CBSE Class 12 Chemistry Notes: Biomolecules – Structure of Proteins

Structure of Proteins

(i) Proteins have three dimensional structures.

(ii) There are number of factors which determine the exact shape of proteins.

Structure of Proteins

Primary structure

(i) This type of structure was given by Friedrich Sanger in 1953 in Insulin.

(ii) Primary structure is conformed by single polypeptide chain in a linear manner.

(iii) All amino acid are attached in a straight chain by peptide bond.

Secondary structure

(i) The fixed configuration of polypeptide skeleton is referred to as the secondary structure of protein.

(ii) It gives information

(a)  About the manner in which the protein chain is folded and bent.

(b)  About the nature of the bond which stabilizes this structure.

(iii) This structure of protein is mainly of two types

(A) a-Helix

(a)  The chain of α-amino acids coiled as a right handed screw (called a-helix) because of the formation of hydrogen bond.

(b)  The spiral is held together by H-bonds between N–H and C = O group vertically adjacent to one another.

(c)  X-Ray studies have shown that there are approximately 3.6 amino acid unit for each turn in helix.

(d)  Such proteins are elastic i.e., they can be stretched.

(e)  On stretching weak H-bonds break up and the peptide act like a spring.

(f)   The hydrogen bonds are reformed on releasing the tension.

e.g. Myosin, Keratin, Tropomysin.

(B) Beta-pleated sheet

(a)  Polypeptide chains are arranged side by side.

(b)  The chains are held together by a very large number of hydrogen bond between  C = O and NH of different chains.

(c)  These sheets can slide over each other to form a three dimensional structure called a beta pleated sheet.

e.g. Silk has a beta pleated structure.

Tertiary structure

(i) It refers to the arrangement and interrelationship of the twisted chain of protein into specific layer or fibres.

(ii) This tertiary structure is maintained by weak interatomic force such as, H-bonds hydrophobic bond, van der Waals’ force and disulphide bonds (eg Insulin).

e.g. Protein of tobacco mosaic virus (TMV); Myoglobin; Hemoglobin

Quarternary structure

(i) When two or more polypeptide chain united by the force other than covalent bond i.e., peptide and disulphide bonds.

(ii) It refers to final three dimensional shape that results from twisting bonding and folding of the protein helix.

(iii) It is most stable structure.

Classification of Proteins

There are two methods for classifying proteins.

(i) Classification according to Composition

(ii) Classification according to Functions

Illustration 1. Secondary structure of proteins is due to

                        (A)  peptide bond                                    (B)  hydrogen bond

                        (C)  covalent bond                                  (D)  co-ordinate bond

Solution: Hydrogen bond.

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