Tag: Enzyme Inhibition

Enzyme Inhibition

ENZYME INHIBTION

Q. 1

The presence of a noncompetitive inhibitor can be implicated in which of the outcome?

 A

Leads to both an increase in the Vmax of a reaction and an increase in the Km

 B

Leads to a decrease in the observed Vmax

 C

Leads to a decrease in Km and Vmax

 D

Leads to an increase in Km without affecting Vmax

Q. 1

The presence of a noncompetitive inhibitor can be implicated in which of the outcome?

 A

Leads to both an increase in the Vmax of a reaction and an increase in the Km

 B

Leads to a decrease in the observed Vmax

 C

Leads to a decrease in Km and Vmax

 D

Leads to an increase in Km without affecting Vmax

Ans. B

Explanation:

In the presence of a noncompetitive inhibitor, Vmax is decreased, whereas Km is unchanged. The steady-state concentration of ES is decreased.


Q. 2

A competitive inhibitor of an enzyme will?

 A

Alter the Vmax of the reaction

 B

Bind to the same site as the substrate

 C

Decrease the apparent Km for the substrate

 D

Decrease the turnover number

Q. 2

A competitive inhibitor of an enzyme will?

 A

Alter the Vmax of the reaction

 B

Bind to the same site as the substrate

 C

Decrease the apparent Km for the substrate

 D

Decrease the turnover number

Ans. B

Explanation:

Substances that reduce the activity of an enzyme are called inhibitors.

Reversible inhibitors bind to an enzyme but rapidly dissociate from it [in contrast to irreversible inhibitors , which bind tightly and dissociate very slowly from the enzyme].

There are several types of reversible inhibitors:
 

Competitive inhibitors usually resemble the substrate and compete with it for binding at the active site.
Thus, increasing the concentration of substrate will decrease the percent inhibition of the enzyme.
The Vmax is unchanged, but the Km is increased.
A noncompetitive inhibitor binds with equal affinity to both enzyme and enzyme-substrate complex.
This binding leads to a distortion of the substrate binding site, so new substrate cannot bind and/or the product cannot be released.
In this kind of inhibition, the Vmax is decreased (choice A), but the Km is not altered.
Adding more substrate will not reverse this type of inhibition.
This is the equivalent of decreasing the turnover number.
 
An uncompetitive inhibitor does not bind to free enzyme, but binds to the enzyme-substrate complex at a site other than the catalytic site.
Once bound by the inhibitor, the enzyme is trapped in the enzyme-substrate complex state until the inhibitor dissociates.
In this kind of inhibition, the slope of the reaction (which is the ratio Km/Vmax) remains the same, but both Vmax and Km are reduced.
 
Ref: Janson L.W., Tischler M.E. (2012). Chapter 5. Enzymes and Amino Acid/Protein Metabolism. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.

Q. 3

True about competitive inhibition of enzyme:

 A

T Km

 B

Km

 C

T Vmax

 D

No change in Km and Vmax

Q. 3

True about competitive inhibition of enzyme:

 A

T Km

 B

Km

 C

T Vmax

 D

No change in Km and Vmax

Ans. A

Explanation:

Quiz In Between


Q. 4

In noncompetitive antagonism, the true statement is:

 A

Km value decrease; V max normal

 B

Km value decreased; V max decreased

 C

Km value normal; V max decreased

 D

Km value decreased; V max increased

Q. 4

In noncompetitive antagonism, the true statement is:

 A

Km value decrease; V max normal

 B

Km value decreased; V max decreased

 C

Km value normal; V max decreased

 D

Km value decreased; V max increased

Ans. B

Explanation:

Q. 5

Non competitive inhibitor of carbonic anhydrase‑

 A

Allopurinol

 B

Acetazolamide

 C

Bimatoprost

 D

Dipivefrine

Q. 5

Non competitive inhibitor of carbonic anhydrase‑

 A

Allopurinol

 B

Acetazolamide

 C

Bimatoprost

 D

Dipivefrine

Ans. B

Explanation:

Ans. is ‘b’ i.e., Acetazolamide

o Acetazolamide is non-competitive , reversible inhibitor of carbonic anhydrase.


Q. 6

Which of the following is known as suicidal enzyme?

 A

Lipoxygenase

 B

Cyclooxygenase

 C

Thromboxane synthatase

 D

5′ nucleotidase

Q. 6

Which of the following is known as suicidal enzyme?

 A

Lipoxygenase

 B

Cyclooxygenase

 C

Thromboxane synthatase

 D

5′ nucleotidase

Ans. B

Explanation:

 

Suicidal enzyme is one, which undergoes self-destruction in order to terminate its own acitivity, e.g. Cyclooxygenase.

Suicidal Inhibition is conversion of a substrate by the enzyme into a metabolite, which is a potent inhibitor of the enzyme; example: Xanthine oxidase converts allopurinol to alloxanthine (oxypurinol), which is a more potent inhibitor of allopurinol.

Quiz In Between



Enzyme Inhibition

ENZYME INHIBITION


ENZYME INHIBITION

  • Enzyme inhibitor binds with enzyme and decreases a catalytic activity.

Types of Enzyme Inhibition-

  1. Reversible
  2. Irreversible
  3. Allosteric
  4. Reversible Inhibition– binds through non-covalent bonds and activity of enzyme is restored. Divided into-

a) Competitive inhibitorKm increased, Vmax unchanged.

  • E.g.- Succinate dehydrogenase by melanate.

b) Non competitive inhibitor- Km unchanged, Vmax decrease, mostly irreversible.

  • E.g.- Cyanide by Cytochrome C Oxidase.
  • Carbonic anhydrase by Acetazolamide

2. Irreversible Inhibition– binds covalently with an enzyme.

a) Suicidal Inhibitor– irreversible binding to enzyme and inhibit enzyme.

  • E.g. Allopurinol inhibit Xanthine oxidase, cyclooxygenase.

3. Feedback Inhibition- called as end product inhibition.

  • E.g. AMP inhibits first step in purine synthesis.

Exam Important

  1. Reversible Inhibition- binds through non-covalent bonds and activity of enzyme is restored.
  2. Competitive inhibitor- Km increased, Vmax unchanged.
  3. Competitive inhibitor- Succinate dehydrogenase by melanate.
  4. Non competitive inhibitor- Km unchanged, Vmax decrease, mostly irreversible.
  5. Non competitive inhibitor- E.g.- Cyanide by Cytochrome C Oxidase.
  6. Suicidal Inhibitor- Allopurinol inhibit Xanthine oxidase.
Type of inhibitor Km Vmax
Reversible inhibbitor Increased No effect
Competitive No effect Decreased
Non-competitive Decreased Decreased
Uncompititive No effect Decreased
Irrversible inhibitor (same as reversible Increased No effect
Don’t Forget to Solve all the previous Year Question asked on ENZYME INHIBITION

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