At IB Biology AHL, tertiary protein structure is stabilized by hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions between amino acid R-groups. These interactions cause and maintain the polypeptide’s specific three-dimensional conformation.
How tertiary structure forms
The primary structure-the sequence of amino acids-determines which R-groups occur and their positions in the polypeptide. As the chain folds, interactions form between R-groups located in different parts of the chain.
| Interaction | How it stabilizes tertiary structure |
|---|---|
| Hydrogen bonds | Form between polar R-groups with regions of partial positive and partial negative charge. Individually weak, but numerous hydrogen bonds provide substantial stability. |
| Ionic bonds | Form through electrostatic attraction between oppositely charged R-groups. The charge of some R-groups depends on pH, so changes in pH can disrupt these bonds. |
| Disulfide bonds | Strong covalent bonds form between the sulfur atoms of two cysteine R-groups. These cross-links hold different regions of the polypeptide together. |
| Hydrophobic interactions | Non-polar R-groups cluster away from water in the interior of water-soluble globular proteins, while polar R-groups tend to face the aqueous environment. |
Because these interactions depend on R-group properties, a change in amino acid sequence can alter folding and therefore protein function. High temperature or extreme pH can disrupt interactions, changing the active site or overall conformation; this is denaturation.
A common misconception is that peptide bonds hold tertiary structure together. Peptide bonds join amino acids in the primary structure; they do not explain the folding that produces tertiary structure. Also, do not call hydrophobic interactions “hydrophobic bonds.”
