Metal ion interactions with bovine prothrombin and prothrombin fragment 1. Stoichiometry of binding, protein self-association, and conformational change induced by a variety of metal ions

The interaction of metal ions with bovine prothrombin fragment 1 was investigated by examining two ion-induced phenomena: a conformational change measured by intrinsic protein fluorescence change or circular dichroic spectral changes and protein self-association. All metal ions tested induced the conformational change. In contrast, self-association varied depending on the metal ion. Magnesium caused essentially no self-association, calcium caused partial (50-75%) dimerization, manganese and cadmium led to a full dimer, and lanthanides caused hexamer formation. Despite these widely differing states of self-association, titration experiments indicated six ions bound per monomer in each case. That is, six magnesium or calcium ions were bound during the conformational change; three manganese ions caused the conformational change, but a total of six manganese ions were required to form a dimer; three lanthanide ions caused the conformational change, but a total of six ions were required to form the hexamer. On the basis of these results and previous data it is postulated that prothrombin fragment 1 contains six metal ion binding sites regardless of its state of aggregation. Two or three ions induce the conformational change, and these sites generally are filled with higher affinity than the sites causing self-association. However, the sequence in which the sites are filled depends on the metal ion, thereby causing overlap or separation of conformational change and self-association. The parent molecule, prothrombin, showed very little self-association in the presence of calcium or manganese. Lanthanides induced formation of a prothrombin trimer. The major difference between prothrombin and its fragment 1 segment is the failure of the former to dimerize. In this one respect it is concluded that fragment 1 is not an ideal model system for studying cation interaction with vitamin K dependent plasma proteins. Hexamers of prothrombin fragment 1 may be formed by dimerization of lanthanide-induced trimers.