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  • Explain the terms primary and secondary structure of proteins What is the difference between alpha helix and beta pleated sheet structure of proteins

Explain the terms primary and secondary structure of proteins. What is the difference between \alpha-helix and \beta-pleated sheet structure of proteins?

 

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Primary structure: Every protein contains one or more polypeptide chains. These chains are made up of a-amino acids linked in a specific sequence. This specificity of sequence attachment is called primary structure.

The successive hydrolysis in the presence of mineral acids leads to the determination of the primary structure of proteins. The reaction results in products with decreasing molecular mass in each step, as shown below:

Proteins → Proteoses → Peptones → Polypeptides →Simple Peptides →  \alpha-Amino acids 

Secondary structure: The folding and arrangement of polypeptide chains result in secondary structures. It also provides a specific shape to the protein molecule. There are two types of structures that arise from this folding -

\alpha-Helix structure: this structure is a very common folding way of polypeptide chains, in this, the h-bonds are twisted in a right-handed manner, with additional -NH group bonded with -C = O group at adjacent turns. Thus, the structure is commonly known as 3.613 helix. The pitch of helix contains 3.16 amino acids. Hydrogen bonds are the backbone of this structure present in between one amide group and a carbonyl group.


\beta-pleated sheet structure: The arrangement of polypeptide chains in in a zigzag manner adjacent to each other and fixed at a common distance. The intermolecular H-bonds hold the chains together, and numerous chains combine to make a sheet. A 3-D arrangement of these sheet on top of each other takes place and due to its resemblance to drapery the structure is named \beta-pleated sheet structure.

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