Evidence for the localization of a calcium-binding site in the amino-terminal disulphide knot of fibrin (ogen).

Human fibrinogen has three calcium-binding sites. Two of these are related to the carboxyl-terminal 13K ends of the gamma-chains. In order to investigate the possible localization of the third binding site in the central domain of fibrinogen, the amino-terminal disulphide knots (NDSK) of human fibrinogen and fibrin were prepared by cyanogen bromide degradation. Their calcium-binding properties were investigated by equilibrium dialysis. The NDSK's of fibrinogen and fibrin appeared to have 0.8 binding sites (Kd = 1.8 X 10(-4) M) and 0.9 binding sites (Kd = 3 X 10(-4) M), respectively. We conclude that the third calcium-binding site of fibrin(ogen) is located in the central symmetrical domain of the molecule (NDSK).