The binding of calcium to fibrinogen: influence on the clotting process.

The influence of Ca2+ on the basic reaction between thrombin and fibrinogen was investigated. The results demonstrate that: (a) A Ca2+-dependent dimeric intermediate is formed during the early step of the clotting process. This dimeric intermeidate is shown to be formed by the association of an intact fibrinogen molecule and a fibrin monomer devoid in only the peptide A, (b) Ca2+ enhances the proteolytic step as illustrated by the measure of the kinetics of H+ release at pH 8.6. On the basis of these observations it is proposed that Ca2+ catalyses the proteolysis of fibrinogen by thrombin through the formation of a Ca2+-dependent dimer.