Regulatory Enzymes and Questions for Exam Like GPAT, NEET, UPSC, SSC

Regulatory Enzymes and Questions for Exam Like GPAT, NEET, UPSC, SSC

In cell metabolism, enzymes work together in a sequential chain to carry out the given metabolic process. In such a system, the intermediate product of first enzyme becomes the substrate for the second enzyme and the cycle goes on. The enzyme activity must be regulated so that they function properly at proper time and place.

In each enzyme system, there is at least one enzyme that regulates the overall metabolic sequence. Such regulatory enzymes not only have catalytic function, but are also responsible for increasing or decreasing the catalytic activity in response to certain signals. In most of the enzyme system, the first enzyme of the system is termed as regulatory enzyme. Enzymes regulate the catalytic process in four ways:

  • Allosteric or non-covalent control of enzymes
  • Isoenzymes
  • Reversible covalent modification of enzymes
  • Induction and repression

Allosteric enzymes or noncovalent regulation of enzymes

Allosteric enzymes are those which have an extra site other than active site. Like other enzymes, allosteric enzymes have a catalytic site where substrate binds, but allosteric enzymes along with catalytic site have allosteric site where regulatory metabolites bind. Such regulatory metabolites are called as effector, modulator or modifier.

These allosteric enzymes are stimulated and inhibited by modulators. Modulators that inhibit the enzyme are known as negative effectors; and those who stimulates the enzyme are known as positive effectors.

Allosteric enzymes do not obey Michaelis-Menten behavior. Allosteric enzymes form sigmoid-shaped curve rather than hyperbolic curve.

Reversible covalent modification

The covalent attachment of a molecule to an enzyme modifies its activity and function. Most of these modifications are reversible. Eg of covalent modification are:

  • Phosphorylation and dephosphorylation
  • Acetylation and deacetylation
  • ADP riboxylation
  • Gamma carboxylation

Induction and Repression of Enzyme Synthesis

  • Apart from the covalent modification mechanism which modifies the activity of existing enzymes; cells can also regulate the amount of enzyme present by altering the rate of enzyme synthesis.
  • Increased in enzyme synthesis is known as induction and decreased in enzyme synthesis is repression. This alteration only alters the total amount of active site present rather than influencing the efficiency of existing enzyme.
  • Enzymes which are regulated by this method are the one which are used only once in the process or the one which work under specific physiologic conditions.


Isoenzymes are the multiple forms of same enzymes like isomers. The different isoenzyme catalyzes the same reaction but differ in their structure and kinetic properties. These are encoded by different genes. Isoenzymes have different chemical and physical properties like:

  • Electrophoretic mobility
  • Kinetic properties
  • Amino acid sequence
  • Amino acid composition

For example:

  • Lactate dehydrogenase have five isomer
    1. LDH1
    2. LDH2
    3. LDH3
    4. LDH4
    5. LDH5
  • Creatine kinase have 3 isomer
    1. CK1
    2. CK2
    3. CK3

Multiple Choice Questions (MCQs)

1. What are regulatory enzymes?
A. One involved in protein synthesis
B. Those involved in enzyme synthesis
C. Those involved In regulating the enzyme activity
D. None of the above

2. Why are regulatory enzymes needed?
A. Control the enzymatic activity
B. Regulate the enzyme synthesis
C. Both
D. None

3. Which enzymes are termed as allosteric enzymes?
A. Those having active site
B. Those having allosteric site
C. Those having modifiers
D. All of the above

4. How many types of effectors are present?
A. 2

B. 3

C. 1

D. 4

5. What are isoenzymes?
A. Same size of enzyme but different function
B. Different function different size
C. Multiple forms of enzyme
D. Both A and B

6. Allosteric enzymes forms which shape of curve?
A. Hyperbolic

B. Bell-shaped

C. Sigmoid shaped

D. Linear

7. Which type of attachment between the molecule and the enzyme changes the enzymatic activity?
A. Metallic

B. Covalent

C. Peptide

D. Glycosidic bond

8. Increased synthesis of enzymes is known as ——?
A. Transcription

B. Induction

C. Translation

D. Repression

9. Match the following-
A. Allosteric enzyme 1. Only have an active site
b. Induction 2. Decreases protein synthesis
c. Repression 3. Increases protein synthesis
d. Catalytic enzyme 4. Have active as well as other site

10. Decreased synthesis of enzyme is known as ——?
A. Transcription

B. Induction

C. Translation

D. Repression

11. Isoenzymes differ in which of the following basis?
A. Structure

B. Function

C. Kinetic property

D. Both A and C

12. Which of the following statement is NOT true?
A. Effectors and modifiers are same
B. Allosteric enzyme do not obey double-reciprocal plot
C. Lactate dehydrogenase have 5 isomers
D. Phosphorylation is an example of covalent modification

13. Which of the following method alters the rate of enzymatic synthesis?
A. Induction and repression

B. Covalent modification

C. Isoenzyme

D. Non-covalent regulation of enzyme

14. How many isomers do creatine kinase have?

A. 2

B. 4

C. 5

D. None of the above

15. Enzymes regulated by which method works only once in the process or works under specific physiological conditions?
A. Induction and repression

B. Covalent modification

C. Isoenzyme

D. Non-covalent regulation of enzyme

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1. Those involved in regulating enzyme activity
2. Both
3. All of the above
4. 2
5. Multiple form of enzymes
6. Sigmoid shaped
7. Covalent
8. Induction
9. a – 4 b – 3 c – 2 d – 1
10. Repression
11. Both A and C
12. Allosteric enzyme do not obey double-reciprocal plot
13. Induction and repression
14. None of the above
15. Induction and repression



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