**1.3 Glycine/arginine rich peptides**

The first purification of glycine rich peptide was done in 1991 (Bulet et al., 1991). As for the proline rich antimicrobial peptide class, the glycine rich peptides have variable sizes and do not show clear sequence signature, apart from the high proportion of glycine in their primary sequence. These peptides are in general longer than AMP from other classes. Between 25 to 50% of their amino acid are glycines. They have disordered structure in water, and tend to self-order when in contact with artificial membranes (Bruston et al., 2007). The structure of bombinin H resembles the influenza hemagglutinin fusion peptide (Zangger et al., 2008). When binding to DPC micelles, a helix is formed that have a glycine ridge on one side. There is an helix-helix interaction that leads to a multimerization process in the bacterial membrane (Zangger et al., 2008).

#### **1.4 Brevinin (hook structure) peptides**

This class of peptides is characterized by having a short amino acid sequence, and a carboxyloterminus disulfide bridge. Some brevinin peptides show post-translational modification. The amino acids included in the carboxyloterminus loop are determinant for the specificity of the antimicrobial activity as well as the length of the loop (Lee et al., 2002) .

Natural Antimicrobial Peptides from Eukaryotic Organisms 55

θ defensin are macrocyclic octadeca peptides connected head to tail. These peptides are present in monkeys but absent in human. There are θ-defensin ortholog pseudo genes in the human genome but the Theta-defensin genes contain a premature stop codon that aborts translation. (Cole et al., 2002) (Cole et al., 2004). Their synthesis implies a head to tail circularization of an octa-peptide (Selsted, 2004). They were found to be active against *S. aureus, E. coli, and C. albicans* as well as HIV virus (Cole et al., 2002). This type of defensins

The insect defensin class have an alpha-helix secondary structure bound to the beta sheet. Their structures are similar to another arthropod peptide class: the scorpion potassium channel blocker toxins (Bontems et al., 1991). Study of the structural features involved in the antimicrobial activity of longicin, a defensin from the hard tick *Haemaphysalis longicornis*, showed that the beta sheet alone was sufficient for antimicrobial activity. This part of the insect peptide is positively charged at neutral pH, as does the alpha helix of the same, nevertheless the role of the alpha-helix, at the antimicrobial level, seems to be restricted to

Plant defensin antimicrobial peptide class present 4 disulfide bridges with a cysteine connectivity of 1-8, 2-5, 3-6, and 4-7. Their three-dimensional structures are similar to the insect defensin structure in that they have a disulfide bonds stabilized alpha-helix. Their structure also show a triple anti-parallel beta-strands. (Sagaram et al., 2011). The strong antifungal activity of the plant defensin has been associated with their alpha helix motif, in a similar fashion as for insect defensin (Lamberty et al., 2001). Conversely, heliomycin, a defensin from *Heliothis virescens*, has more structural communality with plant defensin than

This class of AMP was first found in horseshoe crab (*Polyphemus litoralis*). Gomesin is a tachyplesin type of antimicrobial peptide found in tarantula hemocyte (Silva et al., 2000); and androctonin has been extracted from scorpion hemolymph (Mandard et al., 1999). This type of antimicrobial structure is broadly distributed amongst the genus. The AMP related to this class of peptide present a beta sheet secondary structure stabilized by two disulfide bridges (Nakamura et al., 1988). Tachyplesin have a rigid conformation of antiparallel betasheet connected by a beta-turn (Iwanaga et al., 1994). The tachyplesin family of AMP adopts

RMCKTPSGKFKGYCVNNTNCKNVCRTEGFPTGSCDFHVAGRKCYCYKPCP

also present a disulfide bridges stabilized amphipathic beta sheet structure.

*P. hamadryas* theta-defensin-1 RCVCRRGVCRCVCTRGFC

maintain the globular shape of the peptide (Rahman et al., 2009).

ATCDLLSFRSKWVTPNHAGCAAHCLLRGNRGGHCKGTICHCRK

**1.5.4 θ defensins** 

**1.5.5 Insect defensins** 

*R. prolixus* insect defensin B

with insect defensin (Lamberty et al., 2001).

**1.5.6 Plant defensins** 

*P. sylvestris* defensin 1

**1.6 Tachyplesin** 

The brevinin show alpha helical structure in sodium dodecyl sulfate solution (Lee et al., 2002). Antibacterial activity is favored by structure that group the cationic amino acids of the molecule with on one side an hydrophobic stretch of amino acids and on the other side streches of apolar amino acids (Kumari and Nagaraj, 2001). Liposome disruption activity of the brevinins correlates with the anti-Gram positive bacterial activity, suggesting a lytic activity. None of the peptides showed hemolytic activity making brevinins attractive prospects for broad-spectrum antimicrobial peptide design (Lee et al., 2002).
