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—a-helix, p-pleated and collagen helix. The prefixes a and p signify the first and second types of secondary structure 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), epidermis (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 anti-parallel (e.g., silk fibrin). Straight hydrogen bonds occur between imide (-NH-) group of one polypeptide and carbonyl (-CO-) group of adjacent polypeptide. Cross-linkages help in stabilization of β-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 stabilised by hydrogen bonds amongst the three. The triple helix of collagen is often called tropo- collagen. Its one end is stabilised by -S-S- linkages amongst the three chains. Collagen occurs in those tissues where extensibility is limited, e.g., connective tissue, tendons, bones.
Tertiary Structure (3° Structure) :
Tertiary (3°) structure is bending and folding of secondary strand (2°) of polypeptide (a-helix or β-pleated sheet) in such a way as to form a compact structure with functional sites being established over its surface by coming together of polar regions of specific amino acids. Hydrophobic parts of amino acids generally pass into the interior of protein. When both a-helix and β-pleating occur in the same polypeptide, tertiary structure segregates them into specific areas. Tertiary structure is stabilised by five types of bonds :
1. Disulphide Bonds : Between hydrogen sulphide groups of two cysteine or methionine amino acids.
2. Hydrogen Bonds : They occur between hydrogen and oxygen atoms of various groups like -NH OC- or -OH OC-.
3. Ionic Bonds : They appear between oppositely charged ionised groups of two amino acids, e.g., NH3+ and -COO .
4. Hydrophobic Interactions: They are not true bonds but involve coming together of non-polar R-groups of two amino acids. Hydrophobic interactions are
important in compaction of protein as they exclude water molecules in the area of their occurrence.
5. van derWaals Interactions : They develop between two closely placed polar groups through charge fluctuations between the two.
Functions of Proteins:
• Some proteins as hormones regulate many body functions. For example, the hormone insulin is a protein. It regulates sugar level in the blood.
• Some proteins as enzymes catalyse or help in biochemical reactions. For example, pepsin and trypsin.
