All simulated reaction progress curves, except the positive control curve simulated at [I] 0 = 0, were fit separately to Eqn (4) to determine the best-fit values of the four adjustable model parameters v 0 , v i , k obs , and F 0 . In these regression analyses the enzyme concentration was held fixed at the nominal value [E] 0 = 10 nM. The results of fit are summarized graphically in Figure . The best-fit values of the apparent inhibition constant k obs and the associated formal standard errors (SE) are shown in Table . . The "±" values are formal standard errors from (unweighted) nonlinear regression analysis.