Binding of Calcium to Fibrinogen: Some Related Properties
The calcium binding properties of bovin fibrinogen have been studied using equilibrium dialysis method. At pH 7.5 fibrinogen has 3 specific calcium binding sites of high affinity and several non specific binding sites of low affinity. Direct titration of the calcium induced proton release indicates that the binding center is a chelate. Thermal an acid denaturation is found to be markedly influenced by the presence of Ca++, suggesting that structural features are related to the binding. However the circular dichroism spectra show that no generalized conformational change is induced when Ca++ is bound to the protein.The plasminic digestion of fibrinogen is also found to be specificaly influenced by Ca++. The velocity of the initial cleavages is slightly reduced in the presence of calcium. It is therefore suggested that the C-terminal part of the Aα chain is involved in the binding.Considering the dimeric structure of the fibrinogen molecule, the presence of only 3 calcium binding sites of high affinity suggests the existence of “salt bridges” between the constitutive polypeptide chains.