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Example Questions
Example Question #21 : Enzyme Kinetics And Inhibition
The oxidation of glucose to two molecules of pyruvate produces a net two molecules of ATP during glycolysis. ATP allosterically inhibits the enzyme, PFK-1, that catalyzes the third step of glycolysis. This is an example of which fo the following mechanisms?
Feedback inhibition
Feed-forward activation
Noncompetitive inhibition
Negative cooperativity
Competitive inhibition
Feedback inhibition
This is an example of feedback inhibition, as feedback inhibition is a mechanism in which a molecule binds to an enzyme to decrease its activity. The mechanism is now balanced. Blocking an enzyme typically helps correct a metabolic imbalanace or assists in destruction of a pathogen. In this case the ATP binds to a site other than the protein's active site, and since it blocks PFK-1, a feedback inhibition has occurred.
Example Question #21 : Types Of Inhibition
Which of the following would be observed in the presence of a competitive enzyme inhibitor?
A lower Vmax, with Km unaffected
A higher Km and a higher Vmax
A lower Vmax and a lower Km
None of these
A higher Km, with Vmax unaffected
A higher Km, with Vmax unaffected
Competitive inhibitors bind to the active site of the target enzyme. Km is the substrate concentration at which the reaction rate is at half Vmax. A competitive inhibitor can be outcompeted by adding additional substrate; thus Vmax is unaffected, since it can be accomplished with enough additional substrate. However, since we need to add additional substrate to compete with the inhibitor to get the reaction to the same rate, our Km increases.
Example Question #3 : Identifying Type Of Inhibition
The above graph shows a Lineweaver-Burk plot for an enzyme that has been affected by an inhibitor. The blue line corresponds to an enzyme-catalyzed reaction with no inhibitor, while the red line represents the enzyme-catalyzed reaction in the precence of inhibitor. Based on the parameters shown in the graph, what type of inhibition is happening to this enzyme?
Competitive inhibition
Uncompetitive inhibition
Non-competitive inhibition
Mixed inhibition
Competitive inhibition
The graph shown in the question stem is a Lineweaver-Burk plot, otherwise known as a double-reciprocal plot. In this plot, is plotted along the
-axis and
is plotted along the
-axis. Furthermore, the
-intercept in such a graph is equal to
, and the
-intercept is equal to
.
From the graph shown in the question stem, we can see that there are two lines, each with different slopes. Each line corresponds to a certain concentration of inhibitor. (Note that one of the lines corresponds to no inhibitor, or a concentration of 0.)
It is evident that the two lines intersect each other along the -axis, right on the
value. Consequently, we can conclude that the inhibitor in this case is not having any effect on the value of
for the enzyme. Despite the different slopes for the two reactions, both of them have a common
-intercept but differing
-intercepts. This means that we can conclude the inhibitor in question must be competitive, since the result will be a rise in the
value for the enzyme-catalyzed reaction, but will have no effect on the
value.
Example Question #1 : Identifying Type Of Inhibition
In oxidative phosphorylation, cyanide can block ATP synthesis by binding to cytochrome c oxidase at a site other than its active site, essentially shutting down the electron transport chain. What kind of inhibition is this?
Allosteric inhibition
None of these answers
Competitive inhibition
Mixed inhibition
Reversible inhibition
Allosteric inhibition
This scenario is a classic example of allosteric inhibition. When cyanide binds to a site on cytochrome c oxidase other than the active site, cytochrome C oxidase becomes deactivated, stopping oxidative phosphorylation and causing cells to die since they cannot produce ATP anymore.
Example Question #21 : Enzyme Kinetics And Inhibition
An unknown compound was dropped into a solution of excess enzyme, and it was found that the enzyme's increased but its
remained the same. What kind of inhibition is observed?
Noncompetitive inhibition
Uncompetitive inhibition
Mixed inhibition
Competitive inhibition
Competitive inhibition
In competitive inhibition, the inhibitor binds the active site of the enzyme, competing with the substrate for this binding site. The of a competitively inhibited enzyme remains unchanged, but the
increases. This means that a higher concentration of substrate is required to bring the reaction rate to
. However, since this is competitive inhibition, and the maximum velocity is unchanged, we can overcome this increase in
and achieve maximum velocity if we saturate the enzyme with substrate.
Example Question #652 : Biochemistry
What type of inhibition increases without changing
?
Noncompetitive
Uncompetitive
Incompetitive
Competitive
Allosteric
Competitive
Competitive inhibition involves the substrate's access to the active site. In the case of competitive inhibition, the inhibitor blocks the substrate from the active site. As a result, the is unchanged, but the
is increased. Recall that
is the substrate concentration at which the reaction rate is
. Additionally, the reaction rate will increase with increased concentration of competitive inhibitor and substrate, because they are competing for the active site, causing an increase in reaction rate.
