In the human blood clotting process, which one of the following enzymes or enzyme-like substances plays a key role in initiating the conversion of prothrombin to thrombin?

Introduction:
Blood clotting (coagulation) is a vital protective mechanism that prevents excessive bleeding when blood vessels are injured. The process involves a cascade of reactions and several clotting factors that ultimately convert soluble fibrinogen into insoluble fibrin threads. A key early step is the conversion of prothrombin into the active enzyme thrombin. This question asks you to identify which substance is necessary to bring about this crucial conversion.

Given Data / Assumptions:
- The focus is on the enzyme or enzyme like factor important for clotting. - Options include peptidase, amylase, lipase and thrombokinase. - We assume normal clotting factor function in human blood. - The question specifically mentions the clotting of blood, not digestion or metabolism.

Concept / Approach:
Thrombokinase, also known as thromboplastin, is a clotting factor that helps convert prothrombin into thrombin in the presence of calcium ions and other factors. Thrombin then converts fibrinogen into fibrin to form a clot. Peptidases, amylase and lipase are digestive enzymes that break down proteins, carbohydrates and fats respectively, mainly in the digestive system, not in the coagulation cascade. Therefore, thrombokinase is the correct factor associated with blood clotting.

Step-by-Step Solution:
Step 1: Recall that the clotting cascade involves activation of a series of factors leading to prothrombin activation. Step 2: Understand that thrombokinase (thromboplastin) is released from damaged tissues and platelets and participates in forming prothrombin activator. Step 3: Recognise that this prothrombin activator, together with calcium ions, converts inactive prothrombin into active thrombin. Step 4: Thrombin then converts soluble fibrinogen into insoluble fibrin, which forms the mesh of the blood clot. Step 5: Note that peptidase, amylase and lipase function primarily in digestion within the gastrointestinal tract and are not directly involved in clotting. Step 6: Conclude that thrombokinase is the substance necessary to initiate the clotting pathway at the level of prothrombin activation.

Verification / Alternative check:
Physiology textbooks describe two pathways (intrinsic and extrinsic) that converge into a common clotting pathway, both producing a prothrombin activator complex that includes tissue thromboplastin (thrombokinase) and other factors. Laboratory clotting tests such as prothrombin time and activated partial thromboplastin time depend on activating this cascade. Digestive enzyme chapters, on the other hand, place amylase, lipase and peptidases in the saliva, stomach and intestinal secretions, unrelated to coagulation. These consistent descriptions confirm that thrombokinase is the correct factor for blood clotting.

Why Other Options Are Wrong:
Peptidase: A generic term for enzymes that break down peptides into amino acids; they function mainly in digestion and intracellular protein turnover, not clotting. Amylase: An enzyme that digests starch into maltose and other sugars, found in saliva and pancreatic juice, and not involved in coagulation. Lipase: An enzyme that breaks down fats into fatty acids and glycerol, mainly in digestive juices such as pancreatic lipase, not in the blood clotting cascade.

Common Pitfalls:
Students sometimes focus on the word enzyme and choose a familiar digestive enzyme like amylase or lipase without considering the context of clotting. Others may not recognise the term thrombokinase even though it appears in school textbooks. To avoid errors, always connect the keyword in the question (clotting or coagulation) with specialised clotting factors rather than general digestive enzymes.

Final Answer:
The enzyme like factor necessary to bring about the clotting of blood by converting prothrombin to thrombin is Thrombokinase.