proenzima

A proenzyme, commonly referred to as a zymogen, is an inactive precursor of an enzyme that becomes active following proteolytic cleavage. The proenzyme form allows cells to synthesize and store potent enzymes safely, before their activity is required.

Activation typically occurs through proteolysis by specific proteases that remove a small peptide segment, unveiling the enzyme's active site. In many secretory pathways, a cascade of activations coordinates digestion and other processes. For instance, in the digestive system, trypsinogen is activated to trypsin by the enterokinase (enteropeptidase) in the small intestine; once active, trypsin further activates additional pancreatic zymogens such as chymotrypsinogen and procarboxypeptidases. In the stomach, pepsinogen is converted to pepsin by the acidic environment, with limited autocatalytic activation.

Examples of proenzymes include pepsinogen, trypsinogen, chymotrypsinogen, and the zymogens involved in blood coagulation such as prothrombin, and the plasminogen system. The term proenzyme is thus a broad category that encompasses zymogens across physiology, not limited to digestion.

Clinical and physiological relevance: the proenzyme form is essential to regulate enzyme activity, prevent self-digestion, and allow trafficking to the correct location. Premature or inappropriate activation can cause tissue damage, as seen in pancreatitis, where premature trypsin activation injures pancreatic tissue. Genetic mutations affecting activation steps can alter susceptibility to disease.

Terminology: while proenzyme and zymogen are often used interchangeably, zymogen is commonly used in the context of digestive enzymes, whereas proenzyme emphasizes the general inactive precursor concept.