Question
Explain the terms primary and secondary structure of proteins. What is the difference between α-helix and β-pleated sheet structure of proteins?
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Answer
Primary structure: Proteins may have one or more polypeptide chains. Each polypeptide chain has a large number of a-amino acids linked to one another in a specific sequence. The specific sequence in which the various α-amino acids present in a protein are linked to one another is called its primary structure. Any change in this primary structure, i.e., sequence of amino acids creates a different protein.
The primary structure of a protein is usually determined by its successive hydrolysis with either enzymes or mineral acids into various products having decreasing molecular mass as shown below:
Secondary structure: The secondary structure gives the manner in which the polypeptide chains are folded or arranged. Therefore, it gives the shape or conformation of the protein molecule.
This arises from the plane geometry, of the peptide bond and hydrogen bond between the >C = O and N – H groups of different peptide bonds.
$\alpha-$Helix structure: A helix form is the most common form in which a polypeptide chain forms all possible types of hydrogen bonds by twisting into a right handed screw (helix) with the -NH group of each amino acid residue hydrogen bonded to -C = O group of the adjacent turn of the helix. The a-helix structure is also known as 3.613 helix. This represents that each turn of the helix contains approximately 3.6 amino acids and a 13-member ring is formed by hydrogen bonding. The helix is held in its shape primarily by hydrogen bonds between one amide group and carbonyl group which is 3.6 amino acids units away.
$\beta-$pleated sheet structure: In this conformation, the polypeptide chains lie side by side in a zigzag manner with alternate R groups on the same side situated at fixed distances apart. The two such neighbouring polypeptide chains are held together by intermolecular H-bonds. A number of such chains can be interbounded to form a sheet. These sheets are then stacked one above the other like the pages of this book to form a three-dimensional structure. This structure resembles pleated folds of drapery and hence is also called β-pleated sheet structure.
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