12.8.2 Secondary Structure of Protein

The secondary structure describes how the segments of the backbone chain fold (Scheme 3). These conformations are stabilized by H-bonding between the peptide groups-between NH of one amino acid residue and C=O group of another.

(a) α -Helix

(b)  β -Pleated Sheet

Scheme 3. A segment of a protein in: (a) an α-helix; (b) β-pleated sheet.

α-Helix:

The first type of secondary structure is α-helix, where the backbone coils around the long axis of the protein molecule. The substituents on the α-carbon of the amino acids protrude outward from the helix to minimize the steric hindrance. The H attached to amide nitrogen makes H-bonding with the carbonyl oxygen of an amino acid.