= Fibrinogen
<Fibrinogen> is a soluble <blood plasma> <protein> that supplies the building material for <fibrin> during <coagulation>. <Thrombin> cleaves small <fibrinopeptides> from <fibrinogen>, exposing sites that allow the resulting <fibrin> units to associate. The cleavage is a chemical trigger for assembly; forming the network also requires interactions between different <protein> molecules.<Thrombin EC 3.4.21.5 in IUBMB Enzyme Nomenclature>[Reaction and comments: fibrinopeptide release and fibrin formation]{ref}
A single-molecule experiment used <optical tweezers> to bring prepared <protein> partners together and measure their separation forces. Exposing the “A” binding knobs in <fibrin> produced strong specific interactions with complementary sites in <fibrinogen> or its end fragment. A competing <peptide> that mimicked the exposed knob suppressed those interactions, providing a control for binding-site specificity.<Polymerization of fibrin: specificity, strength, and stability of knob-hole interactions studied at the single-molecule level by Rustem I. Litvinov and others>[2005, abstract: laser tweezers, A-knob exposure and competing peptide]{ref}
This explains how a solution of precursors can become a connected <polymer> network after limited <proteolysis>. The measurements concern defined molecular contacts; a complete clot's mechanics additionally depend on how many such contacts form and how the <fibrin> network is organized.
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