portion of the profile that occurs at higher substrate concentrations. In this
case, CLintis the ratio ofVmax2/Km2but since the profile never saturates
(becomes asymptotic) one cannot estimate the individual parameters and thus,
represents the ratio of these two parameters. Thus, CLintis the slope for this
linear portion of the kinetic profile.
4.5.2.3 Substrate Inhibition Substrate inhibition represents another example
of cooperativity in enzyme kinetic reactions, but of a different profile than
described to this point. With substrate inhibition kinetics, the velocity of a
reaction increases (as expected for hyperbolic profiles) to an apex, however,
beyond this point the velocity of the reaction decreases with increasing
substrate concentrations (Fig. 4.7).
Again, it is believed that this scenario occurs due to simultaneous binding of
a second substrate molecule within the active site resulting in a type of
uncompetitive inhibition. It remains unclear whether binding of the second
substrate molecule results in a partial inhibition of binding of the first substrate
molecule or whether the binding of this second molecule causes a conforma-
tional change in the enzyme active site that inhibits substrate turnover.
Through application of the appropriate equation (Eq. 4.8) that is derived
from the uncompetitive inhibition model using an extra substrate molecule
instead of inhibitor, one can approximately estimate the parametersKmand
Vmax, as well asKsi(the inhibition constant for the second substrate molecule).
Vm½S
Kmþ½þS ½S2
Ksið 4 : 8 ÞIt should be realized that these are only approximations sinceVmaxis never
truly reached and thus, Km is also not exactly discernible. In the past,
FIGURE 4.7 Representative plot depicting substrate inhibition kinetics.98 ENZYME KINETICS