Fig. 2
From: When substrate inhibits and inhibitor activates: implications of β-glucosidases
Effect of inhibitor to an enzyme exerting the nonproductive binding of substrate. a Binding of inhibitor to binding site +2 [designated as the nonproductive (np) binding mode of inhibitor] competes with the nonproductive binding but not with the productive binding of substrate. Binding of inhibitor to subsites −1 or +1 competes with the productive binding of substrate. For simplicity, these binding modes are lumped together (designated by double-headed arrow) in EI and EII(np) complexes. Binding of inhibitor to +1 competes also with the nonproductive binding of substrate, whereas binding to −1 does not. The latter possibility is accounted for by the presence of EIS(np) complex (within yellow box). The stability of all complexes is represented by equilibrium dissociation constants. b Michaelis–Menten equation (Eq. 2) parameters derived using rapid equilibrium treatment. The same equations are applicable for the formation of the first and the second product as well as for the disappearance of substrate. The terms within yellow boxes appear only when EIS(np) complex is included. c Dependency of parameters on inhibitor concentration. d Ratios of rates measured in the presence (v i ) and absence (v i=0) of inhibitor as a function of inhibitor concentration. The concentration of substrate (as a multiple of its K m value at [I] = 0) is shown in the plots. The results corresponding to the three scenarios are depicted in panels, c and d. Following values for parameters were used in analyses of all scenarios: k ES = k ESI(np) = 100 s−1, K s = K s(np) = 10 mM, K i = K i(np) = K ii(np) = K i(np)i = 100 mM. In the case of both, first and second scenario, the values of K is(np) = K s(np)i were set 106 mM. In the first scenario, we used K si(np) = 10 mM and K i(np)s = 100 mM, whereas in the second scenario, the corresponding values were set to 50 and 500 mM, respectively. The parameter values in the third scenario were as in the first scenario except that K s(np)i was set to 10 mM and K is(np) to 100 mM