The big idea: When a chain folds, R-groups from far apart in the sequence end up next to each other. Four kinds of bond between them lock the fold in place.
The points to remember
- Tertiary structure is the way the whole chain folds into its 3-D shape.
- Hydrogen bonds form between polar R-groups.
- Ionic bonds form between R-groups with opposite charges.
- Disulfide bonds are covalent bonds between two cysteines: the strongest of the four.
- Hydrophobic interactions hold non-polar R-groups together, away from water.
Remember it as: H, ionic, S-S and hydrophobic: four ways R-groups hold on.
| Bond or interaction | Between | Strength |
|---|---|---|
| Hydrogen bond | polar R-groups | weak |
| Ionic bond | positive and negative R-groups | weak |
| Disulfide bond | the S atoms of two cysteines | strong (covalent) |
| Hydrophobic interaction | non-polar R-groups | weak |
Bond or interaction?: Say hydrophobic interactions, not hydrophobic bonds, and hydrogen bonds, ionic bonds and disulfide bonds as bonds.
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The big idea: Some R-groups contain an amine or a carboxyl group. Gaining or losing a hydrogen ion gives them a charge, and charged R-groups can form ionic bonds.
The points to remember
- An amine group in an R-group can gain H⁺ and become positive: NH₂ → NH₃⁺.
- A carboxyl group in an R-group can lose H⁺ and become negative: COOH → COO⁻.
- Opposite charges attract: NH₃⁺ and COO⁻ form an ionic bond.
- pH sets how many H⁺ ions there are, so a change in pH changes the charges and can break ionic bonds.
Remember it as: Amine gains H⁺: plus. Carboxyl loses H⁺: minus.
Amine group in an R-group
- Gains H⁺
- NH₂ becomes NH₃⁺
- Positive charge
Carboxyl group in an R-group
- Loses H⁺
- COOH becomes COO⁻
- Negative charge
Real example: hair perms: Hair is the protein keratin, rich in disulfide bonds. A perm lotion breaks them, the hair is set on rollers, and a second lotion forms new disulfide bonds that hold the new curl.
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How it is asked: A two-mark List of R-group interactions, and multiple-choice items that match each level of structure to the bonds that hold it.
Interactions between R-groups in different parts of a polypeptide help to determine the tertiary structure of a protein. List two types of interaction between R-groups.
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And the level-and-bond match from Paper 1A:
Which level of protein structure is stabilised by disulfide, hydrogen and ionic bonds between R-groups?
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