Abnormal platelet function Abnormal platelet phosphatidylserine exposure 2017-06-10T12:34:11Z An abnormality of phosphatidylserine (PS) on activated platelets. PS is normally located on the cytoplasmic face of the resting platelet membrane but appears on the plasma-oriented surface of discrete membrane vesicles that derive from activated platelets. Thrombin, the central molecule of coagulation, is produced from prothrombin by a complex (prothrombinase) between factor Xa and its protein cofactor (factor V(a)) that forms on platelet-derived membranes. This complex enhances the rate of activation of prothrombin to thrombin by roughly 150,000 fold relative to factor X(a) in solution. The negatively charged surface of PS-containing platelet-derived membranes is at least partly responsible for this rate enhancement.