 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.^[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.^[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.