2.2.5Inquiry Process
Levels of Protein Structure — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 46 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Primary and Secondary Structure
Protein structure is described in four levels, each built on the one before.
Primary
- Primary The linear sequence of amino acids held together by peptide bonds, determined by the gene.
Alpha helix
- Alpha helix A coil of the chain stabilised by hydrogen bonds between nearby backbone groups.
Beta pleated sheet
- Beta pleated sheet Segments lie side by side in a zigzag, held by hydrogen bonds between backbone groups.
Backbone bonding
- Backbone bonding Secondary structure arises from hydrogen bonds between the C=O and N-H groups of the backbone, not from R groups.
| Level | Described by | Main bonds |
|---|---|---|
| Primary | Amino acid sequence | Peptide |
| Secondary | Helix or sheet | Hydrogen |
| Tertiary | Overall 3D fold | R-group interactions |
| Quaternary | More than one polypeptide | R-group interactions between chains |
Part 2 of 5Tertiary Structure
Tertiary structure is the final three-dimensional shape of a single polypeptide.
Tertiary Structure
Hydrogen bonds
- Hydrogen bonds Form between polar R groups and help hold the fold.
Ionic bonds
- Ionic bonds Form between positively and negatively charged R groups.
Disulfide bridges
- Disulfide bridges Strong covalent links between cysteine side chains.
Hydrophobic interactions
- Hydrophobic interactions Non-polar R groups gather inside the protein, driving it to fold.
Part 3 of 5Quaternary Structure
Only proteins made of more than one polypeptide chain have a quaternary level of structure.
Definition
- Definition The arrangement of two or more polypeptide chains into one functional protein.
Haemoglobin
- Haemoglobin Made of four polypeptide chains, each carrying a haem group that binds oxygen.
Insulin
- Insulin Consists of two polypeptide chains linked by disulfide bridges.
Collagen
- Collagen Three polypeptide chains wind into a triple helix, giving great tensile strength.
| Protein | