Localization Of A Fibrin Polymerization Site

Abstract

The formation of a fibrin clot is initiated after the proteolytic cleavage of fibrinogen by thrombin. The enzyme removes fibrinopeptides A and B, and generates fibrin monomer which spontaneously polymerizes. Polymerization appears to occur through the interaction of complementary binding sites on the NH2- and COOH- terminal regions of the molecules since Fragment D1, encompassing the COOH-terminals of the β and γ chains, binds to thrombin-treated NDSK which contains N2-terminals of the α , β and γ chains. A peptide of 4,200 molecular weight has been isolated from the y chain remnant of fibrinogen Fragment D1 which has the ability to bind to the NH2- terminal region of fibrin monomer, thus inhibiting fibrin monomer polymerization. The peptide reduces the maximum rate and extent of the polymerization of thrombin or batroxobin fibrin monomer and increases the lag time of the reaction. The D1 peptide does not interact with NDSK, fibrinogen or Fragment D1but it binds to thrombin-treated NDSK with a Kd of 1.45 x 106 m and approximately two binding sites per molecule of NDSK have been found. Fibrin monomer formed either by thrombin or batroxobin binds approximately two molecules of D1 peptide per molecule of fibrin monomer, indicating that the complementary site is revealed by the loss of fibrinopeptide A. The NH2- terminal amino acid sequence (Thr-Arg-Trp) and the COOHterminal sequence (Ala-Gly-Asp-Val) of the D1 peptide were determined. Therefore the γ 373-410 region of the fibrinogen molecule contains a polymerization site which is complementary to the thrombin-activated site on the NH2terminal region of fibrinogen. The binding site on the Dl peptide has the characteristics of the polymerization site which is exposed and available on the COOH-terminal region of the fibrinogen molecule without any participation of thrombin.