While the asymptotic solutions are useful in that they can be employed to make certain predictions about the behavior of the reaction, asymptotic theory fails to yield a visual or geometric understanding of the dynamical behavior of the zymogen activation reaction mechanism (1)-(2). To paint a complete picture of the mathematical structure behind the reaction mechanism, we turn to dynamical systems theory, and analyze this problem from phase space. From this perspective, after the initial buildup of c 1 , the phase space trajectory of the non-observable reaction (10a)-(10b) hugs a slow manifold, M ε , and is asymptotic to M ε in the approach to equilibrium. The time it takes for the trajectory to reach the slow manifold is approximately t c1 , and the time it takes for the trajectory to equilibrium is approximately t s1 . The condition for the validity of the asymptotic solution resides in how well the c 1 -nullcline approximates the slow manifold, and also how straight the phase space trajectory is in its approach to the slow manifold during the initial fast transient. The former of these conditions is known as the QSSA, and the latter is of course the geometrical interpretation of the RSA. We note that if the trajectory is close to M ε , then the complex C 1 is assumed to be in a QSS and the difference between the rate of C 2 depletion is approximately equal to the rate C 2 formation.