Reevaluation of the C3a active site using short synthetic C3a analogues

In 1980 the C-terminal pentapeptide LGLAR (C3a 73-77) was described (Caporale, L. H. et al. J. Biol. Chem. 1980, 255: 10758) as the minimal sequence inducing a C3a-specific activity. We have synthesized C3a-analogue peptides connected to non-peptidic acyl residues known to potentiate biological activity. Starting from the acylated hexapeptide fluorenylmethoxycarbonyl(Fmoc)-aminohexanoyl(Ahx)-ALGLAR+ ++, a related series of shorter peptides was synthesized. C3a-specific activity was measured as ATP release from guinea pig platelets. Even the tripeptide LAR, acylated with Fmoc-Ahx, exhibited C3a-specific activity. With 0.34% C3a activity, it was even more potent than the native LGLAR sequence which has 0.01% activity. N-terminal extension of the acylated tripeptide LAR by adding one to three alanines increased activity tenfold up to 3.26% (Fmoc-Ahx-AAALAR), while N-terminal addition of three glycine residues (Fmoc-Ahx-GGGLAR) only increased activity to 0.83% of native C3a. Furthermore, a stimulus-specific desensitization could be observed. Fmoc-Ahx-R and Fmoc-Ahx-AR exhibited neither activity nor desensitizing capacity, but the addition of four alanines to the dipeptide AR led to a sequence (Fmoc-Ahx-AAAAAR) with a C3a-specific activity of 0.14%. Even arginine prolonged N-terminally with five glycines (Fmoc-Ahx-GGGGGR) exhibited some C3a-specific activity so that for biological activity only the appropriate presentation of arginine seems to be essential.