No abstract

Fibrinogen is a soluble, multi-subunit and multi-domain dimeric protein, which, upon its proteolytic cleavage by thrombin, is converted to insoluble fibrin initiating polymerization that substantially contributes to clot growth. The consentaneous structural view of the soluble form of fibrinogen is relatively straight and rigid-like. However, fibrinogen contains numerous, transiently-accessible "cryptic" epitopes for hemostatic and immunologic proteins, suggesting that fibrinogen exhibits conformational flexibility, which may play functional roles in its temporal and spatial interactions. Hitherto, there has been limited information on the solution structure and internal flexibility of soluble fibrinogen. Here, utilizing an integrative, biophysical approach involving temperature-dependent hydrogen-deuterium exchange mass spectrometry, small angle X-ray scattering, and negative stain electron microscopy, we present a holistic, conformationally dynamic model of human fibrinogen in solution. Our data reveal a high degree of internal flexibility, accommodated by four major and distinct flexible sites along the central axis of the protein. We propose that the fibrinogen structure in solution consists of a complex, conformational landscape with multiple local minima, rather than a single, rigid topology. This is further supported by the location of numerous point mutations that are linked to dysfibrinogenemia, and post-translational modifications, residing near fibrinogen flexions. This work provides a molecular basis for the structural "dynamism" of fibrinogen that is expected to influence the broad swath of functionally diverse macromolecular interactions and fine-tune the structural and mechanical properties of blood clots.