**1.2 Proline rich peptides**

52 Antimicrobial Agents

The first peptide of this family discovered is the cecropin A from the pupae of the moth *Hyalophora cecropiae* (Steiner et al., 1981; Hultmark et al., 1982). This structure of AMP has been encountered in virtually all the multi-cellular organisms. Even if their sequences show some similarity, they are not all evolutionary linked. As such, they cannot be aligned as a whole, and are commonly separated in structural subclasses: Cecropin, magainin and dermaseptin AMP. This AMP class do share general common features: the lack of cysteine bridges, the tendency to form alpha helical secondary structure in relatively hydrophobic solvent, the net positive charge at neutral pH and hydrophobic residues interspersed every 3 amino acid, giving them an amphipatic nature. Indeed, basic amino acid side chains face predominantly one side of the alpha helix and hydrophobic residues are generally on the other side of the molecule. A global alignment of the linear peptides separates three different classes that could be broadly characterised as Dermaseptin, magainin and cecropin class of lineal amphipathic AMP. Most of these peptides share the present a glycine near the

These peptides were extracted from Phyllomedusa genus frog skin secretions. Some of them present a proline-induced kink in the middle of the alpha helix (Shin et al., 2001). Others have a glycine in the same relative position that has been suggested to give the flexibility needed for the membrane lysis activity (Xiao et al., 2006). This structural plasticity has been defined as a molecular determinant for the antimicrobial vs eucaryont membrane specificity (Shin et al., 1999; Shin et al., 2000; Shin et al., 2001), together with overall net positive charge and hydrophibic moment. They present hydrophobic and positively charged amino acid in an alternate pattern. Though mature dermaseptin amino acid sequences are highly variable,

Cecropin peptides were first purified from insect hemolymph, and their expression is usually inducible. Cecropins structural conformations were determined by NMR. Circular dichroism and NMR (Nuclear Magnetic Resonance) data have shown that in aqueous solvent the cecropin structure is largely disordered; but they adopt a stable alpha-helical secondary structure in more hydrophobic environment. This makes the insertion of these peptides in the lipidic membranes entropically favorable. Cecropin usually present a glycine in the middle of their amino acid sequence. This glycine has been proposed to induce a kink between the alpha helical structures that these peptides form in hydrophobic solvent. In turn the deletion of this glycine or its replacement by another amino acid do not eliminate the antimicrobial activity; instead it endows these mutant peptides with hemolytic and cytolytic activity (Moore et al., 1996). The cecropin usually present a tryptophan in one of

Magainin peptides come from the frog genus Xenopus (Duclohier et al., 1989). In this AMP class arthtropod AMP like Opistoporin-2 from Scorpion *Opistophthalmus carinatus* are also

their acidic pro-peptides are strikingly conserved (Azevedo Calderon et al., 2010).

the first two amino acid position as well as a glycine in the first position.

**1.1.4 Magainin/scorpions AMP/cathelicidin peptides** 

**1.1 The lineal amphipathic alpha-helix antimicrobial peptide** 

**1.1.1 General properties** 

middle of their peptidic sequence.

**1.1.2 Dermaseptins peptides** 

**1.1.3 Cecropin peptides** 

This AMP class has been first described in mammals, in the intestine of *Sus scrofa,* (Agerberth et al., 1991). They are also present in Hymenoptera, Lepidoptera and diptera. Some of these peptides from this AMP class have been studied extensively, like drosocin from *D. melanogaster* (Bulet et al., 1993), pyrrhocoricin from the European sap- sucking bug *Pyrrhocoris apterus* (Cociancich et al., 1994) apidaecins from the *Apis mellifera* (Casteels-Josson et al., 1993), and formaecin from the ant *Myrmecia gulosa* (Mackintosh et al., 1998). Mature proline-rich antimicrobial peptides vary in length from 12 to 54 amino acid, and have few common structural feature. Some authors distinguish between glycine/proline rich and alanine/proline rich peptides. The variety of sequence does not allow for straightforward sequence signature recognition for these peptides. Therefore, their antibacterial activity/specificity is not deducible from the sequence analysis.

On the other hand, some of these peptides are able to penetrate the microbial cytoplasm without inducing bacterial lysis, and do not present hemolytic or cytolitic activities (Knappe et al., 2010). Model PR peptides have been designed, using the Ac-(Arg-Pro-Pro-Phe)n-NHCH3 framework, and some essential structural feature, necessary for antimicrobial activity have been determined. The ability to form poly(proline)-II structure in aqueous solution, as well as a critical peptide length are essential for antimicrobial activity (Niidome et al., 1998).
