= Fibrin knob-hole interaction
A <fibrin knob-hole interaction> is the binding of an exposed short <peptide> segment on one <fibrin> molecule to a complementary pocket on another molecule. <Thrombin> removes <Fibrinopeptide A> to expose the A knob and <Fibrinopeptide B> to expose the B knob. The corresponding a and b holes lie in different distal <protein domains>. These intermolecular contacts help assemble a <fibrin> network.<Polymerization of fibrin: specificity, strength, and stability of knob-hole interactions studied at the single-molecule level by Rustem I. Litvinov and others>[Figure1: knob exposure and the locations of complementary holes]{ref}
A decisive test uses a small competing <peptide>. The synthetic sequence Gly-Pro-Arg-Pro mimics the exposed A-knob end: it binds <fibrinogen> and its distal fragment D and inhibits <fibrin> <polymerization>. A soluble imitation can occupy a pocket without connecting two full-sized molecules, disrupting the assembly.<Synthetic peptide derivatives that bind to fibrinogen and prevent the polymerization of fibrin monomers by Andrew P. Laudano and Russell F. Doolittle>[Abstract: binding to fibrinogen/fragment D, lack of binding to fragment E, and inhibition]{ref}
<Optical tweezers> later measured individual rupture events, with forces around $125$–$130\,\mathrm{pN}$ under the study's conditions. Removing the required sites or adding the A-knob mimic suppressed the strong events. The assay supported specific A:a binding; it did not detect a comparable B-mediated force contribution under those conditions.<Polymerization of fibrin: specificity, strength, and stability of knob-hole interactions studied at the single-molecule level by Rustem I. Litvinov and others>[Abstract: rupture-force distributions and peptide-inhibition controls]{ref}
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