Question

In the formation of a blood clot, A. proteolysis of \(\gamma\) -carboxyglutamate residues from fibrinogen to form fibrin is required. B. the clot is stabilized by the cross-linking of fibrin molecules by the action of Factor XIII, transglutaminase. C. thrombin's only role is in activation of Factor VII. D. tissue factor, Factor III or TF, must be inactivated for the clotting process to begin. E. the role of calcium is primarily to bind fibrin molecules together to form the clot.

Short answer

Answer: B. The clot is stabilized by the cross-linking of fibrin molecules by the action of Factor XIII, transglutaminase.

Step-by-step solution

Identify the statement A

Proteolysis of \(\gamma\)-carboxyglutamate residues from fibrinogen to form fibrin is required. Proteolysis is the breakdown of proteins into smaller peptides or individual amino acids. This statement implies that this process is necessary for blood clot formation. Let's keep this as a possibility and evaluate the other statements.

Identify the statement B

The clot is stabilized by the cross-linking of fibrin molecules by the action of Factor XIII, transglutaminase. Cross-linking refers to the joining of molecules via chemical links. In the context of blood clot formation, this statement suggests that Factor XIII plays a vital role in stabilizing the clot. This is an accurate statement about the coagulation process.

Identify the statement C

Thrombin's only role is in the activation of Factor VII. Thrombin is an enzyme that not only activates Factor VII but also activates other clotting factors, such as Factors V, VIII, and XI. It has a wider range of roles in the coagulation process, so this statement is not accurate.

Identify the statement D

Tissue factor, Factor III, or TF, must be inactivated for the clotting process to begin. Tissue factor is indeed necessary for the initiation of the clotting process, but it is not inactivation but rather the exposure of tissue factor that triggers the clotting cascade. This statement is not accurate.

Identify the statement E

The role of calcium is primarily to bind fibrin molecules together to form the clot. Calcium does play a significant role in the coagulation process, such as in the activation of certain clotting factors. However, calcium does not directly bind fibrin molecules together to form the clot. This statement is not accurate.

Select the correct statement

Based on our step-by-step analysis, the correct statement is B. The clot is stabilized by the cross-linking of fibrin molecules by the action of Factor XIII, transglutaminase. This statement accurately reflects the coagulation process, and none of the other statements represent the process as accurately.

Key concepts

Proteolysis in Coagulation

During blood clot formation, an important step involves the transformation of fibrinogen, a soluble plasma protein, into fibrin, an insoluble protein that forms the structural basis of a clot. This transformation is accomplished by the action of thrombin, an enzyme that catalyzes the conversion. This process, known as proteolysis, specifically refers to the cleavage of certain peptides in fibrinogen to release fibrin monomers. These monomers then polymerize to form fibrin strands, which create a mesh that traps blood cells and platelets, forming the initial scaffold of the clot.

Understanding the role of proteolysis is crucial as it initiates the stabilization of the clot through the subsequent actions of other factors such as Factor XIII. Without this initial step, the coagulation process cannot proceed effectively, leading to an inability for the body to halt bleeding.

Factor XIII

Factor XIII, also known as fibrin-stabilizing factor, is a critical enzyme in the final stages of clot formation. After the proteolytic action of thrombin on fibrinogen, Factor XIII is activated by thrombin in the presence of calcium ions. Once activated, Factor XIIIa, the active form of the enzyme, facilitates the cross-linking of fibrin strands. It catalyzes the formation of covalent bonds between the glutamine and lysine residues of different fibrin molecules.

Factor XIIIa's action reinforces the structure of the fibrin clot, making it more resistant to degradation and mechanical stress. This cross-linking process is vital for the clot's longevity and stability, ensuring that it remains intact long enough to allow the underlying tissue to heal properly.

Coagulation Process

The coagulation process, also known as the clotting cascade, is a complex series of events that lead to the formation of blood clots. It is initiated when blood vessels are damaged, exposing tissue factor (TF), which is bound to the endothelial cells lining the blood vessels. This exposure catalyzes a sequence of reactions involving clotting factors, which are mostly proteins circulating as inactive precursors.

The process can be divided into three essential phases: the initiation phase, where tissue factor and clotting Factor VIIa activate Factor X; the amplification phase, where thrombin is generated and activates platelets and additional coagulation factors; and the propagation phase, where large amounts of thrombin are generated and fibrin is formed. The culmination of this process is a stabilized clot, achieved through the cross-linking activity of Factor XIII, which serves as a temporary barrier to control bleeding and facilitate healing.