Enzymes: Biochemical Nature, Classification and MCQs for GPAT, NEET, SSC Exams

Introduction to Enzymes

• Enzymes are the specialized proteins that catalyze the biochemical reactions.
• Almost every reaction that takes place in a cell requires the action of enzyme for its completion.
• Commonly enzymes are proteins but there are some exceptions like ribosomal RNAs and handful of self-cleaving and self-splicing RNA molecules; these are collectively known as ribozymes.
• The catalytic behavior of any protein enzymes depends upon its primary, secondary, tertiary and quandary structure of the proteins. Change in any one of its structure can affect the enzymatic activity.

Zymogen or Proenzyme:- A no. of proteolytic enzymes are found in blood and in the digestive tract. These proteolytic enzymes are present in inactive(precursor) form and are known as proenzyme or zymogen. For eg: Chymotrypsin is secreted in the pancreas as chymotrypsinogen. It is activated in the digestive tract by a proteolytic enzyme known as trypsin. Trypsin breaks the peptide bond from the N-terminal of the chymotrypsinogen. This breakdown changes the conformation of the enzyme and creates a binding site for the substrates.

Classification of Enzymes

Classification of enzymes was done in 1961 by enzyme Commission of International Union of Biochemistry (IUB). According to this classification, each enzyme in assigned a code number known as enzyme code (EC) consisting of four digits. Significance of the four digits of enzyme code:

• The first digit indicates the main class (shows that the enzyme catalyzes the reactions of which type)
• The 2^nd digit indicates the sub-class. This indicates the type of group involved in the reaction
• The 3^rd digit indicates the sub-subclass. This denotes the substrate on which the enzyme acts
• The fourth digit specifies the individual enzyme. It indicates the systemic serial no. and name.

According to IUB, enzymes are classified in 6 main classes

EC-1: Oxidoreductase The enzymes which catalyzes the transfer of electron, oxidation or reduction reaction are included in this category. Enzymes included in this class are:

• Dehydrogenases
• Reductases
• Oxidases
• Peroxidases.

If the reaction involve direct participation of oxygen, the enzyme is known as oxidase. If the reaction involves transfer of hydrogen, the enzyme is called dehydrogenase.

EC-2: Transferase The enzymes which catalyzes the transfer of a group like amino, carboxyl, methyl or phosphoryl from one molecule to another. Common examples include:

• Transaminase or amino transferase
• Kinase
• Transcarboxylase

EC-3: Hydrolase The enzyme which catalyzes the cleavage of a C-C, C-N, C-O etc bond through the addition of water. Examples include:

• Acid phosphatase
• Digestive enzymes like amylase, pepsin, trypsin, chymotrypsin etc.

EC-4: Lyase The enzymes which catalyze the cleavage of a C-C, C-N bond by means other than hydrolysis and oxidation; and give rise to a compound having double don’t or catalyzing a reverse reaction by the addition of a double bond. In cases where reverse reaction is important, synthase enzymes are used and not synthetase. Examples include:

• Aldolase
• Fumerase
• Arginosuccinase
• Carbonic anhydrase etc.

EC-5: Isomerases The enzymes that catalyze intermolecular rearrangements (structural or geometrical changes) in a molecule. These are called as isomerase, mutase or epimerases. Examples include:

• Glucose epimerase
• Phosphohexose isomerase
• Phosphoglucomutase

EC-6: Ligases(synthetases) The enzyme which catalyzes the joining of two molecules through the hydrolysis of ATP. They can form C-C, C-N, C-O bonds. Examples include:

• Glutamine synthetase
• DNA ligase

Multiple Choice Questions (MCQs)

1. Which of the following biomolecule is known as enzymes?

A. Lipids

B. carbohydrates

C. Proteins

D. All of the above

2. On which factor, the catalytic behavior of an enzyme depends?

A. Primary structure of protein

B. Secondary structure of proteins

C. Tertiary structure of proteins

D. All of the above

3. What is the inactive form of enzyme known as?

A. Co-enzyme

B. Zymogen

C. Co-factor

D. Both B and C

4. Which enzymes activates the chymotrypsinogen in digestive tract?

A. Elastase

B. Trypsin

C. Chymotrypsin

D. None of the above

5. In which year, enzymes were classified by the enzyme commission of IUB?

A. 1966

B. 1961

C. 1861

D. 1962

6. What prefix is used for the inactive or precursor enzymes?

A. Pre

B. Pra

C. Pro

D. En

7. What suffix is used for the inactive or precursor enzymes?

A. En

B. Gen

C. ken

D. Sen

8. Match the following-

a. Hydrolase 1. ceruloplasmin

b. Ligase 2. ALT

c. Transferase 3. Pyruvate carboxylase

d. Oxidoreductase 4. lipase

A. a – 4 b – 3 c – 2 d – 1

B. a – 1 b – 2 c – 3 d – 4

C. a – 3 b – 2 c – 1 d – 4

D. a – 1 b – 2 c – 3 d – 4

9. Which enzymes are used for catalyzing the reaction involving transfer of hydrogen?

A. Reductase

B. Dehydrogenase

C. Oxidases

D. Peroxidases

10. All digestive enzymes are the example of which category of enzyme classification?

A. Transferase

B. Oxidoreductase

C. Hydrolase

D. All of the above

11. ligases are also known as?

A. Synthetase

B. Synthase

C. Both

D. None

12. Which of the following statement is NOT true?

A. EC-5 is ligases

B. The 2nd digit indicates the type of group involved

C. The 4th digit denotes the serial no. of the enzyme

D. All enzymes are not proteins, there are exceptions

13. Which of the following enzyme catalyzes the transfer a group from one molecule to another?

A. Transferase

B. Oxidoreductase

C. Hydrolase

D. All of the above

14. Which of the following enzyme forms the double bond in the product?

A. Transferase

B. Oxidoreductase

C. Hydrolase

D. None of the above

15. Phosphoglucomutase is the example of which category of enzyme classification?

A. Ligase

B. Isomerase

C. Lyase

D. Both A and B

ANSWERS:-

1. Proteins

2. All of the above

3. Zymogen

4. Trypsin

5. 1961

6. Pro 7. gen

8. a – 4 b – 3 c – 2 d – 1

9. Dehydrogenase

10. Hydrolase

11. Synthetase

12. EC-5 is ligases

13. Transferase

14. None of the above

15. Isomerase

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REFERENCE:– Pankaja Naik- Biochemistry; 4^th edition; page no:- 83-85.