Biochemistry : Identifying Type of Inhibition

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #22 : Types Of 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?

Possible Answers:

Noncompetitive inhibition

Feed-forward activation

Competitive inhibition

Feedback inhibition

Negative cooperativity

Correct answer:

Feedback inhibition

Explanation:

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 #23 : Types Of Inhibition

Which of the following would be observed in the presence of a competitive enzyme inhibitor?

Possible Answers:

None of these

A lower Vmax, with Km unaffected

A higher Km and a higher Vmax

A lower Vmax and a lower Km

A higher Km, with Vmax unaffected

Correct answer:

A higher Km, with Vmax unaffected

Explanation:

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 #24 : Types Of Inhibition

Inhibition graph 

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?

Possible Answers:

Mixed inhibition

Competitive inhibition

Non-competitive inhibition

Uncompetitive inhibition

Correct answer:

Competitive inhibition

Explanation:

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 #25 : Types 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?

Possible Answers:

None of these answers

Competitive inhibition

Reversible inhibition

Allosteric inhibition

Mixed inhibition

Correct answer:

Allosteric inhibition

Explanation:

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 #26 : Types Of 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? 

Possible Answers:

Competitive inhibition

Noncompetitive inhibition

Uncompetitive inhibition

Mixed inhibition

Correct answer:

Competitive inhibition

Explanation:

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 #27 : Types Of Inhibition

What type of inhibition increases  without changing ?

Possible Answers:

Noncompetitive

Competitive

Allosteric

Uncompetitive

Incompetitive

Correct answer:

Competitive

Explanation:

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 #28 : Types 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,  __________

Possible Answers:

uncompetitive . . . decreases

uncompetitive . . . does not change

competitive . . . does not change

non-competitive . . . decreases

non-competitive . . . increases

Correct answer:

uncompetitive . . . decreases

Explanation:

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 #1 : 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? 

Possible Answers:

Uncompetitive inhibition

Mixed inhibition

Noncompetitive inhibition

Competitive inhibition

Correct answer:

Competitive inhibition

Explanation:

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 #30 : Types 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?

Possible Answers:

Competitive inhibitor

Uncompetitive inhibitor

Noncompetitive inhibitor

Feedback inhibitor

Mixed inhibitor

Correct answer:

Uncompetitive inhibitor

Explanation:

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 #2 : Identifying Type Of Inhibition

Transition state analogs are generally used as what kind of inhibitors for enzymes?

Possible Answers:

Allosteric effectors

Competitive inhibitors

Mixed inhibitors

Noncompetitive inhibitors

Positive feedback effectors

Correct answer:

Competitive inhibitors

Explanation:

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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