The tertiary structure is the overall three-dimensional shape formed when the coiled or folded chain folds further. It is held in place mainly by interactions between the side chains (R groups):• Disulfide bridges: cysteine has the side chain -CH2SH. Two cysteine -SH groups, which may be far apart in the sequence, can be oxidised to form a covalent sulfur-sulfur bond, -CH2-S-S-CH2-. These are the strongest links.
• Ionic interactions: an acidic side chain present as -COO− attracts a basic side chain present as -NH3+.
• Hydrogen bonds: between polar side chains, for example -OH groups (as in serine).
• Van der Waals forces: between non-polar side chains, which tend to cluster in the middle of the protein, away from water.
Worked example. Which interaction forms between the side chains of lysine, -(CH2)4NH2, and aspartic acid, -CH2COOH, at pH 7? At this pH the amine group is protonated, -NH3+, and the acid group is deprotonated, -COO−, so they form an ionic interaction.Heating breaks hydrogen bonds, and changing the pH changes the charges on side chains, so both can destroy the tertiary structure. Disulfide bridges are covalent and much harder to break, which is why they are important in tough proteins such as keratin in hair.The tertiary structure gives each protein its precise shape, which is essential to its function, for example the shape of the active site of an enzyme.
• Ionic interactions: an acidic side chain present as -COO− attracts a basic side chain present as -NH3+.
• Hydrogen bonds: between polar side chains, for example -OH groups (as in serine).
• Van der Waals forces: between non-polar side chains, which tend to cluster in the middle of the protein, away from water.
Worked example. Which interaction forms between the side chains of lysine, -(CH2)4NH2, and aspartic acid, -CH2COOH, at pH 7? At this pH the amine group is protonated, -NH3+, and the acid group is deprotonated, -COO−, so they form an ionic interaction.Heating breaks hydrogen bonds, and changing the pH changes the charges on side chains, so both can destroy the tertiary structure. Disulfide bridges are covalent and much harder to break, which is why they are important in tough proteins such as keratin in hair.The tertiary structure gives each protein its precise shape, which is essential to its function, for example the shape of the active site of an enzyme.