The Blood Clotting Cascade

The clot that seals a wound is made in a cascade: a series of reactions in which each step produces an enzyme that catalyses the next step. Because each enzyme molecule converts many substrate molecules, the response is greatly amplified and a clot forms within minutes.Step 1. Damaged tissue and platelets at the site of injury release an enzyme called thromboplastin.Step 2. Thromboplastin catalyses the conversion of the soluble plasma protein prothrombin into the enzyme thrombin. This step needs calcium ions (Ca²⁺) in the plasma as a cofactor. Prothrombin is made in the liver, and the liver needs vitamin K to make it.Step 3. Thrombin catalyses the conversion of the soluble plasma protein fibrinogen into insoluble fibrin.prothrombin → thrombin (catalysed by thromboplastin, needs Ca²⁺)
fibrinogen → fibrin (catalysed by thrombin)The fibrin molecules form long threads that tangle into a mesh across the wound. Red blood cells and platelets are trapped in the mesh, forming the clot.Because clotting depends on every step, a shortage of any component slows it down. For example, a person lacking vitamin K makes less prothrombin, so their blood takes longer to clot. Some anticoagulant drugs, such as warfarin, work by interfering with vitamin K, so less prothrombin is made.