Essential Cell Biology 5th edition

How Proteins Work
145
enzyme activity. Such regulation allows cells to control
when and how rapidly various reactions occur, a process
we discuss in detail in this chapter.
The effect of an inhibitor on an enzyme’s activity is monitored
in the same way that we measured the enzyme’s
kinetics. A curve is first generated showing the velocity
of the uninhibited reaction between enzyme and substrate.
Additional curves are then produced for reactions
in which the inhibitor molecule has been included in the
mix.
Comparing these curves, with and without inhibitor, can
also reveal how a particular inhibitor impedes enzyme
activity. For example, some inhibitors bind to the same
site on an enzyme as its substrate. These competitive
inhibitors block enzyme activity by competing directly
with the substrate for the enzyme’s attention. They
resemble the substrate enough to tie up the enzyme,
but they differ enough in structure to avoid getting converted
to product. This blockage can be overcome by
adding enough substrate so that enzymes are more
likely to encounter a substrate molecule than an inhibitor
molecule. From the kinetic data, we can see that
competitive inhibitors do not change the V max of a reaction;
in other words, add enough substrate and the
enzyme will encounter mostly substrate molecules and
will reach its maximum velocity (Figure 4–37).
Competitive inhibitors can be used to treat patients who
have been poisoned by ethylene glycol, an ingredient in
commercially available antifreeze. Although ethylene
glycol is itself not fatally toxic, a by-product of its metabolism—oxalic
acid—can be lethal. To prevent oxalic acid
from forming, the patient is given a large (though not
quite intoxicating) dose of ethanol. Ethanol competes
with the ethylene glycol for binding to alcohol dehydrogenase,
the first enzyme in the pathway to oxalic acid
formation. As a result, the ethylene glycol remains mostly
unmetabolized and is safely eliminated from the body.
Other types of inhibitors may interact with sites on the
enzyme distant from where the substrate binds. Many
biosynthetic enzymes are regulated by feedback inhibition,
whereby an enzyme early in a pathway will be shut
down by a product generated later in the pathway (see,
for example, Figure 4–43). Because this type of inhibitor
binds to a separate, regulatory site on the enzyme, the
substrate can still bind, but it might do so more slowly
than it would in the absence of inhibitor. Such noncompetitive
inhibition is not overcome by the addition of
more substrate.
Design
With the kinetic data in hand, we can use computer
modeling programs to predict how an enzyme will perform,
and even how a cell will respond, when exposed
to different conditions—such as the addition of a particular
sugar or amino acid to the culture medium, or
the addition of a poison or a pollutant. Seeing how a
cell manages its resources—which pathways it favors
for dealing with particular biochemical challenges—can
also suggest strategies for designing better catalysts for
reactions of medical or commercial importance (e.g., for
producing drugs or detoxifying industrial waste). Using
such tactics, bacteria have even been genetically engineered
to produce large amounts of indigo—the dye,
originally extracted from plants, that makes your blue
jeans blue. We discuss the methods that enable such
genetic manipulation in detail in Chapter 10.
Harnessing the power of cell biology for commercial
purposes—even to produce something as simple as the
amino acid tryptophan—is currently a multibillion-dollar
industry. And, as more genome data come in, presenting
us with more enzymes to exploit, vats of custom-made
bacteria are increasingly churning out drugs and chemicals
that represent the biological equivalent of pure gold.
(A)
enzyme
(B)
substrate
only
competitive
inhibitor
substrate
v
substrate
+ inhibitor
inactive
enzyme
active
enzyme
products
1/v
[S]
substrate
+ inhibitor
1/[S]
substrate
Figure 4–37 A competitive inhibitor directly blocks
substrate binding to an enzyme. (A) The active site of
the enzyme can bind either the competitive inhibitor or
the substrate, but not both together. (B) The upper plot
shows that inhibition by a competitive inhibitor can be
overcome by increasing the substrate concentration.
The double-reciprocal plot below shows that the V max
of the reaction is not changed in the presence of the
competitive inhibitor: the y intercept is identical for
both the curves.