Example Question #1 : Identifying Type Of Inhibition
Complete the statement about enzymatic inhibition:
In __________ inhibition, the inhibitor can only bind to a complex of the enzyme and its substrate (ES complex). As a result of this type of inhibition, __________.
non-competitive . . . decreases
non-competitive . . . increases
uncompetitive . . . decreases
competitive . . . does not change
uncompetitive . . . does not change
uncompetitive . . . decreases
Uncompetitive inhibitors can only bind the ES complex, whereas competitive and non-competitive inhibitors do not require the enzyme to be complexed with the substrate. , which describes the maximum reaction velocity of the enzyme, is decreased because the inhibitor slows the dissociation of the substrate from the enzyme, thereby slowing the rate at which the enzyme can interact with other substrate molecules.
Example Question #3 : Identifying Type Of Inhibition
Inhibitor I is added to enzyme X. To determine if this inhibitor had any effect on the enzyme, the enzyme is added to a solution that it is known to catalyze. The enzyme's maximum rate of reaction has not decreased.
Therefore, which is the only kind of inhibition that inhibitor I could perform on enzyme X?
Uncompetitive inhibition
Mixed inhibition
Competitive inhibition
Noncompetitive inhibition
Competitive inhibition
All types of inhibitors will induce a change in the of an enzyme except for competitive inhibitors. This is because competitive inhibitors have no effect on the enzyme-subtrate complex. The
may still be reached, but by adding more substrate, since the
is increased by a competitive inhibitor.
Example Question #9 : Identifying Type Of Inhibition
Suppose that for a given enzymatic reaction, the addition of a certain chemical was found to result in a reduction in both the maximum reaction rate (), as well as the concentration of substrate necessary to achieve half the maximum rate (
). Which of the following phrases best describes this added chemical?
Noncompetitive inhibitor
Competitive inhibitor
Uncompetitive inhibitor
Mixed inhibitor
Feedback inhibitor
Uncompetitive inhibitor
This question is presenting us with a situation in which a chemical is being added to a mixture of enzyme and substrate, and its effects on the kinetic parameters of the reaction are observed. We're told that the and
for this reaction both become reduced. We then are asked to identify which term best describes the added chemical.
To begin with, let's take note that all of the answer choices are some kind of inhibitor. Thus, we know that the chemical we're adding to the mixture is an inhibitor of some type. The challenge is in identifying which type of inhibition is happening. For this question, we'll need to have familiarity with each type of inhibition in order to identify the correct answer.
Let's start with what we know. Both the and the
are being decreased. Right away, we can rule out competitive inhibition because the
should remain the same.
We should also be able to rule out feedback inhibition right off the bat, as this kind of inhibition involves the products of a reaction putting a halt on the reaction that led to its production.
Next, we can also realize that two of the answer choices are so similar that they are actually saying nearly the same thing. Mixed inhibition is a case in which the inhibitor binds to the enzyme regardless of whether substrate is also bound to the enzyme. However, with mixed inhibition, the inhibitor shows greater affinity for either the free enzyme or the enzyme-substrate complex. In such a case, the for the reaction is expected to fall, but the
can either increase or decrease.
Noncompetitive inhibition is a special type of mixed inhibition, in which the inhibitor binds both the free enzyme and the enzyme-substrate complex with equal affinity. In such a situation, the of the reaction will fall, but the
will remain unchanged.
And finally, we look at uncompetitive inhibition, which is the correct answer. In this type of inhibition, the inhibitor binds only to the enzyme-substrate complex rather than the free enzyme. It does so by binding to an allosteric site, which is distinct from the active site to which substrate binds. Thus, there is no way to out-compete the inhibitor by adding more and more substrate, as can be done in competitive inhibition. The end result of this is that the becomes irrecoverably lowered. And since this value becomes less, the substrate concentration needed to obtain half of that reduced value (the
) also becomes decreased.
Example Question #1 : Identifying Type Of Inhibition
Transition state analogs are generally used as what kind of inhibitors for enzymes?
Competitive inhibitors
Allosteric effectors
Mixed inhibitors
Positive feedback effectors
Noncompetitive inhibitors
Competitive inhibitors
Enzymes bind to and stabilize transition states. So a molecule that resembles the transition state of a reaction will be able to bind to the enzyme for that reaction very readily and compete with the binding of the actual transition state. Therefore transition state analogs are competitive inhibitors.
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