The Fibrinogen-Fibrin Conversion

The Fibrinogen-Fibrin Conversion Harold A. Scheraga and Michael Laskowski, Jr. 

Limited Proteolysis and Aggregation in the Fibrinogen-Fibrin Conversion... 

Essentially all the experiments on the fibrinogen-fibrin conversion to be discussed here were carried out with the less pure preparations of Seegers and Alkjaersig (1956) before the introduction of the Rasmussen (1955) procedure for the purification of thrombin. It is hoped that the traces of impurities thereby introduced do not vitiate any of the conclusions derived from these experiments. Further discussion of the earlier work on thrombin will be found in the reviews of Seegers (1955) and Scheraga and Laskowski (1957). 

We may thus summarize the reaction scheme for the fibrinogen-fibrin conversion in terms of the following steps, each of which is thought to be reversible (Laskowski et al, 1952). 

As indicated in Chapter III, step 1 of the fibrinogen-fibrin conversion is an example of a limited proteolytic reaction in which hydrolysis does not go to completion. Side-chain hydrogen bonding may stabilize the peptide bond in the manner indicated in Chapter III. We shall therefore discuss the reversibility of step 1 and the thermodynamic parameters for the equilibrium (Laskowski et al., 1960b). As in the case of the kinetic experiments discussed in Section 5b, the medium used was 1 molar NaBr at pH 5.3 to prevent polymerization of fibrin monomer, and the analysis for f was carried out using TAMe as a thrombin inhibitor, as already mentioned The equilibrium position was approached from both directions. 

Fig. 83. Sedimentation patterns of protein species involved in the fibrinogen-fibrin conversion. The solvent in all cases is 1 Af NaBr. A fibrinogen, F, at pH 6.3 B Fibrin monomer, f, at pH 5.3 (the same pattern is obtained either for a solution of fibrin at pH 6.3 or for a mixture of thrombin and fibrinogen at pH 5.3) C fast and slow peak pattern in a system of intermediate polymers (the same pattern is obtained either for a solution of fibrin at pH 6.1 or for a mixture of thrombin and fibrinogen at pH 6.1) (Donnelly et al., 1955).

In conclusion, the fibrinogen-fibrin conversion provides an excellent example of a set of reactions which illustrate the role of side-chain hydrogen bonding in limited proteolysis and in protein association. Further work on step 3 may indicate to what extent the ideas of Chapter IV may be applicable. Finally, the key role which side-chain hydrogen bonding can play, not only in denaturation, but also in the modification of pK s, in limited proteolysis, and in protein association, should be noted. 

Thrombin, the two-chain derivative of the prothrombin molecule, has a molecular weight of approximately 37,000 daltons. Its proteolytic properties induce the conversion of fibrinogen to fibrin to produce the initial visible manifestation of coagulation, the soluble fibrin clot. In addition, thrombin influences the activity of Factors V, VIII, and XIII and plasmin. Thrombin affects platelet function by inducing viscous metamorphosis and the release reaction with subsequent aggregation. 

Structural Aspects of the Fibrinogen to Fibrin Conversion R. F. Doolittle... 

Figure 9.7. Diagrammatic representation of the fibrinogen molecule and its conversion to the soft clot of fibrin. Reproduced (in modified form) by permission from Textbook of Biochemistry with Clinical Correlations (3rd Ed.) Devlin (1992). This material is used by permission of John Wiley Sons, Inc.

Doolittle, R. F. (1973). Structural aspects of the fibrinogen to fibrin conversion. Adv. Protein Chem. 27, 1-109. 

During the conversion of fibrinogen to fibrin, 20% of the sialic acid is released in a bound form. Furthermore, preincubation of fibrinogen with neuraminidase enhances the effect of thrombin. Attempts have been made to establish which molecular fraction is susceptible to the action of thrombin. We have already seen that the polysaccharide must be close to the site of action of thrombin on the fibrinogen molecule. The physiological role of the released polypeptide has not been elucidated, but it has been observed that fibrino-peptide B sensitizes smooth muscle to stimulation. 

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