Peptides and the four levels of protein structure

A peptide bond is an amide, –CO–NH–, formed when the carboxyl group of one amino acid joins the amino group of another with loss of water. Glycine's –COOH with alanine's –NH₂ gives the dipeptide glycylalanine.

Three amino acids joined by two peptide bonds make a tripeptide; then tetra-, penta-, hexapeptide. More than ten amino acids make a polypeptide.

A polypeptide with over a hundred residues and molecular mass above 10,000 u is called a protein. The line is not sharp: insulin, with only 51 amino acids, counts as a protein because it has a definite protein-like shape.

By shape. Fibrous proteins: parallel chains held by hydrogen and disulphide bonds form fibres, generally insoluble in water, e.g. keratin (hair, wool, silk) and myosin (muscles). Globular proteins: chains coil into a sphere and usually dissolve in water, e.g. insulin and albumins.

Primary structure: the exact sequence of amino acids in each chain. Change the sequence and you have a different protein.

Secondary structure: the shape of the backbone, held by hydrogen bonds between the C=O and –NH– groups of peptide bonds. Two kinds occur: α-helix and β-pleated sheet.

α-Helix: the chain twists into a right-handed screw so that each residue's –NH is hydrogen-bonded to a C=O on the next turn, making the most hydrogen bonds possible.

β-Pleated sheet: chains stretched almost fully lie side by side, held by hydrogen bonds between chains; the folds look like pleated drapery.

Tertiary structure: the overall folding of the chain beyond the secondary structure, giving the fibrous or globular shape. Hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic attraction stabilise 2° and 3° structure.

Quaternary structure: the arrangement in space of two or more chains (sub-units) in one protein. Haemoglobin is NCERT's example.