Biologically active Arg-Gly-Asp oligopeptides assume a type II beta-turn in solution

The sequence Arg-Gly-Asp (RGD) has been found to be the consensus sequence of matrix proteins for binding cell surface receptors (integrins). Studies with synthetic peptides containing the RGD sequence show that the biological activity of these oligopeptides is removed upon a conservative substitution of Glu for Asp in the RGD sequence. Two-dimensional 1H NMR methods were used to investigate the secondary structures in aqueous solution for two such oligopeptides of differing biological activity. The sequence Tyr-Gly-Arg-Gly-Asp-Ser-Pro, which binds to selected integrins, is found to assume a type II beta-turn at both pH 4 and 7. In contrast, the sequence Tyr-Gly-Arg-Gly-Glu-Ser-Pro, which does not interfere with integrin-mediated cell attachment, is found to assume a type I or III beta-turn at both pH 4 and 7. This comparison confirms not only that oligopeptides can assume a secondary structure in aqueous solution, but also that these structures may be important to biological functions.