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Example Questions
Example Question #1 : Inhibitors
Which of the following is true regarding competitive inhibition?
I. They form covalent bonds with the active site
II. They are reversible
III. They are similar to allosteric inhibitors
II only
III only
II and III
I and II
II only
Competitive inhibition decreases enzyme activity by binding to the active site of the enzyme. Recall that active sites are sites on enzymes where the substrates bind. Upon binding to the enzyme, the substrates undergo changes that facilitate and speed up the chemical reaction. A competitive inhibitor binds to this active site and prevents the substrate from binding. With no binding, the substrate will not undergo the necessary changes and, subsequently, the chemical reaction. A key characterisitic of competitive inhibitors is that the bond between the inhibitor and the active site is reversible. This means that the chemical bonds involved here are weak, reversible noncovalent bonds such as hydrogen bonds and van der Waals forces. Covalent bonds are very strong and are usually found in irreversible interactions.
Allosteric inhibitors are molecules that bind to enzymes at their allosteric site(s). In this way, the allosteric inhibiton is very similar to, and is a subset of, another type of enzyme inhibition, noncompetitive inhibition.
Example Question #1 : Inhibitors
A researcher is analyzing the effects of an unknown inhibitor. He observes that the inhibition can be overcome by increasing the concentration of the substrate. What can you conclude about this inhibitor?
The inhibitor increases the Michaelis constant
The inhibitor decreases the
The inhibitor decreases the Michaelis constant
The inhibitor increases the
The inhibitor increases the Michaelis constant
Recall that competitive inhibition can be overcome by increasing substrate concentration. Competitive inhibitors alter the Michaelis constant, , but maintain the
(maximum reaction rate). Inhibitors act to decrease the reaction rate. To figure out the effect of competitive inhibitors on the Michaelis constant, we need to look at the Michaelis-Menten equation.
where is reaction rate,
is maximum reaction rate,
is substrate concentration, and
is the Michaelis constant. Since reaction rate is inversely proportional to the
, competitive inhibitors will increase
and, thereby, decrease reaction rate.
Example Question #1 : Inhibitors
The slope of a Lineweaver-Burk plot is and the x-intercept is
. Upon addition of a noncompetitive inhibitor the slope increases to
. Which of the following is the correct value of
(Michaelis constant) after the addition of the inhibitor?
Cannot be determined from the given information
To answer this question we need to first figure out the equation for slope and x-intercept of Lineweaver-Burk plot. The Linweaver-Burk plot is a graphical way to plot the Michaelis-Menten equation. It is defined as the reciprocal of Michaelis-Menten equation. Michaelis-Menten equation is as follows.
where is reaction rate,
is maximum reaction rate,
is substrate concentration, and
is the Michaelis constant. Taking the reciprocal of this gives us
The slope, therefore, is . The x-intercept can be found by plugging in zero for the Y value (the reaction rate,
). The x-intercept is
.
The question states that the slope is and the x-intercept is
. Using the equation for x-intercept we can solve for
.
Using the equation for slope we can solve for
Recall that the addition of a noncompetitive inhibitor alters the but not the
; therefore,
is still
after the addition of noncompetitive inhibitior.
Note that if we were asked to solve for the , we would have had to use the new slope (
) and the same
value (
).
Example Question #1 : Inhibitors
Which of the following is true regarding noncompetitive inhibitors?
I. They do not form covalent bonds with the active site
II. They alter both Km and Vmax
III. They alter the shape of the active site
II and III
I only
I and III
III only
III only
Noncompetitive inhibitors bind to enzymes and prevent the formation of the enzyme-substrate complex. These inhibitors bind to a location other than the active site. Upon binding, the inhibitors alter the conformation of the active site and prevent the binding of substrate. They form covalent bonds with the enzyme; therefore, these are irreversible inhibitors and are hard to remove. and
are altered by both types of inhibitors (competitive and noncompetitive). Competitive inhibitors alter the
whereas the noncompetitive inhibitors alter the
.
This implies that competitive inhibition can be overcome by increasing substrate concentration whereas noncompetitive inhibition cannot. Molecularly this makes sense. Competitive inhibitors bind reversibly to the active site and prevent binding of substrate. If we were to drastically increase its concentration, substrate will compete with and remove the competitive inhibitor from the active site. Noncompetitive inhibitors, on the other hand, alter the conformation of the active site, making it hard for substrates to bind to the active site; therefore, the substrate will not be able bind, regardless of the substrate concentration.
Example Question #1 : Inhibitors
Upon addition of an inhibitor, which of the following is expected to happen in a catalytic reaction?
The amount of reactants will increase because the the forward reaction is halted
It will take longer to reach equilibrium
The amount of products will decrease because the equilibrium is shifted to the left
The amount of reactants will increase because the reverse reaction will be favored
It will take longer to reach equilibrium
Inhibitors are molecules that prevent the action of catalysts. They bind to catalysts and prevent substrate binding, thereby halting the catalytic action. Since catalysts increase the speed of a reaction, addition of an inhibitor will lower the speed of the reaction. This does not mean that the reaction will stop proceeding; it simple means that it will take longer for the reaction to complete (reach equilibrium).
Remember that a catalyst speeds up both the forward and the reverse reaction; therefore, inhibitors will slow down both reactions. As mentioned, inhibitors will only slow down the reaction. The amount of products produced (equilibrium) will not change, although it will take longer for products to form.
Example Question #2 : Inhibitors
__________ inhibitors bind to the active site and __________ inhibitors alter the binding affinity of substrate and catalyst.
Competitive . . . noncompetitive
Competitive . . . competitive
Noncompetitive . . . noncompetitive
Noncompetitive . . . competitive
Competitive . . . competitive
There are two main types of inhibitors. Competitive inhibitors bind to the active site of the catalyst and prevent substrate from binding. This phenomenon causes a decreasing in the binding affinity of substrate and catalyst. However, competitive inhibitors can be overcome by adding excess substrates. The substrates will dissociate the competitive inhibitor and carry out the reaction; therefore, the reaction can still be carried out at a faster rate and the maximum rate of reaction is not altered.
Noncompetitive inhibitors bind to the catalyst at an allosteric site. They alter the conformation of the active site and prevent substrate binding. They cannot be overcome by addition of excess substrate; therefore, they lower the maximum rate of reaction.
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