Biochemical reactions inside cells are generally catalyzed by enzymes, which accelerate the conversion of substrates into products under physiological conditions. Most of the complex chemical processes occurring inside cells or organisms that are necessary for the maintenance of life are catalyzed by enzymes. Consequently, the experimental measurement of enzyme activity through in vitro assays plays a substantial role in understanding the dynamics of biochemical processes inside cells . Often however, the activity of numerous enzymes cannot be observed experimentally through direct observation of their reaction. Instead, these non-observable enzyme catalyzed reactions must be observed indirectly by coupling it to another enzyme catalyzed reaction that is used to indicate the progress of the non-observable reaction. The observable enzyme catalyzed reaction is known as the indicator or monitor reaction . The non-observable enzyme catalyzed reaction can be of varying complexity; it may contain linear inhibition or exhibit enzyme degradation . Regardless of the nature of the non-observable reaction, there are two general mechanisms employed to couple enzyme reactions: the coupled auxiliary enzyme reaction mechanism , and zymogen activation coupled to its enzyme catalyzed reaction .