Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter VI. BIOCHEMISTRY OF PHYSIOLOGICAL FUNCTIONS AND SPECIALIZED TISSUES

CHAPTER 29. BIOCHEMISTRY OF BLOOD COAGULATION AND FIBRINOLYTIC SYSTEMS

29.2. BLOOD COAGULATION SYSTEM: COMPONENTS, ACTIVATION MECHANISMS

The Blood Coagulation system includes enzymatic and non-enzymatic Proteins of plasma and Tissues, supramolecular complexes, and Calcium Ions.

The process of blood clotting and blood clot formation represents a cascade of sequential enzymatic Reactions Catalyzed by specialized proteins known as coagulation factors. In the coagulation cascade, each protein factor activates the subsequent component following a proenzyme (inactive) —► enzyme (active) principle, which ensures a sequential, avalanche-like Amplification of the process and triggers a rapid protective response to vascular injury.

Nomenclature (numeration and trivial designations) of blood coagulation factors (According to the recommendations of the International Nomenclature Committee):

Factor I (fibrinogen).

Factor II (prothrombin).

Factor III (tissue thromboplastin).

Factor IV (calcium ions).

Factor V (proaccelerin).

Factor VII (proconvertin).

Factor VIII (antihemophilic globulin A, von Willebrand factor).

Factor IX (antihemophilic globulin B, Christmas factor).

Factor X (Stuart-Prower factor).

Factor XI (Rosenthal factor, or plasma thromboplastin antecedent).

Factor XII (Hageman factor).

Factor XIII (fibrin-stabilizing factor).

Along with the aforementioned main coagulation factors found predominantly in Blood Plasma, there are platelet coagulation factors that participate at various stages of vascular-platelet hemostasis, blood coagulation, and Fibrinolysis. Platelet coagulation factors are typically designated by the Latin letter P (from platelets) with numerical indices: P₁-P₁₁.

Class="center">Mechanisms of activation and functioning of the blood coagulation cascade system

The blood coagulation cascade can be triggered through the activation of two alternative mechanisms: the intrinsic and extrinsic pathways (mechanisms) of coagulation. These pathways differ in their initial reactions and converge into a common final pathway of coagulation, which begins with the activation of factor X.

The central molecular event in the execution of blood coagulation is the generation of active factor X (Stuart-Prower factor).

Factor X is a Ca²⁺-dependent glycoprotein synthesized in the Liver with the participation of vitamin K. It consists of light and heavy polypeptide chains connected by Disulfide Bonds and is activated via Limited proteolysis. Activated factor X (f.X_a) is a Serine proteinase that converts prothrombin (f.II) into active Thrombin (f.II_a), which is essential for transforming fibrinogen (f.I) into fibrin—the foundation of the fibrin clot or thrombus.

The interrelation between the intrinsic, extrinsic, and common final pathways in blood coagulation is illustrated in Fig. 29.1.

Fig. 29.1. Diagram illustrating the relationship between the intrinsic, extrinsic, and common pathways of coagulation.

The intrinsic pathway of coagulation is triggered when blood interacts with a "foreign" or wettable surface. In vivo, this surface is provided by the endothelium (when blood flow is sluggish near an abnormal vascular wall) or by adhered platelets. In vitro, such wettable surfaces are typically the Glass walls of containers that come into contact with blood.

Blood coagulation via the intrinsic pathway proceeds through the following sequential steps:

1. Activation of factor XII (Hageman factor) — occurs upon contact between blood and a surface, facilitated by the proteolytic action of kallikrein, which cleaves a 28 kDa peptide fragment from factor XII, yielding the active factor XII_a.

2. Activation of factor XI — mediated by factor XII_a, which converts factor XI into factor XI_a (active plasma thromboplastin).

3. Activation of factor IX (Christmas factor) — driven by factor XI_a, which cleaves a 9 kDa peptide fragment from factor IX to produce the active serine protease, factor IX_a. This process requires Ca²⁺ ions.

4. Activation of factor X — in the intrinsic pathway, factor X is activated through the proteolytic action of factor IX_a. Factor IX_a cleaves peptide fragments from factor X, generating active serine protease forms known as factor X_a (comprising two molecular variants, X_a-α and X_a-β). This reaction requires the modifier protein factor VIII (antihemophilic globulin A) and Ca²⁺ ions.

