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出典:『Wikipedia』 (2011/03/22 18:09 UTC 版)

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Cyclic peptides (or cyclic proteins) are polypeptide chains whose amino and carboxyl termini are themselves linked together with a peptide bond that forms a circular chain. A number of cyclic peptides have been discovered in nature and they can range anywhere from just a few amino acids in length, to hundreds. Cyclic peptides can be classified according to the types of bonds that comprise the ring. Homodetic cylcic peptides, such as cyclosporine A, are those in which the ring is composed exclusively of normal peptide bonds (i.e. between the alpha carboxyl of one residue to the alpha amine of another). Cyclic isopeptides contain at least one non-alpha amide linkage, such as a linkage between the side chain of one residue to the alpha carboxyl group of another residue, as in microcystin and bacitracin. Cyclic depsipeptides, such as aureobasidin A and HUN-7293, have at least one lactone (ester) linkage in place of one of the amides. Some cyclic depsipeptides are cyclized between the C-terminal carboxyl and the side chain of a Thr or Ser residue in the chain, such as kahalalide F, theonellapeptolide, and didemnin B. Bicyclic peptides such as the amatoxins amanitin and phalloidin contain a bridging group, generally between two of the side chains. In the amatoxins, this bridge is formed as a thioether between the Trp and Cys residues. Other bicyclic peptides include echinomycin, triostin A, and Celogentin C. There are a number of cyclic peptide hormones which are cyclized through a disulfide bond between two cysteines, as in somatostatin and oxytocin.