Dynamic regulation of fibrinogen: integrin ?IIb?3 binding.

ABSTRACT: This study demonstrates that two orthogonal events regulate integrin ?IIb?3's interactions with fibrinogen, its primary physiological ligand: (1) conformational changes at the ?IIb-?3 interface and (2) flexibility in the carboxy terminus of fibrinogen's ?-module. The first postulate was tested by capturing ?IIb?3 on a biosensor and measuring binding by surface plasmon resonance. Binding of fibrinogen to eptifibatide-primed ?IIb?3 was characterized by a k(on) of ~2 × 10(4) L mol(-1) s(-1) and a k(off) of ~8 × 10(-5) s(-1) at 37 °C. In contrast, even at 150 nM fibrinogen, no binding was detected with resting ?IIb?3. Eptifibatide competitively inhibited fibrinogen's interactions with primed ?IIb?3 (K(i) ~0.4 nM), while a synthetic ?-module peptide (HHLGGAKQAGDV) was only weakly inhibitory (K(i) > 10 ?M). The second postulate was tested by measuring ?IIb?3's interactions with recombinant fibrinogen, both normal (rFgn) and a deletion mutant lacking the ?-chain AGDV sites (rFgn ??408-411). Normal rFgn bound rapidly, tightly, and specifically to primed ?IIb?3; no interaction was detected with rFgn ??408-411. Equilibrium and transition-state thermodynamic data indicated that binding of fibrinogen to primed ?IIb?3, while enthalpy-favorable, must overcome an entropy-dominated activation energy barrier. The hypothesis that fibrinogen binding is enthalpy-driven fits with structural data showing that its ?-C peptide and eptifibatide exhibit comparable electrostatic contacts with ?IIb?3's ectodomain. The concept that fibrinogen's ?IIb?3 targeting sequence is intrinsically disordered may explain the entropy penalty that limits its binding rate. In the hemostatic milieu, platelet-platelet interactions may be localized to vascular injury sites because integrins must be activated before they can bind their most abundant ligand.

SUBMITTER: Hantgan RR 

PROVIDER: S-EPMC3210020 | biostudies-literature | 2010 Nov

REPOSITORIES: biostudies-literature