Influence of Calcium Ion on the Binding of Fibrin Amino Terminal Peptides to Fibrinogen

The affinity of the amino terminal tetrapeptide of the β chain of fibrin, Gly-His-Arg-Pro, for fibrinogen dramatically increases in the presence of 2 millimolar calcium ion. In contrast, there is no significant increase in the affinity of peptides beginning with the amino terminal sequence of the fibrin α chain, Gly-Pro-Arg, in the presence of calcium ions, although the number of binding sites increases. In the latter case, the increased number of sites is due to the α chain analogs binding to the site ordinarily occupied by the β chain analogs. These results indicate that structures at the amino terminus of the fibrin β chain play a more important role in fibrin polymerization when calcium ions are present.