Proteins: Definition, Classification and Functions and MCQs for GPAT, NEET, CSIR NET, SSC Exams

Proteins are the macromolecules and are composed of one or more polypeptide chains. Each of these polypeptide chains contains a specific sequence of amino acids. These polypeptide chains are linked by peptide bonds.

Proteins are classified in several ways. Mainly classified on the basis of three criteria:

Classification of proteins on the basis of function

Catalytic proteins:- Also known as enzymes accelerate the biochemical reactions. Eg include:
- Glucokinase
- Dehydrogenases etc.
Transport proteins: These proteins bind and carry specific molecules from one organ to another. Eg are:
- Hemoglobin transports oxygen
- Apolipoproteins carry lipids from liver to other organ
- Transferrin transports iron
- Ceruloplasmin carries copper
Storage proteins:- Many proteins works as storage form of many components
- Apoferritin stores iron
- Myoglobin stores oxygen
Contractile proteins:- Some proteins work for the contraction of skeletal muscles. Eg include:
- Actin
- Myosin
Structural proteins:- Some proteins serve as supporting filaments. Eg include:
- Collagen, a fibrous protein of tendon
- Cartilage
- Keratin
Defense protein:- These proteins work for protection purpose against the invading microbes and foreign substances
- Immunoglobin
- Fibrinogen

Classification based on molecular shape of proteins

Fibrous proteins:- Also known as sclero proteins. These are insoluble and have high molecular weight. These have axial ratio (length/breadth) greater than 10.
- Collagen
- Keratin
- Elastin
Globular proteins:- Also known as spheroproteins. These are compactly folded and coiled. These have axial ratio less than 10. Eg include:
- Albumin
- Globulin
- Histones
- Protamine

Classification of proteins based on composition of proteins

Simple proteins:- These proteins on hydrolysis yields amino acid and its derivatives
- Albumins:- soluble in water and coagulated by heat
- Globulins: – soluble in neutral salts and are heat coagulable.
- Glutelins:- soluble in dilute acids and alkalies and are insoluble in neutral solvents.
- Protamines:- soluble in water but are not heat coagulable.
Conjugated proteins:- These are simple proteins combined with non-protein substances.
- Nucleoproteins:- Composed of simple basic proteins with nucleic acid as prosthetic group.
- Glycoproteins:- Consist of simple proteins and carbohydrate as prosthetic group.
- Phosphoproteins:- Consist of combination of simple proteins and phosphoric acid as prosthetic group.
- Lipoproteins:- combination of proteins and lipids.
Derived proteins:- These proteins are derived from the combination of simple and conjugated proteins
- Primary derived proteins:- Also known as denatured proteins. These are derived from agents like heat, acids, alkalis etc. these are again divided in two:
  - Proteans:- These are earliest product of protein hydrolysis by action of dilute acids.
  - Metaproteins:- These are formed by further action of acids and alkalies on proteans.
- Secondary derived proteins:- These are formed in the progressive hydrolytic cleavage of the peptide bonds of metaproteins. Eg:- proteoses, peptones and peptides

Functions of proteins

It provides amino acid which are useful for the synthesis of body proteins
The carbon-skeleton of amino acid is catabolized to yield energy.
The carbon and nitrogen of proteins can be used for the synthesis of other nitrogen containing constituents as well as non-nitrogenous metabolites.

Multiple choice questions (MCQs)

1. Proteins are composed of which biomolecule?
A. Fats

B. Amino acids

C. Lipids

D. Vitamins

2. Amino acids in proteins are linked through which bond?
A. Covalent bond

B. Peptide bond

C. Glycosidic

D. None of the above

3. Which of the following proteins are also known as scelro-proteins?

A. Fibrous proteins

B. Globular proteins

C. Regulatory proteins

D. Contractile proteins

4. Which of the following is the example of defense proteins?
A. Dehydrogenase

B. Myoglobin

C. Hemoglobin

D. Immunoglobin

5. Which of the following is the example of storage proteins?
A. Dehydrogenase

B. Myoglobin

C. Hemoglobin

D. Immunoglobin

6. Which of the following is the example of transport proteins?
A. Dehydrogenase

B. Myoglobin

C. Hemoglobin

D. immunoglobin

7. Match the following-
a. Fibrous protein 1. peptones
b. Globular protein 2. metaproteins
c. Primary protein 3. albumin
d. Secondary protein 4. keratin

8. Which of the following proteins are coagulated by heat?
A. Albumin

B. Globulin

C. Glutelins

D. All of the above

9. Which of the following are soluble in neutral salts solution?
A. Albumin

B. Glutelins

C. Protamine

D. None of the above

10. Which of the following statement for fibrous protein is NOT true?
A. Fibrous protein are insoluble
B. They have high molecular weight
C. There peptide chain is coiled
D. Their axial ratio is more than 10

11. Which type of proteins are also known as denatured proteins?
A. Primary proteins

B. Secondary proteins

C. Conjugated proteins

D. None of the above

12. Ceruloplasmin protein carries which metal?

A. Magnesium

B. Copper

C. Iron

D. Sodium

13. The secondary structure of protein is held together by which type of bonding

A. Covalent bonding

B. 3-D stereochemical folding

C. Arrangement in space

D. Hydrogen bonding

14. High density lipoproteins is an example of which kind of protein?

A. Regulatory proteins

B. Conjugated proteins

C. derived proteins

D. none of the above

15. Which type of prosthetic group is present in nucleoproteins?
A. Neurons

B. Nerve cells

C. Nucleic acid

D. Axons of nerve cells

ANSWERS:-
1. Amino acid
2. Peptide bond
3. Fibrous proteins
4. Immunoglobin
5. Myoglobin
6. Hemoglobin
7. a – 4 b – 3 c – 2 d – 1
8. All of the above
9. None of the above
10. There peptide chain is coiled
11. Primary proteins
12. Copper
13. Hydrogen bonding
14. Conjugated Proteins
15. Nucleic acid

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REFERENCE:- Pankaja Naik- Biochemistry; 4^th edition; page no:- 67-71.