Europium(III) binding to bovine prothrombin residues 1-39 and to bovine prothrombin fragment 1.

Changes in the Eu3+ luminescence decay constant observed for Eu3+ bound to prothrombin fragment 1 and the peptide containing residues 1-39 of prothrombin result solely from changes in the number of water molecules in the primary hydration sphere of the ion. Highly coordinated sites which bind Eu3+ in a different manner from simple Gla-containing peptides exist on both the fragment 1 and 1-39 molecules. Metal ions bound at some of these sites do not appear to undergo rapid exchange with metal ions in solution. The binding of other lanthanide ions and calcium ions in the presence of europium ions cause a change in the conformation of the macromolecules, resulting in even tighter coordination of fragment 1 or residues 1-39 to Eu3+. Hydrophilic groups other than the actual Eu3+ binding sites on fragment 1 help to maintain its water solubility when complexes to Eu3+ or Ca2+. Because the 1-39 peptide does not contain as many of these hydrophilic groups, the peptide precipitates upon binding to di- and trivalent cations. Self-association of the 1-39 molecules appears to occur at high peptide concentrations which result in a peptide concentration effect on Eu3+ binding not seen in fragment 1. pH titration yields results which suggest a role for Gla carboxyl groups in Eu3+ complexation to both fragments 1 and 1-39. Although the two molecules are not identical with regard to Eu3+ binding behavior, enough similarities exist that residues 1-39 appear to be a reasonable model of the fragment 1 molecule for the purposes of metal ion complexation studies.