Prothrombin-membrane interaction. Effects of ionic strength, pH, and temperature

Abstract

The effects of ionic strength, pH and temperature on 3 separate aspects of [bovine] prothrombin-phospholipid membrane binding were studied. The 3 parameters include a Ca-dependent protein transition, a Ca-membrane interaction and binding of Ca-saturated protein to a Ca-saturated phospholipid membrane. The results are consistent with Ca binding to carbonyl groups in the protein and to phosphate in the phospholipids. These interactions show the expected pH profiles and sensitivity to ionic strength. Temperature effects indicate a small negative enthalpy change for each process. Binding of Ca-saturated membrane shows very little variation between pH 6 and pH 9, is accompanied by no detected enthalpy change and is relatively insensitive to ionic strength. Ionic Ca bridging between the protein and membrane is apparently unimportant. A chelation model for prothrombin-membrane binding is proposed where the 2 interacting species have no net charge; ligands on the protein complete the coordination sphere of membrane-bound Ca and vice versa.