A new approach to inhibit human β-tryptase by protein surface binding of four-armed peptide ligands with two different sets of arms.

A series of six new tetravalent ligands (1-6) with two different sets of arms bind to the surface of β-tryptase, a tetrameric enzyme with an A(2)B(2) arrangement of its four monomers and two different binding sites on its protein surface (as suggested by a docking study). Besides proteinogenic amino acids also the guanidiniocarbonyl pyrrole cation (abbreviated as GCP), as an artificial arginine analog, was introduced into the arms of the ligands to investigate its influence on protein surface binding and enzyme inhibition. Furthermore, four ligands (7-10) with four identical arms also containing the GCP group were additionally synthesized to study the influence of the GCP moiety on the inhibition properties compared to related ligands previously identified by us in earlier work. The best ligand from this new series (RWKG)(2)(GCP-LFG)(2) (6) indeed contains the artificial GCP group and with a K(i)-value of 67 nM is two orders of magnitude more efficient than the analogous ligand (RWKG)(2)(RLFG)(2) (1) derived solely from proteinogenic amino acids. Hence, four-armed ligands with two different arms are indeed efficient inhibitors for β-tryptase and the artificial GCP group can improve the binding affinity of this type of ligand to the protein, demonstrating the advantage of tailor-made binding motifs to increase affinity.