Proteins: peptide bond, structure and denaturation
Proteins are long chains of α-amino acids joined by peptide bonds. Their four levels of structure decide their shape; denaturation destroys the 2° and 3° structure but not the sequence.
Forming a peptide bond
- 1H₂N-CH₂-COOH (glycine) + H₂N-CH(CH₃)-COOH (alanine) → H₂N-CH₂-CO-NH-CH(CH₃)-COOH (glycylalanine, Gly-Ala) + H₂OThe -CO-NH- amide link is the peptide bond.
- 2Two amino acids: a dipeptide. Three: a tripeptide. More than ten: a polypeptide.n amino acids in a chain are joined by n − 1 peptide bonds.
- 3★A polypeptide with more than 100 amino acid residues and molar mass above 10,000 u is called a protein.The line is not sharp: insulin has only 51 amino acids but is called a protein because it has a definite folded shape.
★ key step
Fibrous vs globular proteins
Four levels of protein structure
| Level | What it is | Held by |
|---|---|---|
| Primary | The sequence of amino acids in each chain. Changing even one amino acid gives a different protein. | Peptide bonds (covalent) |
| Secondary | Regular folding of the chain into an α-helix or a β-pleated sheet | Hydrogen bonds between C=O and N-H of peptide groups |
| Tertiary | Further folding of the whole chain, giving a fibrous or globular shape | Hydrogen bonds, disulphide links, van der Waals forces and electrostatic attraction |
| Quaternary | The arrangement of two or more chains (subunits) relative to each other, e.g. the four subunits of haemoglobin | The same forces, acting between the subunits |
α-Helix vs β-pleated sheet
Denaturation
A protein in its natural form, with its unique 3D shape and biological activity, is a native protein. A physical change such as heating, or a chemical change such as a change in pH, disturbs the hydrogen bonds. The globules unfold, the helix uncoils, and the protein loses its biological activity. This is denaturation. The secondary and tertiary structures are destroyed, but the primary structure stays intact. Examples: egg white coagulates on boiling, and milk curdles when bacteria in it make lactic acid, which lowers the pH.
How to draw the levels of protein structure
- Primary: a straight line of beads, each bead labelled with a three-letter symbol: Gly-Ala-Val-Ser.
- Secondary, α-helix: a coiled spring, with dotted lines between the turns for hydrogen bonds.
- Secondary, β-sheet: two or three zigzag strands side by side, with dotted hydrogen bonds between them.
- Tertiary: the helix and sheet parts folded into a compact ball; mark one S-S bridge.
- Quaternary: two to four such balls packed together, each labelled as a subunit.
Making paneer uses denaturation: lemon juice or vinegar added to hot milk lowers the pH, and the milk proteins coagulate. The cooked protein has the same amino acids, so its food value stays.
2 marks: “What is denaturation of a protein? Give one example.” Loss of biological activity when heat or a pH change breaks hydrogen bonds and unfolds the protein; the 2° and 3° structures are lost, the primary structure stays. Example: egg white coagulating on boiling.
3 marks: “Distinguish between the primary, secondary and tertiary structures of proteins, naming the bonds that hold each.” Primary: amino acid sequence, peptide bonds. Secondary: α-helix or β-pleated sheet, hydrogen bonds between C=O and N-H. Tertiary: overall folding, hydrogen bonds, disulphide links, van der Waals and electrostatic forces.
2 marks: “Keratin in hair does not dissolve in water, but albumin in egg white does. Explain using the two shapes of protein molecules.” Keratin is a fibrous protein: parallel chains held by hydrogen and disulphide bonds, so it is insoluble. Albumin is globular: chains coiled into a sphere, so it is soluble.
Primary = sequence, secondary = spring or sheet, tertiary = tangle, quaternary = team of chains. Heat breaks the hydrogen bonds, not the peptide bonds.