(i) Secondary Structure (2° structure): It is development of new stearic relationships amongst the amino acids for protecting their peptide bonds through formation of intrapolypeptide and interpolypeptide hydrogen bonds. Secondary structure is of three types — α-helix, β-pleated and collagen helix. The prefixes α and β signify the first and second types of secondary structures discovered by Pauling and Corey (1951).
α-Helix : The polypeptide chain is spirally coiled, generally in a clockwise or right¬handed fashion (Fig ). There are 3.6 amino acid residues per turn of the spiral. The . spiral is stabilized by straight hydrogen bonds between imide group (-NH-) of one amino acid and carbonyl group (-CO-) of fourth amino acid residue. In this way, all the imide and carbonyl groups become hydrogen bonded. R-groups occur towards the outer side of a-helix. a-helix is the final structure in certain fibrous proteins,
e.g., keratin (hair, nail, horn), epidermin (skin).
β-Pleated Sheets : Two or more polypeptide chains come together and form a sheet. Condensation is little. However, twisting does occur. The same polypeptide may fold over itself to form two strands for β-pleating. Adjacent polypeptide chains may occur in parallel (e.g., p-keratin) or antiparallel (e.g., silk fibroin). Straight hydrogen bonds occur between imide (-NH-) group of one polypeptide and carbonyl (-CO-) group of adjacent polypeptide. Cross-linkages help in the stabilisation of p-pleated sheets.
Collagen Helix : Collagen has a large amount of glycine (25%) and proline (and hydroxyproline, 25%). It cannot form a-helix due to them. Three of its polypeptide each having about 1000 amino acid residues, come together with each forming an extended left-handed helix. They run parallel, form a right-handed super-helix that is stabilized by hydrogen bonds amongst the three. The triple helix of collagen is often called tropocollagen. Its one end is stabilized by -S-S- linkages amongst the three chains. Collagen occurs in those tissues where extensibility is limited,
e.g., connective tissue, tendons, bones.
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