions. One feature which differentiates the toroidal-pore model from the barrel-stave is the complete arrangement of the lipid membrane. The hydrophobic and hydrophilic arrangement of the bilayer is kept intact in the barrel-stave mechanism, while in toroidal pores this arrangement of the lipids is disrupted, due to which a lipid head and lipid tail groups start interacting with each other. Some peptides traverse through the cytoplasmic membrane enter inside the cytoplasm and start attacking intracellular components as the pores are transient upon the destruction of the membrane [47]. Various AMPs act by toroidal pore models like magainin 2, lacticin Q, and melittin (**Figure 4(a)**) [6].

#### **3.3 Carpet model**

The carpet model, originally described by Shai [40], is the widely studied model for destabilization of the membrane by AMPs. AMPs can also perform the antimicrobial activity without pores formation in the membrane. Carpet model is one such model [17, 41, 48]. Similar to the other two models the mechanism occurs when cationic AMPs are initially attracted with strong electrostatic interaction to a negatively charged phospholipid membrane. AMPs are oriented in a parallel direction to the lipid bilayer membrane surface. Peptides accumulate themselves until they reach to critical threshold concentration, to form a "carpet" on the membrane, leading to unnecessary binding interactions on the outer surface of the membrane, thus rupture of the membrane occurs by creating an effect just like detergent, which leads to micelle formation [23]. There are a few models which can't be distinguished. The carpet model is one of them and it has been proposed as a necessary step for the toroidal pore model [30]. The membrane bilayer is broken into micelles is referred to as a detergent-like model. The carpet mechanism does not need peptide–peptide interactions of the peptide individuals bound to the membrane; nor does it need the peptide to embed itself into the hydrophobic region to create transmembrane channels [33]. Some peptides' antimicrobial activity is independent of their amino acid or sequence length; such peptides use the carpet model to demonstrate their action [41], and they perform their action when they are in large amounts because of their amphiphilicity [28]. AMPs performing their activity with the mechanism of carpet model are e.g., cecropin [49] and aurein 1.2 (**Figure 4(a)**) [50].

#### **3.4 Other models**

The models which have ATP-independent cellular uptake mechanisms involve the barrel-stave model, carpet model, or toroidal model, which we have already discussed. ATP-dependent uptake mechanism involves macropinocytosis. Macropinocytosis is the ATP-dependent uptake method of action of AMPs, where the target cell's plasma membrane folds inward along with the peptide to generate macropinosomes. Furthermore, the AMPs in the vesicles are exudated inside the cytoplasm and show the antibacterial effect (**Figure 4(b)**) [18, 45]. There are several other models by which peptides perform their antimicrobial action. In models like sinking raft and electroporation, unstable holes emerge in the membrane, altering the charge on both sides of the membrane and eventually developing holes [44].

#### **4. Mechanism of action against other targets**

Mechanism of AMPs is widely studied against other targets as well like viruses, fungi, and cancer. Gram-positive bacteria and Gram-negative bacteria are the most commonly studied targets though [2].
