METAL-ION INDUCED CONFORMATIONAL TRANSITIONS OF PROTHROMBIN AND PROTHROMBIN FRAGMENT-1

Circular dichroism experiments indicate that prothrombin fragment 1 undergoes essentially the same secondary structural change whether in the presence of Ca2+, Mg2+, or Mn2+. Titration with any of these metal ions results in a sigmoidal titration curve indicative of cooperative binding. Mg2+ and Ca2+ have nearly identical transition midpoints, while that for Mn2+ is 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 second transition corresponding to dimerization at high calcium concentrations. The present circular dichroism experiments show 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, indicate 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. This suggests that caution must be used in interpreting the results of metal ion binding studies using fragment 1 as an analogue for prothrombin. © 1978, American Chemical Society. All rights reserved.