Page - (000240) - in Biomedical Chemistry: Current Trends and Developments

positive surface charge of cholesterol present in the membrane bilayer decreases the channel formation potency of cecropins (Christensen, 1988). Cholesterol increases the thickness and condensation of membranes, so cecropins have little to no effect on eukaryotic cells, which contain a high amount of cholesterol in contrast to bacteria (Beevers & Dixon, 2010). A second class of linear, cysteine-free peptides, the magainins, is exclusively found in amphibians. Magainin 1 and magainin 2 adopt an α- helical conformation in solution (Zasloff, 1987). Magainins are proposed to induce toroidal pores in bacterial membranes (Ludtke, 1996). The nonhemolytic feature of magainin 2 and its protocidal activity make it highly interesting for the examination of its activity against human parasites. Another family of amphibian AMPs is the dermaseptin superfamily. These AMPs exhibit a broad range of antimicrobial activity and some aggregate on the membrane surface in a carpet-like manner (Pouny, 1992; Raghuraman & Chattopadhyay, 2007). Melittin is an α- helical cationic peptide, composed of 26 amino acid residues in which the amino-terminal region is predominantly hydrophobic and the carboxy-terminal region is hydrophilic due to the presence of a stretch of positively charged amino acids (Raghuraman & Chattopadhyay, 2007), and the membrane permeabilization mechanism is proposed to result from pore formation according to the toroidal model (Yang, 2001). Cationic antimicrobial peptides are a class of small charged peptides, imparted by the presence of multiple Lys and Arg but with a substantial portion (50% or more) of hydrophobic residues. They are known for their broad-spectrum antimicrobial activity and, most recently, the ability to modulate the innate immune response (Powers & Hancock, 2003). 3.1.3.1.3 Peptides Containing Disulfide Bridges Defensins are peptides from mammalian phagocytes that contain 6 Cys residues (or for some insect defensins, eight Cys) that stabilize the peptide structure by forming three intramolecular disulphide bridges (Selsted, 1985). The mechanism of action of these peptides is based on membrane permeabilization, probably by pore formation, and defensins are more active against negatively charged phospholipids (Lehrer, 1989;