The phase-plane description of the zymogen activation assay is markedly different than that of the auxiliary enzyme assay. For example, the condition µ 1 establishes the presence of a slow manifold when the indicator reaction is fast. However, unlike the auxiliary enzyme reaction, the s 2 and c 2 -nullclines of (39a)-(39b) only intersect at the origin. This means that when 1, and the depletion timescale of the indicator reaction is much smaller than t s 1 , that