SECONDARY STRUCTURE OF PROTEINS: This focuses on the regular folding patterns of the polypeptide backbone. Polypeptide chains are stabilized in two pattern- α-helix and β-sheet conformations.
α-Helix: In this structure, the polypeptide backbone is tightly wound around an imaginary axis drawn longitudinally through the middle of the helix, and the R groups of the amino acid residues protrude outward from the helical backbone. The amino acid residues in anα- helix have conformations with ѱ= 45 to 50 and ф=60, and each helical turn includes 3.6 amino acid residues. The helical twist of the α-helix found in all proteins is right-handed. An α- helix makes optimal use of internal hydrogen bonds.
Β-Sheet: The β-conformation arranges polypeptide chains into sheets. In the β- conformation, the backbone of the polypeptide chain is extended into a zigzag rather than helical structure. The zigzag polypeptide chains can be arranged side by side to form a structure resembling a series of pleats. In this arrangement, called a β-sheet, hydrogen bonds are formed between adjacent segments of the polypeptide chain. The R groups of adjacent amino acids protrude from the zigzag structure in opposite directions, creating the alternating pattern. The adjacent polypeptide chains in a β-sheet can be either parallel or antiparallel.