Dvik

FIGURE 9.22 The steady-state dependence of various isotope effects.

A, Diagram for a hypothetical enzyme catalyzing a reaction described as an ordered ternary complex kinetic mechanism with asterisk marking the isotopically sensitive step. Braces indicate the region of the mechanism that affects the indicated isotope effect. B, Regions of v versus [S] plot the yield key rate parameters. The shaded area shows the substrate concentration range typically studied in initial-rate enzyme kinetic experiments.

FIGURE 9.22 The steady-state dependence of various isotope effects.

A, Diagram for a hypothetical enzyme catalyzing a reaction described as an ordered ternary complex kinetic mechanism with asterisk marking the isotopically sensitive step. Braces indicate the region of the mechanism that affects the indicated isotope effect. B, Regions of v versus [S] plot the yield key rate parameters. The shaded area shows the substrate concentration range typically studied in initial-rate enzyme kinetic experiments.

Taking the limit of this equation as (k3H/k5H) and (k3H/ k5H) approach zero (i.e., k3 becomes so small so that the isotopically sensitive step is rate-limiting), we get:

Vd k3D

We can see that in this case the isotope effect will be fully expressed in terms of VH/VD when the isotopically sensitive step is the rate-limiting step. One can now ask what would happen if k5H is not much larger than k3H. Consider the hypothetical case where the primary isotope is assumed to be 10: (a) if k3H/k5H = 1, then VH/VD value would equal {10+ 1}/ (1 + 1) = 5.5; and (b) if k3H/k5H = 10, then the VH/VD value would equal {10 + 10}/(1 + 10) = 1.8. These straight

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