An unusual structural motif of antimicrobial peptides containing end-to-end macrocycle and cystine-knot disulfides

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Abstract
Four macrocyclic cystine-knot peptides of 29–31 residues, kalata, circulin A and B (CirA and CirB), and cyclopsychotride, have been isolated from coffee plants but have undetermined physiological functions. These macrocycles and 10 of their analogs prepared by chemical synthesis were tested against nine strains of microbes. Kalata and CirA were specific for the Gram-positiveStaphylococcus aureuswith a minimum inhibition concentration of ≈0.2 μM. They were relatively ineffective against Gram-negative bacteria such asEscherichia coliandPseudomonas aeruginosa. However, CirB and cyclopsychotride were active against both Gram-positive and Gram-negative bacteria. In particular, CirB showed potent activity againstE. coliwith a minimum inhibitory concentration of 0.41 μM. All four cyclic peptides were moderately active against two strains of fungi,Candida kefyrandCandida tropicalis, but were inactive againstCandida albicans. These macrocycles are cytotoxic and lysed human red blood cell with a lethal dose 50% of 400 μM. Modifying the Arg residue in kalata with a keto aldehyde significantly reduced its activity againstS. aureuswhereas blocking the arg in CirA produced no significant effect. The two-disulfide variants and their scrambled disulfide isomers exhibited antimicrobial profiles and potency similar to their native peptides. However, in high-salt assays (100 mM NaCl), few of these macrocyclic peptides, natives or analogs, retained antimicrobial activity. These results show that the macrocyclic peptides possess specific and potent antimicrobial activity that is salt-dependent and that their initial interactions with the microbial surfaces may be electrostatic, an effect commonly found in defensin antimicrobial peptides. Furthermore, their end-to-end cyclic structure with a cystine-knot motif represents a molecular structure of antimicrobials and may provide a useful template for the design of novel peptide antibiotics.