Dissociation of Bimolecular αllbβ3-Fibrinogen Complex under a Constant Tensile Force

The regulated ability of integrin alpha llb beta 3 to bind fibrinogen plays a crucial role in platelet aggregation, adhesion, and hemostasis. Employing an optical-trap-based electronic force clamp, we studied the thermodynamics and kinetics of alpha llb beta 3-fibrinogen bond formation and dissociation under constant unbinding forces, mimicking the forces of physiologic blood shear on a thrombus. The distribution of bond lifetimes was bimodal, indicating that the alpha llb beta 3-fibrinogen complex exists in two bound states with different mechanical stability. The alpha llb beta 3 antagonist, abciximab, inhibited binding without affecting the unbinding kinetics, whereas Mn(2+) biased the alpha llb beta 3-fibrinogen complex to the strong bound state with reduced off-rate. The average bond lifetimes decreased exponentially with increasing pulling force from similar to 5 pN to 50 pN, suggesting that in this force range the alpha llb beta 3-fibrinogen interactions are classical slip bonds. We found no evidence for catch bonds, which is consistent with the known lack of shear-enhanced platelet adhesion on fibrinogen-coated surfaces. Taken together, these data provide important quantitative and qualitative characteristics of alpha llb beta 3-fibrinogen binding and unbinding that underlie the dynamics of platelet adhesion and aggregation in blood flow.