FIGURE 3-24 K_m and V_max for an enzyme-catalyzed reaction. K_m and V_max are determined from analysis of the dependence of the initial reaction rate on substrate concentration. The shape of these hypothetical kinetic curves is characteristic of a simple enzyme-catalyzed reaction in which one substrate (S) is converted into product (P). The initial reaction velocity is measured immediately after addition of enzyme to substrate, before the substrate concentration changes appreciably. (a) Plots of initial reaction velocity at two different concentrations of enzyme [E] as a function of substrate concentration [S]. The [S] that yields a half-maximal reaction rate is the Michaelis constant K_m, a measure of the affinity of E for turning S into P. Quadrupling the enzyme concentration causes a proportional increase in the reaction rate, so the maximal velocity V_max is quadrupled; K_m, however, is unaltered. (b) Plots of initial reaction velocity versus substrate concentration with a substrate S for which the enzyme has a high affinity and with a substrate S′ for which the enzyme has a lower affinity. Note that V_max is the same with both substrates because [E] is the same, but that K_m is higher for S′, the low-affinity substrate.