Blood clotting cascade mechanism

A number of iron-containing, ascorbate-requiring hydroxylases share a common reaction mechanism in which hydroxylation of the substrate is linked to decarboxylation of a-ketoglutarate (Figure 28-11). Many of these enzymes are involved in the modification of precursor proteins. Proline and lysine hydroxylases are required for the postsynthetic modification of procollagen to collagen, and prohne hydroxylase is also required in formation of osteocalcin and the Clq component of complement. Aspartate P-hydroxylase is required for the postsynthetic modification of the precursor of protein C, the vitamin K-dependent protease which hydrolyzes activated factor V in the blood clotting cascade. TrimethyUysine and y-butyrobetaine hydroxylases are required for the synthesis of carnitine. 

The catalytic mechanism of the subtilisins is the same as that of the digestive enzymes trypsin and chymotrypsin as well as that of enzymes in the blood clotting cascade, reproduction and other mammalian enzymes. The enzymes are known as serine proteases due to the serine residue which is crucial for catalysis (Kraut, 1977 and Polgar, 1987)... 

Antihemophilic factor, Factor VIII, a protein (Mr 265 kDa) acting as component of the blood clotting cascade in humans. Activated Factor VIIT acts as an accessory factor during the activation of Factor X Stuart factor) by activated Christmas factor IXa. Factor VIII forms a complex with the Willebrandt factor during circulation in blood [T. Halkier, Mechanisms in Blood Coagulation, Fibrinolysis and the Complement System, Cambridge University Press, 1991]. 

The concept that different structural domains on the heparin chains are principally involved for optimal activity in the foregoing interactions could not be perceived in early work on structure-activity correlations, because the activity of heparin has been most frequently evaluated only with whole-blood-clotting tests (such as the U.S.P. assay). Development of assays for specific clotting-factors (especially Factor Xa and thrombin) has permitted a better insight into the mechanism of action of heparin at different levels of the coagulation cascade. 

Activated factor X, along with Ca++ ion, factor V, and PF3 (collectively referred to as the prothrombin activator), catalyzes the conversion of prothrombin into thrombin. Thrombin then catalyzes the conversion of fibrinogen into fibrin, an insoluble, thread-like polymer. The fibrin threads form a meshwork that traps blood cells, platelets, and plasma to form the blood clot. The clotting cascade may be elicited by means of two mechanisms (see Figure 16.1) ... 

Laskey, R.A., Honda, B.M., Mills, A.D., Finch, J.T. (1978). Nucleosomes are assembled by an acidic protein which binds histones and transfers them to DNA. Nature (London) 275,416-420. Lederberg, J. Tatum, E.L. (1946). Gene recombination in E. coli. Nature (London) 158, 558. Macfarlane, R.G. (1964). An enzyme cascade in the blood clotting mechanism and its function as a biochemical amplifier. Nature (London) 202,498-499. 

Interactions between serine proteases are common, and substrates of serine proteases are usually other serine proteases that are activated from an inactive precursor [66]. The involvement of serine proteases in cascade pathways is well documented. One important example is the blood coagulation cascade. Blood clots are formed by a series of zymogen activations. In this enzymatic cascade, the activated form of one factor catalyzes the activation of the next factor. Very small amounts of the initial factors are sufficient to trigger the cascade because of the catalytic nature of the process. These numerous steps yield a large amplification, thus ensuring a rapid and amplified response to trauma. A similar mechanism is involved in the dissolution of blood clots. A third important example of the coordinated action of serine proteases is the intestinal digestive enzymes. The apoptosis pathway is another important example of coordinated action of other types of proteases. 

MacFarlane RG, An enzyme cascade in the blood clotting mechanism and its function as a biochemical amplifier. Nature 1964 202 498-499. 

There are other ways to stop blood flow from wounds, but those ways are not step-by-step precursors to the clotting cascade. For example, the body can constrict blood vessels near a cut to help stanch blood flow. Also, blood cells called platelets stick to the area around a cut, helping to plug small wounds. But those systems cannot be transformed gradually into the blood-clotting system any more than a glue trap can be transformed into a mechanical mousetrap. 

As a result of the contact of blood with none-ndothelial surfaces, several humoral and cellular systems can be activated. Exposure of blood proteins and cells to blood contacting medical devices can activate plasma proteolytic systems (coagulation (blood clotting system), fibrinolysis (process by which clot is broken down), complement cascade (a system of soluble proteins involved in microbiocidal activity and the release of inflammatory components), Kallekrein-kinin and contact systems) and at least three cellular elements (leukocytes, endothelial cells, and platelets). Contrary to the normal situations whereby these mechanisms are localized and intended to promote wound healing, activation of these systems by medical devices can result in nonlocalized systemic reactions. The preclinical and clinical assessments of hemocompatibility are designed to minimize modification of these systems. 

There is a fine line between hemorrhage and thrombosis. Clots must form rapidly yet remain confined to the area of injury. What are the mechanisms that normally limit clot formation to the site of injury The lability of clotting factors contributes significantly to the cont ol of clotting. Activated factors are short-lived because they are diluted by blood flow, removed by the liver, and degraded by proteases. For example, the stimulatory protein factors V , and VIIC are digested by protein C, a protease that is switched on by the action of thrombin. Thus, thrombin has a dual function it catalyzes the formation of fibrin and it initiates the deaclivation of the clotting cascade. 

McFarlane, R.G., An Enzyme Cascade in the Blood Clotting Mechanism, and its Function as a Biochemical Amplifier. Nature 1964, 202, 498-499. 

A blood clot is formed mainly from a network of crosslinked fibrin molecules that traps platelets, erythrocytes, and other materials to form a solid clot. The aggregation and cross-linking of fibrin is the final stage of a proteolytic cascade or pathway which is triggered by one or both of two mechanisms the intrinsic pathway and the extrinsic pathway. 

Research in many different laboratories has shown that the clotting of blood results from two series of stepwise reactions ultimately leading to the formation of a tough insoluble fibrin clot (1,2). Depending on the mechanism of activation, two clotting cascades can occur (Figure 1). Exposure to a foreign... 

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