Cross-linked gamma-chains in a fibrin fibril are situated transversely between its strands

Structural Basis of Interfacial Flexibility in Fibrin Oligomers

Fibrin is a filamentous network made in blood to stem bleeding; it forms when fibrinogen is converted into fibrin monomers that self-associate into oligomers and then to polymers. To gather structural insights into fibrin formation and properties, we combined high-resolution atomic force microscopy of fibrin(ogen) oligomers and molecular modeling of crystal structures of fibrin(ogen) and its fragments. We provided a structural basis for the intermolecular flexibility of single-stranded fibrin(ogen) oligomers and identified a hinge region at the D:D inter-monomer junction. Following computational reconstruction of the missing portions, we recreated the full-atomic structure of double-stranded fibrin oligomers that was validated by quantitative comparison with the experimental images. We characterized previously unknown intermolecular binding contacts at the D:D and D:E:D interfaces, which drive oligomerization and reinforce the intra- and inter-strand connections in fibrin besides the known knob-hole bonds. The atomic models provide valuable insights into the submolecular mechanisms of fibrin polymerization.

Fibrinogen and fibrin structure and functions

Fibrinogen molecules are comprised of two sets of disulfide-bridged Aalpha-, Bbeta-, and gamma-chains. Each molecule contains two outer D domains connected to a central E domain by a coiled-coil segment. Fibrin is formed after thrombin cleavage of fibrinopeptide A (FPA) from fibrinogen Aalpha-chains, thus initiating fibrin polymerization. Double-stranded fibrils form through end-to-middle domain (D:E) associations, and concomitant lateral fibril associations and branching create a clot network. Fibrin assembly facilitates intermolecular antiparallel C-terminal alignment of gamma-chain pairs, which are then covalently 'cross-linked' by factor XIII ('plasma protransglutaminase') or XIIIa to form 'gamma-dimers'. In addition to its primary role of providing scaffolding for the intravascular thrombus and also accounting for important clot viscoelastic properties, fibrin(ogen) participates in other biologic functions involving unique binding sites, some of which become exposed as a consequence of fibrin formation. This review provides details about fibrinogen and fibrin structure, and correlates this information with biological functions that include: (i) suppression of plasma factor XIII-mediated cross-linking activity in blood by binding the factor XIII A2B2 complex. (ii) Non-substrate thrombin binding to fibrin, termed antithrombin I (AT-I), which down-regulates thrombin generation in clotting blood. (iii) Tissue-type plasminogen activator (tPA)-stimulated plasminogen activation by fibrin that results from formation of a ternary tPA-plasminogen-fibrin complex. Binding of inhibitors such as alpha2-antiplasmin, plasminogen activator inhibitor-2, lipoprotein(a), or histidine-rich glycoprotein, impairs plasminogen activation. (iv) Enhanced interactions with the extracellular matrix by binding of fibronectin to fibrin(ogen). (v) Molecular and cellular interactions of fibrin beta15-42. This sequence binds to heparin and mediates platelet and endothelial cell spreading, fibroblast proliferation, and capillary tube formation. Interactions between beta15-42 and vascular endothelial (VE)-cadherin, an endothelial cell receptor, also promote capillary tube formation and angiogenesis. These activities are enhanced by binding of growth factors like fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor (VEGF), and cytokines like interleukin (IL)-1. (vi) Fibrinogen binding to the platelet alpha(IIb)beta3 receptor, which is important for incorporating platelets into a developing thrombus. (vii) Leukocyte binding to fibrin(ogen) via integrin alpha(M)beta2 (Mac-1), which is a high affinity receptor on stimulated monocytes and neutrophils.