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. 

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

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

Thromboplastin Formation 400 Conversion of Prothrombin to Thrombin 401 Properties of Fibrinogen 403 Conversion of Fibrinogen to Fibrin 404 Polymerization of Fibrin 405... 

Fibrinogen, Fibrin, and Clotting. The key feature of blood clotting is the conversion of soluble fibrinogen into insoluble fibrin, a skeleton of protein fibers. Certainly not all details of blood clotting have been explained, but at present ten different factors, designated by Roman numerals, are implicated. We shall restrict ourselves to the principal components of the system. 

VI (accelerin). The accelerins in turn activate prothrombin. In this way, all prothrombin is very quickly transformed to thrombin, which can then catalyze the conversion fibrinogen —> fibrin, as mentioned. Initially, monomeric fibrin (or activated fibrinogen) is produced, w hich then polymerizes spontaneously. The last reaction is the clotting phase. 

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. 

Factor II. Prothrombin is a vitamin K-dependent compound synthesized by the Hver. When prothrombin is activated it is cleaved at two sites, resulting in a two-chain molecule linked by a disulfide bond that has a molecular weight of 37,000 daltons. Thrombin is the serine protease that initiates the conversion of soluble fibrinogen into fibrin. 

FIGURE 15.5 The cascade of activation steps leading to blood clotting. The intrinsic and extrinsic pathways converge at Factor X, and the final common pathway involves the activation of thrombin and its conversion of fibrinogen into fibrin, which aggregates into ordered filamentous arrays that become cross-linked to form the clot. 

Fibrin is formed from fibrinogen synthesized by the liver and secreted into the circulation. The conversion of fibrinogen to fibrin is initiated by a serine protease, thrombin. Thrombin, at the same time, can activate a transglutaminase enzyme, factor XIII present in... 

Heparin inhibits the formation of fibrin clots, inhibits the conversion of fibrinogen to fibrin, and inactivates several of the factors necessary for the clotting of blood. Heparin cannot be taken orally because it is inactivated by gastric acid in the stomach therefore, it must be given by injection. Heparin has no effect on clots that have already formed and aids only in preventing the formation of new blood clots (thrombi). The LMWHs act to inhibit clotting reactions by binding to antithrombin HI, which inhibits the synthesis of factor Xa and the formation of thrombin. 

Ginseng Rg2 has shown inhibitory effects on platelet aggregation similar to aspirin, and R0 reportedly inhibits the conversion of fibrinogen to fibrin [27]. The amount of ginseng administered may also influence the effect(s) produced. In rats and mice, small doses of ginseng extract result in increased spontaneous motor activity, whereas larger doses produce an inhibitory effect on the central nervous system [28]. 

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