Alternative Role of B/b Knob-Hole Interactions in the Fibrin Assembly

The self-assembly of fibrin is a vital process in blood clotting, primarily facilitated by the interactions between knobs "A" and "B" in the central E region of one molecule and the corresponding holes "a" and "b" in the peripheral D regions of two other fibrin molecules. However, the precise function of the interactions between knob "B" and hole "b" during fibrin polymerization remains a subject of ongoing debate. The present study focuses on investigating intermolecular interactions between knob "B" and hole "b". We investigated the D-E-D interactions within the fibrin protofibril to accomplish this objective. Our investigation involved studying the formation of supramolecular complexes involving desAB fibrin with fibrin(ogen) fragments, specifically the D-dimer and D fragment. The research utilized analytical size-exclusion chromatography, SDS-PAGE and densitometry of SDS-PAGE images, dynamic light scattering measurements, turbidity studies, electron microscopy, and computer modeling. Our findings indicate that the interference of the D fragment into classical D-E-D interaction occurs through knob "B" of the fibrin molecule. Molecular dynamics simulations elucidate the binding of only one D region, attributed to the shift of the D-dimer toward the fibrin desAB molecule. The formation of such a complex can be considered evidence supporting the potential mechanism of the branching of protofibrils. According to this theoretical mechanism, the inclusion of the D region from an external fibrin molecule into D-E-D interactions is facilitated through "B/b" contacts.