STRUCTURAL STUDIES OF A FAMILY OF HIGH-AFFINITY LIGANDS FOR GPIIB/IIIA

A class of potent orally active cyclic peptide antagonists of the glycoprotein IIb/IIIa adhesion molecule have been prepared by linking a tetrapeptide, RGD-containing sequence between the two ends of a semirigid linker, m-(aminomethyl)benzoic acid (Mamb). To determine how this amino acid constrains the conformation of the intervening peptide sequence and to shed some light on the receptor-bound conformation of the peptide, we examined the solution and solid-state structures of nine cyclic analogues whose receptor binding constants span approximately 4 orders of magnitude. The general structure of these analogues is cyclo(Xxx-Arg-Gly-Asp-Mamb). The backbone conformations of each compound trace out a rectangular shape with a beta-turn centered at the Xxx-Arg bond. In the most potent compounds in this series Xxx is a small, aliphatic D-amino acid, and N-alpha of the Arg residue is methylated: peptides containing these features are highly rigid and contain a type II' beta-turn centered at the D-Abu-N-MeArg dipeptide, a highly extended Gly residue, and a C-7 turn centered at the Asp. Peptides lacking the N-methyl group and/or with reversed chirality at Xxx are more flexible. The N-alpha-methyl group also restricts the conformation of the Arg side chain. In addition the aliphatic side chain of the D-amino acid, the N-alpha-methyl of N-MeArg, and the phenyl group of the Mamb linker form a continuous hydrophobic surface, which presumably interacts favorably with the receptor. Finally, the effects of conformational constraints introduced at the Mamb and the Asp residues were investigated to determine their effects on receptor binding.