Specificity of Soluble Phospholipid Binding Sites on Human Factor X_a^†

We explore here the specificities of lipid regulatory sites on factor X_a that affect the rate of factor X_a-catalyzed prothrombin activation. We examined a series of 11 phosphatidylserine (PS) analogues in order to map the structural features of a lipid molecule that are needed to elicit both the structural response and the full increase in activity that can be obtained with the PS molecule. Our observations are interpreted in terms of a model in which factor X_a is regulated by sequential occupancy of a pair of linked lipid binding sites, each of which have different minimum ligand structural requirements to induce structural changes. The first site is apparently of higher affinity and recognizes diacylglycerol (DAG) as a minimal binding structure. The second site is occupied with an affinity slightly less than the first site only when the first is occupied, but binds PS with very low affinity otherwise. It recognizes glycerophosphorylserine (GPS) as the minimal ligand. To test this interpretation, experiments were performed in which more than one lipid species was present. It was necessary to invoke the existence of factor X_a species containing different lipids at each site, each having different structural and functional responses. For optimal activity enhancement, both binding sites must be occupied, the first by PS, although the second can be occupied with other lipids.