Structure of proteins

The structure of proteins is very complex. The primary structure of a protein refers to the number and sequence of the amino acids in its polypeptide chains (discussed in the formation of proteins). The primary structure is represented beginning with the amino acid whose amino group is free (the N-terminal end) and it forms the one end of the chain. Free carboxyl group (C-terminal end) forms the other end of the chain.

         Side chains may have basic groups or acidic groups as -NH₂ in lysine and -COOH in aspartic acid. Because of these acidic and basic side chains, there are negatively and positively charged centers. Though the peptide linkage is stable, the reactivity is due to these charged centres in the side chains.

         Primary structure tells us nothing about the shape or conformation of the molecule. Most of the bonds in protein molecules being single bonds can assume infinite number of shapes due to free rotation about single bonds. However, it has been confirmed that each protein has only a single three dimensional conformation. The fixed configuration of a polypeptide skeleton is referred to as the secondary structure of a protein. It gives information:

          · About the type in which the protein chain is folded and bent;

         · About the nature of the bonds which stabilise this structure.

Secondary structure of protein is mainly of two types

(i) α-helix : This structure is formed when the chain of  α -amino acids coils as a right handed screw (called a-helix) because of the formation of hydrogen bonds between amide groups of the same peptide chain, i.e., NH group set in one unit is related to carbonyl oxygen of the third unit by hydrogen bonding. This hydrogen bonding between different units is responsible for holding helix in a position. The side chains of these units project outward from the coiled backbone.

Such proteins are elastic, i.e., they can be stretched. On stretching weak hydrogen bonds break up and the peptide chain acts like a spring. The hydrogen bonds are reformed on releasing the tension. Wool and hair have  α -helix structure.

(ii) β-pleated sheet : A different type of secondary structure is possible when polypeptide chains are arranged side by side. The chains are held together by a very large number of hydrogen bonds between C = O and NH of different chains. Thus, the chains are bonded together forming a sheet. These sheets can slide over each other to form a three dimensional structure called a beta pleated sheet. Silk has a beta pleated structure.

         Globular proteins possess tertiary structure. In general globular proteins are very tightly folded into a compact spherical form.

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