The binding of calcium to bovine fibrinogen

The determination of the number of calcium binding sites of fibrinogen was carried out by means of equilibrium experiments. At pH 7.5 fibrinogen has 3 binding sites of high affinity and several binding sites of low affinity. In the presence of MgCl2 (10(-2)M) the sites of low affinity are eliminated, suggesting that they are not specific and due to weak interactions. Study of the effect of the pH have demonstrated that the 3 sites of high affinity are not identical. At pH values below 7.5 one site is eliminated. This could be due either to an abnormal protonation or to a conformational change. No cooperativity between the sites was found. Partial identification of the binding site by means of direct titration of the calcium induced proton release have shown that the calcium is tightly bound through a chelate system. From the apparent dissociation constant obtained a possible involvement of histidine residues in this chelate system is suggested.