Binding of RGD-peptide mimics to intact human platelets : an NMR study

The interactions of small peptides with biological membranes is central to a number of biological processes. In contrast to soluble proteins there is comparatively little information available about ligandreceptor interactions that occur at membrane surfaces. The biophysical environment of a membrane is considerably different from the isotropic extracellular medium. It is therefore desirable to investigate membrane proteins and their binding specificity directly in living cells. The integrin aIIbb3 is the most abundant platelet cell surface glycoprotein and plays a key role in adhesion of platelet to protein-coated surfaces and platelet/platelet aggregation. Therefore, aIIbb3 receptor is an excellent target for drug design and delivery. The ligands are cyclic pentapeptide mimics incorporating stereoisomeric 5,6- and 5,7-fused bicyclic lactams and the tripeptide sequence Arg-Gly-Asp (RGD)[1], a common amino acid motif found in a number of adhesive proteins. In order to study these binding processes at molecular level, we used trNOE experiments directly on whole human platelets.[2] Conformational properties of the free and bound mimics are investigated by computational and NMR studies. Utilizing structural information by NMR tr-NOE experiments and docking studies, we have discovered a new higher affinity aIIbb3 ligand, which is able to inhibit the adhesion of platelets.