2.3.22 · Biological Molecules
The Four Levels of Protein Structure — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
2.3.2 The Four Levels of Protein Structure — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe primary, secondary, tertiary and quaternary protein structure and the bonds at each level.Four levels — three apply to a single polypeptide and the fourth to proteins…
- Size range
- Definition
- Importance
- Alpha-helix
- Beta-pleated sheet
- Occurrence
Part 1 of 7Overview
Four levels
- Four levels three apply to a single polypeptide and the fourth to proteins with two or more chains.
Size range
- Size range from 3 amino acids (glutathione) to over 34,000 (titin).
Part 2 of 7Primary structure
Primary structure
Definition
- Definition the sequence of amino acids joined by peptide bonds.
Control
- Control the DNA of the cell specifies the type, number and order of amino acids.
Importance
- Importance one change in the sequence can alter the function of the whole protein.
Part 3 of 7Secondary structure
Secondary structure
Bonding
- Bonding hydrogen bonds between the weakly negative N and O atoms and weakly positive H atoms of the backbone.
Alpha-helix
- Alpha-helix hydrogen bonds between every fourth peptide bond.
Beta-pleated sheet
- Beta-pleated sheet the chain folds so parallel sections hydrogen bond to each other.
Occurrence
- Occurrence most fibrous proteins, such as collagen and keratin, have secondary structure.
Sensitivity
- Sensitivity these hydrogen bonds break with high temperature and pH changes.
Part 4 of 7Tertiary structure
Tertiary structure
Definition
- Definition further folding of the secondary structure, with bonds between R groups.
Bonds and interactions
- Bonds and interactions hydrogen bonds, disulfide bonds, ionic bonds, and weak hydrophobic interactions.
- (only between cysteines)
Occurrence
- Occurrence common in globular proteins such as antibodies.
Part 5 of 7Quaternary structure
Quaternary structure
Definition
- Definition a protein made of more than one polypeptide chain working as one macromolecule.
Subunits
- Subunits each chain is a subunit; haemoglobin has four.
Part 6 of 7Bonds at each level
| Bond | Primary | Secondary | Tertiary |
|---|---|---|---|
| Peptide | Yes | – | – |
| Hydrogen | – | Backbone (N–H and C=O) | Between R groups |
| Disulfide | – | – | Yes |
| Ionic | – | – | Yes |
| Hydrophobic interactions | – | – | Yes |
Part 7 of 7More from the specification
More from the specification
Primary Protein Structure and Peptide Bonds
- The primary structure is the sequence of amino acids joined by covalent peptide bonds, formed by condensation between the amine group (-NH2) of one amino acid and the carboxyl group (-COOH) of another, eliminating water.
Secondary Protein Structure
- Hydrogen bonds between carbonyl oxygen atoms and amide hydrogen atoms of the polypeptide backbone cause chains to fold into regular α-helices or β-pleated sheets.
Tertiary Structure Interactions and Shape
- The specific 3D shape of a protein is maintained by interactions between amino acid R groups,
- including ionic bonds, hydrophobic interactions avoiding water, hydrogen bonds,
- and covalent disulfide bridges.
Quaternary Structure and Prosthetic Groups
- Quaternary structure forms when multiple polypeptide subunits associate, sometimes incorporating a non-amino acid prosthetic group to yield a functional conjugated protein.
- (such as the iron-containing haem group in haemoglobin)
Primary Protein Structure and Peptide Bonds
- The primary structure is the sequence of amino acids joined by covalent peptide bonds, formed by condensation between the amine group (-NH2) of one amino acid and the carboxyl group (-COOH) of another, eliminating water.
Secondary Protein Structure
- Hydrogen bonds between carbonyl oxygen atoms and amide hydrogen atoms of the polypeptide backbone cause chains to fold into regular α-helices or β-pleated sheets.
Tertiary Structure Interactions and Shape
- The specific 3D shape of a protein is maintained by interactions between amino acid R groups,
- including ionic bonds, hydrophobic interactions avoiding water, hydrogen bonds,
- and covalent disulfide bridges.
Quaternary Structure and Prosthetic Groups
- Quaternary structure forms when multiple polypeptide subunits associate, sometimes incorporating a non-amino acid prosthetic group to yield a functional conjugated protein.
- (such as the iron-containing haem group in haemoglobin)
Prosthetic group
- A non-protein component, such as haem in haemoglobin.
Amine group
- The –NH2 functional group.
Triple helix
- Three polypeptides wound together.
Key terms
| Term | Meaning |
|---|---|
| Primary structure | The sequence of amino acids in a polypeptide. |
| Secondary structure | Alpha-helices and beta-pleated sheets held by hydrogen bonds. |
| Tertiary structure | The three-dimensional folding of one polypeptide through R-group interactions. |
| Quaternary structure | The arrangement of two or more polypeptide subunits. |
| Alpha-helix | A coiled secondary structure. |
| Beta-pleated sheet | A folded sheet-like secondary structure. |
Avoid these mistakes
1
Saying secondary-structure hydrogen bonds involve R groups — they are between amino and carboxyl groups.
2
Saying disulfide bonds form between any amino acids — only between cysteines.
3
Saying a protein with one chain has quaternary structure — it needs two or more chains.
4
Forgetting that a single changed amino acid can alter function — primary structure determines folding.
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