Equation ( ) holds provided ε 1 and the non-observable reaction is in a QSS for the duration of the reaction. Therefore, the rate expression for the product formation is equivalent to the rate expression for the single-enzyme, single-substrate reaction when the indicator reaction is extremely fast. Notice that it is not necessary that λ 1 in order to impose the QSSA, and the restriction that e 0 2 be less than s 0 1 is not required: the QSSA will be valid as long as the phase-plane trajectory adheres to x * , and this will occur provided the indicator reaction is sufficiently fast, even if the initial auxiliary enzyme concentration (e 0 2 ) is large.