Hairpin-like peptides

Finally, the hairpin-like peptides, hepcidins, isolated from human urine and liver [120], and from the gill of hybrid striped bass [121], are more complex, with eight cysteine residues forming four disulphide bridges. They form an unusual distorted P-sheet. Interestingly, besides their antimicrobial activity, hepcidins are the principal hormonal-regulators of iron homeostasis in humans [124]. 

The role of the stereochemical configuration has been evaluated in several 3-hairpin-like peptides. For example, D enantiomers proved to be as potent as the native molecules in the cases of androctonin [128], protegrin 1 [133], and gomesin [134], implying that the peptides do not act via a stereo-specific receptor. 

Several cyclic peptides have between two and eight cysteine residues. They adopt a triple-stranded 3-sheet structure e.g. vertebrate defensins) or a P-hairpin-like structure (e.g thanatin, androctonin, gomesin, and tachyplesin from arthropods and protegrin from vertebrate) or a mixed a-helix/p-sheet conformation (e.g. invertebrate and plant defensins, including some vertebrate defensins). Several reviews have been published in the past years discussing the structure and the mode of action of cyclic AMPs from animals. The reader is referred to the reviews written by Powers and Hancock [108], Bulet et al. [4], Ganz [8], and Yount [88]. In this chapter, only cyclic peptides with a P-hairpin-like structure will be discussed. 

The hairpin-like structure has been conserved in the course of evolution, since it was found in many peptides isolated from various classes of arthropods, such as the primitive horseshoe crabs (tachyplesins [110,111] and polyphemusins [112,113]), arachnids (androctonin in scorpion [114] and gomesin in spider [115]) insect (thanatin [116]), in two classes of vertebrates, mammalian (protegrin [117,118], lactoferricin B [119] and hepcidins [120]) and fish (hepcidins [121]), and in plants (76-AMPl[122]). 

The hairpin-like P-turn has recently been examined in more detail from the standpoint of conformational energy (28). This investigation revealed that there are two types of conformation of the L L bend which accomodate the sequences L -> L, L G, G - L, G G, while only one type is possible for the L D bend which accomodates the sequences L - D, L G, G D, G G. The new investigation supported and extended the earlier considerations, and has been successfully compared with the experimental data. These investigations revealed that the formation of the /1-turn is a determining factor in the cyclization of linear peptides and explained why a particular amino acid such as D-amino acid or a constrained amino acid like proline are often found at the comer of the p-tum in thetic and naturally occurring cyclic peptides. 

Template-o , which is a designed coiled coil. To encourage a conformational switch, residues at / positions were changed from glutamine to threonine to produce Template-ofT. Below 70 °C, the peptide is cK-helical and above this temperature it forms -structured amyloid-like fibrils. Cross-linking the peptides to achieve a )8-hairpin-like conformation increases amyloid fibril formation. 

Subclass Ila are pediocin-like bacteriodns with a conserved Al-terminal sequence motif known as the pediocin box and one or two intrachain disulfide bonds (Lozano et al. 1992 Nes and Holo 2000). The peptides form a very characteristic stmcture, consisting of a conserved Al-terminal antiparallel P-sheet connected by a flexible hinge to the more variable C-terminal hairpin-like domain... 

Model peptides that can adopt the (3-sheet conformation have until recently been confined to ones that form intermolecular (3-sheets, t96,104,105 Peptides that can form intramolecular (3-sheets have been avidly sought because the coil-(3 transition1 06 in such peptides would provide thermodynamic data on the effects of sequence and individual residues on (3-sheet stability like those obtainable from model a-helical peptides. Two types of models have been developed 107 (3-hairpins, i.e. two-stranded (3-sheets, and three-stranded (3-sheets. 

Besides hairpin turns and broader U-tums, a protein chain may turn out and fold back to reenter a P sheet from the opposite side. Such crossover connections, which are necessarily quite long, often contain helices. Like turns, crossover connections have a handedness and are nearly always right-handed (Fig. 2-25).117/219 Most proteins also contain poorly organized loops on their surfaces. Despite their random appearance, these loops may be critical for the functioning of a protein.220 In spite of the complexity of the folding patterns, peptide chains are rarely found to be knotted.221... 

Like some other membrane active proteins and peptides [95-97], Cry toxins bind to the cell siuface in a water soluble form, followed by an irreversible conformational change converting into a form capable of inserting into the membrane [35]. The Cry toxin structures revealed that putative membrane-spaiming amphipathic helices located in domain I might be involved in pore formation [30,31]. Since the amphipathic helices predicted to span the membrane are buried in the helical bundle in domain I, a conformational change was predicted to expose a relatively non-polar/hydrophobic hairpin composed of helices a4 and a5 to initiate membrane insertion [30,31]. 

The cystatins, which are a superfamily of proteins that inhibit papain-like cysteine proteases, are a classic example of these inhibitors. The cystatins (Fig. 3) insert a wedge-hke face of the inhibitor that consists of the protein N-terminus and two hairpin loops into the V-shaped active site of a cysteine protease. The N-terminal residues bind in the S3-S1 pockets in a substrate-like manner, but the peptide then turns away from the catalytic residues and out of the active site. The two hairpin loops bind to the prime side of the active site, which provides most of the binding energy for the interaction. Thus, both the prime and the nonprime sides of the active site are occupied, but no interactions are actually made with the catalytic machinery of the enzyme (23). 

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