The determination of a calcium-dependent binding constant of the bovine prothrombin Gla domain (residues 1-45) to phospholipid vesicles.

Calcium-mediated binding of the radioiodinated peptide representing residues 1-45 of bovine prothrombin to single bilayer phospholipid vesicles composed of phosphatidylserine from bovine brain and synthetic 1-palmitoyl-2-oleoyl-phosphatidylcholine (25:75 PS/PC) has been studied over peptide concentrations from 0.33 microM to 3.75 microM and at a calcium concentration of 1.0 mM. The binding isotherm for the interaction between the radioiodinated peptide and PS/PC vesicles fits a model in which there is noncooperative binding of the peptide to non-interacting sites on the phospholipid bilayer. A dissociation constant determined at these conditions is 11.8 microM compared to 1.0 microM for prothrombin fragment 1.