Explain the primary, secondary, tertiary and quaternary structures of proteins. What happens to them when a protein is denatured? Give two examples of denaturation.

1. 1 · Define a protein. A polymer of α-amino acids joined by peptide bonds (-CO-NH-).
2. 2 · Primary structure. The sequence of amino acids; changing one amino acid gives a different protein.
3. 3 · Secondary structure. α-helix (right-handed coil) or β-pleated sheet, held by hydrogen bonds between C=O and N-H.
4. 4 · Tertiary and quaternary structures. Tertiary: overall folding, held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces. Quaternary: arrangement of subunits, as in haemoglobin.
5. 5 · Draw the levels. A bead chain, a coiled spring, a zigzag sheet, a folded ball with an S-S bridge, and two to four balls together.
6. 6 · Explain denaturation. Heat or a pH change breaks hydrogen bonds: 2° and 3° structures are lost, the primary structure stays, and activity is lost. Examples: boiled egg, curdling of milk.
7. 7 · Link to real life. Paneer is made by acid and heat denaturation of milk proteins.
8. 8 · Close in one line. So a protein's job depends on its folded shape, not just on its sequence.

Scoring tip: Name the bonds for every level. 'Held by hydrogen bonds' is the phrase examiners look for in the secondary structure.