Refer to the given figure.\ Formation of structures $A$ and $B$ could be due to\

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Professor PC Thomas Classes · Answer

(b) : The folding of a linear polypeptide chain into a specific coiled structure is referred to as the secondary structure of the protein. Such coiling or folding is produced by hydrogen bonds. The importance of such folding lies in the fact that amino acid residues which may lie far apart in their primary sequence are brought together in close proximity by secondary folding. There are two types of secondary structures- $\alpha$-helix and $\beta$-pleated sheets. In $\alpha$-helix, hydrogen bonds between oxygen of carboxylic group of one amino acid residue and NH group of next amino acid residue. In $\beta$-pleated secondary structure, two or more polypeptide chains get interconnected by hydrogen bonds. Adjacent strands of polypeptides may run in the same direction (parallel $\beta$-sheet, e.g., $\beta$-keratin) or in opposite directions (antiparallel $\beta$-sheet, e.g., fibroin of silk).