Structure and Rheology of Fibrin Networks

Abstract

Fibrin, the structural component of a blood clot, is formed by polymerization of fibrinogen, a megamonomer (molecular weight 340 000) present in blood plasma at a concentration of about 0·25%. The enzyme thrombin splits off certain polypeptides from fibrinogen to expose association sites (A and B), and the resulting rod-like fibrin monomers assemble with strong non-covalent bonding in a staggered overlapping pattern to form long straight protofibrils. At high pH and ionic strength, the protofibrils associate laterally to some extent and a network is formed in which branching appears to occur by intermittent twisting of the protofibrils around each other. This gel-like ‘fine’ clot formed from pure fibrinogen is remarkably close to perfectly elastic, obeying Hooke’s Law in small shearing deformations and having very little viscoelastic loss over many decades of time scale. The mechanism for elasticity is not rubber-like and is tentatively ascribed to bending of the fibrils. At very long times, the clot undergoes creep and creep recovery with some irrecoverable deformation; the Boltzmann superposition principle is obeyed and there is no net structural damage, as evidenced by constant differential modulus measured with intermittent application of additional stress. At large strains, there is pronounced strain-hardening of the structure followed by some structural damage that appears to be largely reversible. Introduction of a certain tetrapeptide (glycine—proline—arginine—proline, GPRP), that competes with A association sites for binding, lowers the shear modulus and increases enormously the creep rate and irrecoverable deformation; the tetrapeptide apparently catalyzes interchange of intrafibril junctions so that the clot flows with a high but finite viscosity, again with no net structural damage. Under physiological conditions, another enzyme (fibrinoligase) introduces covalent bonds by a chemical reaction after the protofibrils have assembled. The resulting ligated clot experiences practically no irrecoverable deformation in creep nor susceptibility to GPRP.