Fibrinogen γ-chain C-terminal peptide HHLGGAKQAGDV (γ12) and α-chain peptide GRGDSP are known to inhibit fibrinogen-mediated platelet cell aggregation via competitive interactions with platelet integrin receptor GPIlb/IIIa. NMR studies of γ12 in the presence of purified GPIIb/IIIa in SDS/water solution have demonstrated the presence of two γ12 binding states, one of which is eliminated by GRGDSP (RGD) up to a RGD: γ12 ratio of 2:1. RGD: γl2 ratios greater than 2:1 produce multiple sets of γ12 NMR signals in TOCSY spectra. At a ratio of 4:1, two to four such resonance sets can be resolved for A405, Q407, A408, G409, D410 and V411 spin systems. The number of multiple resonances remains unchanged at ratios of 6:1 and 8:1. Addition of γ12 to reverse the ratio to 8:8 (1::1) has no apparent effect on the RGD-induced distribution. Results suggest that RGD irreversibly induces a conformational transition(s) in GPIIb/IIIa to produce multiple γ12 binding sites on the receptor.
