Catalyzed chemical reactions are often carried out experimentally as a pair consisting of an initial non-observable reaction that is followed by an indicator reaction. The latter (indicator) reaction is used to detect (monitor) the preliminary non-observable reaction . We consider a coupled reaction that consists of a non-observable component in which a substrate and an enzyme reversibly bind in complex and irreversibly release a second substrate. In the indicator reaction that follows, the second substrate then binds with a second enzyme (auxiliary enzyme) and synthesizes a product. Denoting the primary enzyme, substrate, and complex of the non-observable reaction as E 1 , S 1 , and C 1 respectively, and the substrate, auxiliary enzyme, complex and product associated with the indicator reaction as S 2 , E 2 , C 2 , and P (respectively), the complete assay consists of two chemical equations: