Reactions of the form (3)-( ) often occur in vivo. For example, the physiologic response to a vascular lesion entails a number of enzymatic steps that lead to clot formation. These enzymatic steps are a cascade of enzyme catalyzed reactions that follow a sequence of zymogen activation steps as described above . In the laboratory, the activity of thrombin, one of the blood coagulation enzymes, has been studied with a zymogen activation coupled to an enzyme catalyzed reaction assay. Thrombin catalyzes the activation of protein P, which is non-observable using steady-state kinetic laboratory assays. However, the formation of p-nitroaniline from substrate S2266 is catalyzed by activated protein P, and is observable through steady-state kinetic progress curve experiments. By coupling the two reactions, thrombin function is studied with a zymogen activation coupled to its enzyme catalyzed reaction .