Our analysis supports the claim that this system of interacting MM equations supplies an excellent approximation to the QSS phase of the coupled (or auxiliary) enzyme reaction (1)-( ) when the RSA is valid. This requires setting up initial experimental conditions where s 0 1 is in excess relative to both e 0 1 and e 0 2 . The RSA criteria can also be valid when s 0 1 ≈ e 0 1 ≈ e 0 2 as long as the initial concentrations of enzymes are small compared to the Michaelis constants.