A cascade of zymogen activations occur within both the intrinsic and extrinsic blood coagulation pathways.
A zymogen is an inactive enzyme precursor, a proenzyme, that displays no catalytic activity until it is transformed into its active form by a process such as the cleavage of one or more peptide bonds. Pancreatic digestive enzymes are secreted as zymogens partly to prevent the enzymes from digesting proteins in the cells in which they are synthesized. An additional important role of zymogen activations occurs in blood clotting. For blood clotting, the response time must be fast in order to achieve clotting at the right spot and time to prevent excessive bleeding. Zymogen cascades are therefore employed to achieve that rapid response in which activation of one clotting factor is then responsible for activating another clotting factor.
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Conceptual Vocabulary for Zymogens
Zymogens
A zymogen is an inactive precursor of an enzyme that requires cleavage for it to become an active enzyme.
Trypsinogen is the precursor form of trypsin, a digestive enzyme.
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids.
Pepsin is expressed as a zymogen called pepsinogen, whose primary structure has an additional 44 amino acids compared to the active enzyme.
Enteropeptidase converts trypsinogen into its active form trypsin, resulting in the subsequent activation of pancreatic digestive enzymes.
A biochemical cascade is a series of chemical reactions that occur within a biological cell in which one event triggers the next, in a linear fashion.
A single chemical reaction is said to be autocatalytic if one of the reaction products is also a catalyst for the same or a coupled reaction.
Thrombin in acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin.
Caspases are a family of protease enzymes playing essential roles in programmed cell death.
Factor X, also known as Stuart–Prower factor, is an enzyme of the coagulation cascade that acts by cleaving prothrombin to yield the active thrombin.
Serpins are a superfamily of proteins with protease inhibition activity, notable for their unusual mechanism of action, in which they irreversibly inhibit their target by undergoing a large conformational change to disrupt its active site.