Antibodies As Probes Of The Conformation Of Fibrinogen
Thrombosis and Haemostasis
In order for an antibody to react with a protein, the recognized antigenic determinant must be available at the hydrated surface and be a conformation permissive for antibody binding. The validity of the capacity of antibodies to identify surface-oriented regions has been verified with a number of protein molecules of known three-dimensional structure. Based upon this concept, we have utilized antibodies to defined structural loci of fibrinogen to identify exposed and inaccessible regions of
... sible regions of the molecule. The following information has been derived from such studies. I) Regions of the molecule including the C-terminal of the A α chain and the N-terminal of the B β chain, which are accessible to proteases such as plasmin, are surface oriented. The accessibility of protease-sensitive regions to antibodies would be anticipated and serves to validate the approach. 2) The D and E domains are capable of interacting with one another. This interaction qppears to be a native conformational element of fibrinogen and reflects more than the mere abutting of the C-terminal of the E domain with N-terminal of D domain. 3) Fibrin degradation products are immunochemically distinguishable from fibrinogen derivatives suggesting that the fibrin transition results in conformational change. 4) Plasmic degradation is a dynamic process resulting in conformational changes in both the D and the E domains. This suggests either a spatial proximity between a number of structurally distant regions such as the C-terminal of the A α chain and the N-terminal of the γ chain or a remarkable capacity to transmit allosteric effects from one domain to another. 5) Antigenic determinants (μ 10) located within the N-terminal two-thirds of the γ chain are unreactive with antibody suggesting an interiorization or unique conformation which hinders antibody binding. One interpretation of this latter observation is that the γ chain provides a "nucleation center" which is circumscribed by the other Constituent chains of fibrinogen. These results delineate a set of constraints on the conformational format of fibrinogen in solution. With additional antibody probes to specific structural loci within the molecule, it should be possible to establish additional constraints and to further define exposed and interiorized regions of fibrinogen.