Interaction of bovine coagulation factor X and its glutamic-acid-containing fragments with phospholipid membranes. A surface plasmon resonance study.

The interaction of blood coagulation factor X and its Gla-containing fragments with negatively charged phospholipid membranes composed of 25 mol% phosphatidylserine (PtdSer) and 75 mol% phosphatidylcholine (PtdCho) was studied by surface plasmon resonance. The binding to 100 mol% PtdCho membranes was negligible. The calcium dependence in the membrane binding was evaluated for intact bovine factor X (factor X) and the fragment containing the Gla-domain and the N-terminal EGF (epidermal growth factor)-like domain, Gla-EGFN, from factor X. Both proteins show the same calcium dependence in the membrane binding. Calcium binding is cooperative and half-maximum binding was observed at 1.5 mm and 1.4 mm, with the best fit to the experimental data with three cooperatively bound calcium ions for both the intact protein and the fragment. The dissociation constant (Kd) for binding to membranes containing 25 mol% PtdSer decreased from 4.6 microm for the isolated Gla-domain to 1 microm for the fragments Gla-EGFN and Gla-EGFNC (the Gla-domain and both EGF-like domains) fragments and to 40 nm for the entire protein as zymogen, activated enzyme or in the active-site inhibited form. Analysis of the kinetics of adsorption and desorption confirmed the equilibrium binding data.