Selective recognition of cyclic RGD peptides of NMR defined conformation by alpha IIb beta 3, alpha V beta 3, and alpha 5 beta 1 integrins

The binding of purified fibrinogen receptor alpha IIb beta 3, vitronectin receptor alpha V beta 3, and fibronectin receptor alpha 5 beta 1 to their corresponding ligands in solid-phase binding assays was used to examine the inhibitory activity of various linear and cyclic penta- and hexapeptides of different conformation containing RGD or RAD sequences. Cyclic peptides with different defined backbone conformations were designed by introducing a single D-amino acid or a proline at different positions in the ring. The data were calibrated for alpha IIb beta 3 integrin incorporated into a planar lipid bilayer by a physical method (total internal reflection fluorescence microscopy) which yielded KD = 1.7 microM for a linear RGD peptide and KD = 0.03 microM for fibrinogen. With this integrin, three cyclic hexapeptides ([GRGDFL], [ARGDFV], [GRGDFV]) were 2-4-fold more inhibitory than the linear GRGDS peptide in solid-phase assays and showed similar inhibition as the fibrinogen ligand. Six peptides had the same or a 2-fold lower activity as the linear reference peptide, and three peptides were up to 7-fold less active. Replacement of Arg or Asp by their stereoisomers or Gly by Ala resulted in a 100-1000-fold reduction in activity. With the two other integrins, a single cyclic pentapeptide [RGDFV] was 10-fold more active (alpha V beta 3) or equal in activity (alpha 5 beta 1) to linear GRGDS, while all of the other cyclic peptides were moderately or distinctly less active. Changes in the RGD sequence caused a less dramatic reduction in binding strength for alpha V beta 3 and alpha 5 beta 1 than for alpha II beta 3. Inhibitory activity was compared with the distance between the C beta atoms of Arg and Asp residues as determined by NMR and indicated that the optimum distance is in the range of 0.75-0.85 nm for alpha IIb beta 3 and at or below 0.67 nm for alpha V beta 3 and alpha 5 beta 1. This indicates that alpha IIb beta 3 less sensitive to variations in the RGD backbone structure and can accommodate a larger distance than the integrins alpha V beta 3 and alpha 5 beta 1.