**1.9 Mixed structure peptides**

Some peptides share structural communality with more than one class of AMP; the sum of the different AMP part activity are not additive; and these peptides classes do show unique activity not present in the separated structural part of the molecule. Scorpine type AMP represent a class on its own, as several homologous proteins have been found. The structural defensin part of the molecule resembles the insect peptides. It has been determined in later studies that once separated from the linear cecropin-like amino

beta-hairpin-like structures when in contact with hydrophobic solvent. NMR studies revealed a largely unordered structure in water, but a transition to a regular beta-hairpin backbone conformation in the presence of dodecylphosphocholine micelles. The cysteine null mutant of protegrin, a mammal tachyplesin type peptide, revealed that the cystein bridges were not necessary for antimicrobial activity. Aside from their antimicrobial activity, tachyplesin have also a scavenger capability. They bind lipo-polysacharide with high affinity (Niwa et al., 1990). The structure of tachyplesin I also interacts with Vesicular

The archetypical W rich peptide is the indolicin, this peptide comes from *Bos Taurus* neutrophils and is the result of proteolysis. Unlike the amphipathic alpha helical structure of the cecropin class of peptides, their linear structure (no disulfid bridges) has no particular secondary structure in water. Indolicin is globular and amphipathic in aqueous solution, while it adopts a wedge shape when in contact with micelles. Indolicin shows a high affinity for neutral POPC and anionic POPG vesicles(Hsu et al., 2005). The author suggests that the structure changes and the strong membrane affinity are key to the antimicrobial activity of indolicin (Ladokhin and White, 2001). Tryptophan rich AMP contains more than 25% of the aminoacid. Indolicin, the archetypical tryptophan rich antimicrobial peptide, has a globular secondary structure in water, but show a wedge shape when in contact with in lipid micelles (Rozek et al., 2000). This peptide has the ability to permeate bacterial membranes and, depending of its tridimensional shape, inhibits DNA synthesis by binding

Histidine-rich amphipathic cationic peptides are peptides with ¼ of their amino acids represented by histidine. They show a global cationic amphipathic helical structure. They trigger microorganism membrane disruption when the peptide adopts an alignment parallel to the membrane surface. Even though, pore formation is not essential for their high antimicrobial activity (Mason et al., 2009). Clavinin and daptomycin are other studied members of this antimicrobial peptide class. Some Histine rich peptide, like LH4, also have the capability to enhance transfection, a feature that is related to membrane perturbation

Some peptides share structural communality with more than one class of AMP; the sum of the different AMP part activity are not additive; and these peptides classes do show unique activity not present in the separated structural part of the molecule. Scorpine type AMP represent a class on its own, as several homologous proteins have been found. The structural defensin part of the molecule resembles the insect peptides. It has been determined in later studies that once separated from the linear cecropin-like amino

stomatitis virus envelope, inactivating the virus (Murakami et al., 1991).

Tachyplesin III *Tachypleus gigas*: KWCFRVCYRGICYRKCR

**1.7 Tryptophan rich antimicrobial peptides** 

to it (Hsu et al., 2005).

Indolicin: H-ILPWKWPWWPWRR-NH2

**1.8 Histidine rich glycoprotein peptides** 

capability (Georgescu et al., 2010).

**1.9 Mixed structure peptides** 

terminus, this peptide could effectively block these channels. Even though, the antimicrobial activity of the complete molecule was dependent of the presence of this toxin/defensin motif (Diego-Garcia et al., 2008).

The penaeidin class of peptide consist in proline-rich N-terminus and of a C-terminus containing six cysteine residues engaged in three disulfide bridges (Destoumieux et al., 2000). The proline-rich domain of penaeidin class AMP suffices to confer target specificity and antimicrobial activity of penaeidin (Cuthbertson et al., 2004). The carboxyl end cysteinerich domain consists of an amphipathic helix linked to the upstream and the downstream coils by two disulfide bonds. The peptide shows a highly hydrophobic core of globular and compact structure, that has 2 arginines exposed on each side (Yang et al., 2003).

Another example of hybrid antimicrobial peptide is Hyastatin, isolated from the spider crab (*Hyas araneus*) hemocytes (Cuthbertson et al., 2008). This AMP combines a Glycine rich motif N-terminal region, a short Pro/Arg-rich region, and a panaeidin like C-terminal region containing 3 disulfid bridges (Sperstad et al., 2009).

The chicken beta defensin 11 is formed by the repeat of two defensin motif, therefore having 6 disulfid bridges. This defensin show a nanomolar range of anti *E.coli* activity, being one of the most effective antimicrobial peptide for this microorganism (Herve-Grepinet et al., 2010).

Microplusin, is a *Rhipicephalus* (*Boophilus*) microplus anti-microbial peptide (AMP). Microplusin has a cysteine-rich AMPs structure with histidine-rich regions at the N- and Ctermini. Microplusin consists of five alpha-helix and has been shown to bind copper and iron (Silva et al., 2009).
