Influence of calcium ion on the binding of fibrin amino terminal peptides to fibrinogen.

The affinity of the amino terminal tetrapeptide of the beta 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 alpha 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 alpha chain analogs binding to the site ordinarily occupied by the beta chain analogs. These results indicate that structures at the amino terminus of the fibrin beta chain play a more important role in fibrin polymerization when calcium ions are present.