(PROTEIN) WATER MOLECULE AMINO GROUP

Figure 3–7
The effect of an activator upon the
velocity and substrate affinity of an
allosteric enzyme. ➤ When the activator
is bound (left hand curve). The apparent
K is decreased (i.e., the affinity is
increased) while the velocity is
increased.
Figure 3–8
The inability of a Lineweaver-Burk plot
to give accurate information about the
apparent K of an allosteric enzyme. ➤
The degree of curvature and any
intersection with the x-axis cannot be
determined.
Enzymes • 61
ceeding from a T-form (T = tense) to an R-form (R = relaxed). A second
way that the kinetics is influenced is with the binding of activator substances
that occupy sites on the enzyme away from the active sites themselves.
In this way, such an enzyme is also induced to change from its
T-form to its R-form at much lower substrate concentrations. This is a
biological way of jump-starting an enzyme to high velocities at lower
substrate concentrations. Figure 3–6 shows a hypothetical enzyme in the
T- and R-forms when bound to activators or multiple substrates
(inhibitors, also shown, are described later).
Figure 3–7 indicates a graph of velocity versus [S] concentration for
an allosteric enzyme without (on the right) and with (on the left) bound
activator substances. Note that with such enzymes K m becomes known as
K apparent (also known as K app or K 0.5) since the K value with allosteric
enzymes is dependent upon activators as well. In the eye, sodiumpotassium
activated ATPase, whose role is of special significance in the
cornea and the ciliary body, serves as an example of an allosteric enzyme.
When one attempts to make a Lineweaver-Burk plot with allosteric
enzymes, however, the line becomes nonlinear and it is impossible to
determine K app (Figure 3–8). There are other graphing techniques that can
solve this problem such as an Eadie-Hofstee plot (Figure 3–9),
(Whikehart, Montgomery, Hafer, 1987).