5. Activation of factor II (prothrombin) — The conversion of prothrombin to thrombin (factor II_a) is catalyzed by protease X_a in the presence of Ca²⁺ ions and the active form of proaccelerin (factor V₃). Prothrombin activation takes place on the platelet surface, utilizing platelet membrane Phospholipids; platelet accelerin (factor V₃) is essential for binding factor X_a, accelerating the reaction rate by tens of thousands of times. The resulting product is thrombin, the primary enzyme of the blood coagulation system. Thrombin is a serine protease with a molecular mass of 34 kDa and Trypsin-like activity. Human prothrombin consists of two chains—light and heavy (comprising 36 and 264 amino acid residues, respectively); the peptide chains in the thrombin molecule are bound to glucosamine and sialic acid.

6. Conversion of fibrinogen to fibrin — the final step of the coagulation cascade. Fibrinogen is a glycoprotein with a molecular mass of 340 kDa, consisting of six polypeptide chains (two Aα-chains, two Bβ-chains, and two γ-chains; its molecular Structure is denoted as (Aα)₂ (Bβ)₂γ₂). Thrombin cleaves four -Arg-Gly- peptide bonds within the fibrinogen molecule, releasing free fibrin monomer Peptides.

Fibrin monomer molecules spontaneously aggregate to form long, insoluble strands of fibrillar fibrin, thereby forming a fibrin clot. Subsequently, the enzyme transglutaminase (factor XIII) cross-links individual fibrin monomer molecules to form a fibrin polymer. This stabilization of the fibrin meshwork is known as blood clot retraction.

The Extrinsic pathway of coagulation is initiated by injury to Blood Vessels and surrounding tissues, resulting in the release of lipoprotein tissue factor into the bloodstream, referred to in modern literature as factor III (tissue thromboplastin).

Acting as a modifier protein, factor III triggers the activation of factor VII (proconvertin), converting it into the active protease factor VII_a (convertin). Together with Ca²⁺ ions, convertin activates factor X, thereby initiating the common pathway of coagulation as described above:

Following the Formation of tissue thromboplastin—which is the rate-limiting factor in extrinsic blood coagulation—subsequent clotting proceeds extremely rapidly, forming a blood clot within mere seconds.

The overall scheme of the blood coagulation cascade encompassing both the intrinsic and extrinsic pathways is presented in Figure 29.2.

Fig. 29.2. Cascade of blood coagulation reactions via the intrinsic (a, activation of factor XII) and extrinsic (b, formation of tissue thromboplastin, i.e., factor III) pathways.

The Role of Vitamin K in Coagulation Cascade Reactions

Vitamin K is a fat-soluble vitamin existing in two vitamer forms (K₁ and K₂) and acts as an essential cofactor in coagulation reactions. In its absence, the synthesis of functionally active coagulation factors—factors II, VII, IX, and X—is impaired. The biochemical mechanism of vitamin K action involves its participation in an enzyme system that converts glutamic acid residues in the peptide chains of these coagulation factors into γ-carboxyglutamic acid. Specifically, γ-carboxylation of glutamate occurs at positions 7, 8, 15, 17, 20, 21, 26, 27, 30, and 33 within the prothrombin molecule.

γ-Carboxylation of protein coagulation factors enhances their affinity for Ca²⁺ ions, which are necessary for binding these proteins to membrane phospholipids and triggering the coagulation cascade. Vitamin K deficiency leads to increased bleeding tendencies, and decalcified blood fails to clot.

Inherited Disorders of Blood Coagulation

Impairments in the blood-clotting system—known as coagulopathies—can arise from Genetic Defects in the Synthesis of Plasma or platelet coagulation factors. Clinically, they are characterized by reduced blood clotting activity and a predisposition to hemorrhages. The most common inherited coagulopathies include:

1. Hemophilias — coagulopathies caused by the hereditary absence of one or more plasma clotting factors. Hemophilias manifest as severe bleeding episodes that occur even after minor vascular injuries and pose a life-threatening risk. They are classified into:

- hemophilia A (von Willebrand disease) — develops due to a deficiency of factor VIII, the synthesis of which is linked to the X chromosome; it is inherited in a recessive manner and manifests in males;

- hemophilia B (Christmas disease) — develops due to impaired synthesis of factor IX;

- hemophilia C — develops due to impaired synthesis of factor XI, clinically characterized by less severe bleeding than hemophilias A and B.

2. A-(hypo-)fibrinogenemia — characterized by the complete or partial absence of fibrinogen in the plasma. The condition is inherited as an autosomal recessive disorder, presenting with severe bleeding due to a complete lack of blood clotting ability.

3. Dysfibrinogenemia — coagulopathies caused by Amino Acid Substitutions in the Introduction/19.html">Primary Structure of fibrinogen molecules. These abnormal fibrinogen molecules exhibit an altered conformation, which hinders the normal conversion of fibrinogen into fibrin.