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Tertiary Structure: How R-Group Interactions Shape Protein Folding
Definition
Tertiary structure
The tertiary structure of a protein refers to the complete three-dimensional folding of a single polypeptide chain.
- The tertiary structure of a protein is the complete three-dimensional shape of a single folded polypeptide chain.
- Secondary structure comes from hydrogen bonds along the polypeptide backbone.
- Tertiary structure instead depends on interactions between the R-groups (side chains) of amino acids scattered along the sequence.
- Four types of interaction stabilise tertiary structure: hydrogen bonds, ionic bonds, disulfide bonds, and hydrophobic interactions.
- Each operates differently, and together they hold the protein in a precise conformation.
Analogy
- Think of tertiary structure like a coat held together by different fasteners.
- Some fasteners are weak but numerous, like hydrogen bonds.
- Some are strong but sensitive to conditions, like ionic bonds.
- Some are permanent, like disulfide bonds.
- The overall shape depends on which parts are water-repellent versus water-loving, driven by hydrophobic interactions.
Hydrogen Bonds Between Polar R-Groups
- Hydrogen bonds form between polar R-groups, when a slightly positive hydrogen atom (for example in an -OH group) is attracted to an electronegative atom like the oxygen in a C=O group.
- Each individual hydrogen bond is weak, but a protein contains hundreds of them.
- Collectively they provide a significant stabilising force.
Tip
- Hydrogen bonds are highly sensitive to changes in temperature and pH.
- This is why enzyme activity depends so heavily on environmental conditions.
- Exam questions frequently test this link.
Ionic Bonds Between Oppositely Charged R-Groups
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- B1.2.9 Dependence of tertiary structure on functional groups (HL)
- B1.2.10 Effect of polar and non-polar amino acids on tertiary structure of proteins (HL)
- B1.2.11 Quaternary structure of non-conjugated and conjugated proteins (HL)
- B1.2.12 Relationship of form and function in globular and fibrous proteins (HL)