Metal ion induced conformational transitions of prothrombin and prothrombin fragment 1

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Abstract
Circular dichroism experiments indicated that [bovine and human] prothrombin fragment 1 underwent essentially the same secondary structural change whether in the presence of Ca2+, Mg2+ or Mn2+. Titration with any of these metal ions resulted in a sigmoidal titration curve indicative of cooperative binding. Mg2+ and Ca2+ had nearly identical transition midpoints, while that for Mn2+ was an order of magnitude less. These results correlate well with the results of previous metal ion intrinsic fluorescence quenching experiments. Fragment 1 has previously been shown to undergo a 2nd transition corresponding to dimerization at high Ca2+ concentrations. The present circular dichroism experiments showed that this transition does not result in a gross alteration of secondary structure in the fragment 1 molecule. Studies with prothrombin, similar to those with fragment 1, indicated a similar metal ion dependent conformational change but of smaller magnitude. As apparently only the fragment 1 portion of the molecule undergoes the transition, it would appear that the covalently linked fragment 1 is constrained from attaining the same conformation as the purified entity. Caution must be used in interpreting the results of metal ion binding studies using fragment 1 as an analog for prothrombin.