Rational design of antimicrobial C3a analogues with enhanced effects against Staphylococci using an integrated structure and function-based approach.

The anaphylatoxin C3a and its inactivated derivative C3adesArg, generated during complement activation, exert direct antimicrobial effects, mediated via its C-terminal region [Nordahl et al. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 16879-16884]. During evolution, this region of C3a displays subtle changes in net charge, while preserving a moderate but variable amphipathicity [Pasupuleti et al. (2007) J. Biol. Chem. 282, 2520-2528]. In this study, we mimic these evolutionary changes, employing a design approach utilizing selected amino acid substitutions at strategic and structurally relevant positions in the original human C3a peptide CNYITELRRQHARASHLGLA, followed by structure-activity studies incorporating sequence-dependent QSAR models as tools for generation of C3a peptide variants with enhanced effects. While the native peptide and related amphipathic analogues of moderate positive net charge were active against the Gram-negative Escherichia coli, activity against the Gram-positive Staphylococcus aureus was primarily observed for peptides characterized by a combination of a relatively high net charge (+6-7) and a propensity to adopt an alpha-helical conformation with amphipathic character. Such increased helicity and charge also conferred activity against the fungus Candida albicans. A central histidine residue (H11), evolutionarily conserved among vertebrates, conferred high selectivity toward microbes, while substitutions with leucine rendered the peptides hemolytic. Selected C3a analogues retained their specificity against staphylococci in the presence of human plasma, while showing low cytotoxicity. The work illustrates structure-activity relationships underlying the function and specificity of antimicrobial C3a and related analogues and provides insights into the forces that drive evolution of antimicrobial peptides.