Differential roles of two consecutive phenylalanine residues in thrombin receptor-tethered ligand peptides (SFFLRNP) in thrombin receptor activation

A synthetic heptapeptide H-Ser-Phe-Phe-Leu-Arg-Asn-Pro-NH₂, which corresponds to the ligand peptide latent in rodent thrombin receptors, was able to activate the thrombin receptor with no thrombin. In order to evaluate the structural requisites of two consecutive phenylalanines, three sets of analogs with substitutions at either position 2 or 3 were synthesized and examined for their stimulatory activity in phosphoinositide turnover in SH-EP epithelial-like cells. The replacement of Phe-2 by Ala completely eliminated the activity, while that of Phe-3 retained about 50% activity with a full stimulation. The Phe/Leu substitution resulted in a large increase (37-fold) in EC₅₀ value for Phe-2, but in insignificant change for Phe-3. Substitution of para-fluorophenylalanine ((p-F)Phe) for Phe-2 enhanced strongly (4-fold) the activity, in contrast to a reduction by the Phe-3/(p-F)Phe substitution. Elimination of either Phe-2 or Phe-3 resulted in a complete loss of activity. These results indicated that Phe-2 and Phe-3 play different roles in the receptor activation. A highly specific aromatic π-π interaction was suggested between Phe-2 phenyl and thrombin receptor binding site, while Phe-3 appeared to be important for retaining a bioactive conformation.