Many enzyme catalyzed reactions that occur in physiological processes require an activation step in which a precursor of a zymogen (inactive enzyme precursor or pro-enzyme) is converted to an active enzyme. This process, known generally as zymogen activation , is typically the first step in a cascade of coupled enzyme catalyzed reactions . The activation step of the zymogen is itself an enzyme catalyzed reaction, and the inactive enzyme precursor is activated by a functional enzyme. The active enzyme can be generated by enzyme-catalyzed proteolosis or enzyme activation by phosphorylation . For example, the digestive enzyme trypsin, which is the activate form of trypsinogen, is activated by the enzyme enterokinase; trypsin can then bind with trypsinogen to convert remaining trypsinogen into trypsin . Likewise, plasminogen is activated by streptokinase to form plasmin (an enzyme), which then degrades fibrin (a substrate) to break down clots in blood coagulation . Denoting the active enzyme, zymogen, activated enzyme, and intermediate complex of the activation reaction as E 1 , E i 2 , E 2 , and C 1 respectively, the preliminary zymogen activation step coupled with its secondary enzyme-catalyzed reaction can be expressed with the following reaction mechanism: