Binding of human blood-coagulation Factors IXa and X to phospholipid membranes

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Abstract
A simple centrifugation technique was developed to study the interaction of human coagulation Factors IXa and X with phospholipid membranes. In the presence of Ca2+, equimolar phosphatidylserine/phosphatidylcholine membranes form tight complexes with Factor X (KD = 2.8 .times. 10-8 M); the KD is independent of the phospholipid concentration. Binding sites are available for about 2 mmol of Factor X/mol of phospholipid. Factor IXa has a slightly higher affinity for the phospholipid membrane (KD = 1.2 .times. 10-8 M), and competes with Factor X for binding. The experimentally observed competition between Factor X and Factor IXa is in agreement with a model that describes the binding of 2 distinct ligands to a single class of independent binding sites.