Conformational equilibria in the gamma chain COOH terminus of human fibrinogen

The conformations of the gamma chain COOH terminus of intact fibrinogen and various fragments containing this region have been compared by an immunochemical analysis. Location of a major epitope in the sequence gamma 391-405 was successfully predicted from a hydrophobicity profile. An antibody population specific for the native epitope within the gamma 391-405 segment was isolated by immunoadsorption. Between 19.2 and 22.8% of antibodies were obtained from three different antisera, indicating that this region represents one of the major epitopes of native fibrinogen. Anti-gamma 391-405(N) antibodies were used to determine the value of Kconf, the equilibrium constant for the interconversion of the non-native and native conformations of this epitope. The measurements were done using native fibrinogen, fragments D1 and DD, gamma chain, and gamma 391-405. In addition, the effect of 5 M guanidine HCl on the conformation of fragments D1 and DD, which is known to abolish their antipolymerizing activity, was studied. Radioiodinated fibrinogen was used in the determination of Kconf, CI50%, and CIs (quantitative analytical parameters calculated from competitive inhibition radioimmunoassays) by measuring the competition between 125I-fibrinogen and the fibrinogen derivatives under study for binding to the immunochemically purified antibody. The measurements indicated that the epitope is unperturbed by iodination of fibrinogen and that 38.5% of fragment D1, 8.9% of fragment DD, 3.6% of the gamma chain, and less than 0.008% of the gamma 391-405 molecules adopt in aqueous solution the native conformation within the epitope. Denaturation of fragment D1 with 5 M guanidine HCl affected only slightly the conformation of this gamma chain determinant. More significant changes in the conformation were observed when fragment DD was denatured. The results suggest that long-range interactions are necessary for the stabilization of the native structure in the region of fibrinogen that interacts with the antibody and which is in close vicinity to the polymerization site, cross-linking site, and platelet recognition site.