Kinetic and hydrodynamic analysis of blood clotting factor V-membrane binding

The kinetics and hydrodynamic properties of factor V-membrane interaction were characterized. Factor V bound to membranes containing acidic phospholipids with a high collisional efficiency. For membranes of 20% phosphatidylserine-80% phosphatidylcholine, an association rate constant of (1.13 ± 0.10) × 10⁸ M^-1 s^-1 was obtained. These membranes contained about 20 factor V binding sites per vesicle of 3.6 × 10⁶ daltons. This association rate represented about a 30% collisional efficiency. Dissociation of factor V was measured by a fluorescence energy transfer method with a dissociation rate constant of 0.0055 s^-1 at 10°C. The equilibrium dissociation constant for binding to these membranes at 10°C and 0.14 M ionic strength was 5 × 10^-11 M. Ionic strength, pH, calcium, and charge density in the membrane had large effects on the rate of factor V-membrane dissociation, indicating a strongly ionic interaction between protein and membrane. In contrast, the association rate was nearly insensitive to ionic strength. The membrane-binding properties were relatively unchanged after thrombin digestion of factor V or after long-term protein storage which resulted in loss of procoagulant activity. Other proteins of the prothrombinase reaction greatly decreased the rate of factor Va-membrane dissociation. At protein saturation, factor V increased the hydrodynamic radius of phospholipid vesicles by 11.4 nm. In contrast, factor Va increased the hydrodynamic vesicle radius by only about 5 nm. The mass of membrane-bound protein was comparable for both proteins.