Figure 6.

Simulated plots for a reaction with a rate-limiting step following product formation. The model used in KinTek Explorer 8.0 was

$$\mathrm{E}+\mathrm{S}\underset{{10}^2{\mathrm{s}}^{\text{-}1}}{\overset{{10}^7{\mathrm{M}}^{\text{-}1}{\mathrm{s}}^{\text{-}1}}{\leftrightarrows }}\text{ES}\stackrel{{}^{{}^{50{\mathrm{s}}^{\text{-}1}}}}{\to }\text{EP}\stackrel{{}^{{}^{1{\mathrm{s}}^{\text{-}1}}}}{\to }\mathrm{E}+\mathrm{P}$$

$$\mathrm{E}+\mathrm{I}\underset{5{\mathrm{s}}^{\text{-}1}}{\overset{{10}^7{\mathrm{M}}^{\text{-}1}{\mathrm{s}}^{\text{-}1}}{\leftrightarrows }}\text{EI}$$

and the S concentrations used were 1, 2, 4, 8, 15, 30, 100, and 200 μM, with E = 0.1 μM and the time indicated in seconds. Rates of product formation increased with the substrate concentration. A, traces of P formation versus time without I. B, traces of product formation versus time in the presence of 5 μM I. C, rates of product formation in the absence of I. D, rates of product formation in the presence of 5 μM I. The K_m in the absence of I was 0.41 μM (Part C) and the K_m, apparent in the presence of 5 μM I was 4.4 μM, and using the standard equation for competitive inhibition

$$v=\frac{k_{cat}\left[S\right]}{Km\left(1+\frac{\left[I\right]}{Ki}\right)+\left[S\right]}$$

the K_i was calculated to be 0.056 μM. These values may be compared to the actual K_d values for S (10 μM) and I ((0.5 μM) from the model.