Terbium probe of calcium-binding sites on the prothrombin-membrane complex

Terbium was used as a probe of Ca²⁺-binding sites on the prothrombin-phospholipid complex. Stoichiometric titrations of prothrombin binding to phospholipid vesicles with either Tb³⁺ or Ca²⁺ showed that a minimum of 8 metal ions were needed for binding prothrombin to vesicles (3 Mn²⁺ + 5 Ca²⁺ for prothrombin or 8 Tb³⁺ for F-1). When Ca²⁺ alone was used, a total of about 11 metal ions were needed for complete binding. These stoichiometries indicated 3 classes of metal ions: one class needed to induce the conformational change, a second required for protein-membrane contact, and a third class bound at other sites on the protein that are not involved in membrane binding. By adding Tb³⁺ to solutions containing both protein and phospholipid, undesirable Tb³⁺-induced events, such as irreversible aggregation of prothrombin or vesicle fusion, were avoided. Protein-vesicle binding apparently prevented protein aggregation or vesicle fusion. The protein-vesicle binding affinity was severalfold greater in the presence of Tb³⁺ compared to Ca²⁺. CoEDTA quenching of Tb³⁺ bound to the prothrombin-phospholipid complexes indicated that all metal ions were at least partially exposed to the quencher. Some populations of Tb³⁺ showed lower quenching constants when all of the prothrombin was bound. Tb³⁺ emission lifetimes revealed that some Tb³⁺ ions in the protein-membrane complex were in a different environment from those bound to the protein alone. The results indicated that the metal ions in the prothrombin-membrane complex are relatively open to the solvent yet do affect the characteristics of the protein-membrane binding equilibrium.