heteroactivation, as well as additional parameters that shed light into the
changes that have occurred in the enzyme kinetics.
v¼Vmax½SKm1 þ½KBB1 þba½KBBþ½S1 þa½BKB1 þab½KBBð 4 : 11 ÞIn this case,KmandVmaxrepresent the values obtained in the absence of
effector andKBis the binding constant of the effector. Additionally, interaction
factors includinga, which is the effect onKmdue to the presence of effector
(a<1 is indicative of activation anda>1 is indicative of inhibition) and
b, which is the effect onVmaxdue to the presence of effector (b<1 is indicative
of inhibition andb>1 is indicative of activation) can be estimated.
4.6 Graphical Analysis of Atypical Kinetic Data
As mentioned previously, use of the Eadie–Hofstee plot can be beneficial in
discerning whether atypical kinetic profiles are operational. When plotted in
this fashion, data that is of a hyperbolic nature (i.e., follows Michaelis–Menten
kinetics) will give a straight line (Fig. 4.9a). However, in the case of sigmoidal
kinetics, an Eadie–Hofstee style plot will yield a characteristic hook (Fig. 4.9b)
whereas a plot of this type to data that is exhibiting biphasic kinetics will give a
concave shape (Fig. 4.9c). Though not necessarily obvious from the standardn
versus [S] plot, these Eadie–Hofstee graphical representations of the data can
help to discern whether the data is nonhyperbolic. Finally, in the case of
substrate inhibition, the Eadie–Hofstee plot of these types of data will be of a
convex nature, analogous to thenversus [S] plot (Fig. 4.9d).
4.7 Enzyme Inhibition Kinetics
4.7.1 Overview
When a compound is added to an incubation reaction mixture and its presence
decreases the rate of substrate turnover, the process is termed inhibition. In
drug discovery and development, the study of inhibition potentialin vitrois of
great importance as the results may help to screen out compounds with
significantin vivoinhibition potential and correspondingly, the potential to
elicit drug–drug interactions. This chapter will deal primarily with reversible
inhibition as the concepts of irreversible inhibition are more extensively
covered in a later chapter. If the enzyme has no residual catalytic activity in the
presence of saturating concentrations of the inhibitor, this is termed complete
inhibition. However, more often than not, some residual enzyme catalytic
ENZYME INHIBITION KINETICS 101