Dynamic light scattering study of fine semiflexible fibrin networks.

Fine fibrin networks have been investigated using the dynamic light scattering (DLS) technique. At the shortest delay times, t, the dynamic structure factor s(q,t) is found to depend on time according to an exponential function and, at intermediate delay times (up to 1 ms), to a stretched exponential. At longer times (t > 1 ms), a progressively increasing deviation from the stretched exponential behaviour has been observed. These results are in agreement with the theoretical predictions of a recently forwarded model for semiflexible polymers in semidilute solutions [K. Kroy and E. Frey, Physical Review E 55 (1996) p. 3092.], despite the fact that fibrin networks are made up of crosslinked branched polymers. The model, moreover, allows the calculation from the initial decay rate gamma q(0) of the average diameter of the fibrin fibres, a. The value of a = 30 +/- 2 nm, at fibrinogen concentration c(f) = 1676 nM and ionic strength 0.5, fits well into the data reported in electron microscopy studies. A concentration dependence of the average diameter of the fibrin fibres has been observed which saturates at the highest concentrations. The diameter of fibrin fibres is an important component in determining the physical properties of the fibrin networks, since the radial growth of fibrin fibres is limited by twisting during protofibrils aggregation. Our results indicate the importance of taking into account intrinsic semiflexibility in studying the physical properties of 'real' polymers and emphasize the high sensitivity of the DLS technique to investigate biological polymers also at the lowest concentrations where the systems are very fragile.

Selective loss of fibrinogen clotting in a loop-less thrombin.

The autolysis loop of thrombin comprises nine residues, from Glu146 to Lys149e, five of which (Ala149a-Lys149e) are inserted relative to trypsin and chymotrypsin. Deletion of the insertion Ala149a-Lys149e causes no significant change in the properties of the enzyme, except for a slight enhancement of protein C activation. Deletion of the entire Glu146-Lys149e loop, however, reduces fibrinogen clotting 240-fold, but decreases protein C activation only 2-fold. This loop-less mutant is de facto an exclusive activator of protein C, having lost the primary procoagulant function of thrombin. Because the autolysis loop affects fibrinogen binding, but not protein C activation, it provides a target for new drugs designed to suppress exclusively the procoagulant activity of thrombin.