Conformation of fibrinogen: calorimetric evidence for a three-nodular structure.

Solutions of fibrinogen show two endothermal (denaturing) transitions, at 61 degrees and at 100 degrees , when heated in a differential scanning calorimeter. Similar transitions are observed for a mixture of the fragments D and E obtained by limited proteolysis of fibrinogen. Isolated fragment E shows only a single transition, at 97 degrees . The independent thermal denaturation of these portions of fibrinogen supports the three-nodular model proposed for fibrinogen. The D and E subunits retain their characteristic denaturation behavior when fibrinogen is clotted by thrombin addition, but over a period of about one hundred times the clotting time, the denaturation temperature of the D subunit increases by 9 degrees and its enthalpy of denaturation by one-third. Since this change takes place in the absence of Factor XIII activity, and its rate is proportional to thrombin concentration, it is presumed to be mediated by a proteolytic cleavage distinct from those which liberate the A and B fibrinopeptides.