FVIII binding to PS membranes differs in the activated and non-activated form and can be shielded by annexin A5.

Binding of Factor VIII to phosphatidylserine (PS)-expressing platelets is a key process in the intravascular pathway of the blood coagulation cascade. Activated by thrombin, FVIIIa acts as a cofactor on the surface of platelets. It is under debate whether and how annexin A5 influences FVIIIa binding to platelets. Here, we investigate FVIII binding to PS-containing vesicles as model platelets and its interplay with annexin A5 in buffer using fluorescence correlation spectroscopy (FCS). We find that activated FVIIIa, in contrast to inactivated FVIII, exhibits a striking binding anomaly as a function of PS content, marked by a sharp maximum of the binding constant around 11% PS, which is close to the natural PS content of platelets. Furthermore, we show that the addition of annexin A5 can both increase or decrease this FVIIIa binding depending on whether the relative PS content is lower or higher than the maximum binding value. We demonstrate in theory that the observed binding diagram supports the hypothesis that annexin shields PS, indicating a possible indirect regulatory role of annexin A5 in blood coagulation. The overall PS- and annexin-dependent binding behavior of activated FVIIIa is preserved in experiments in blood plasma, confirming the validity of our results under more physiological conditions.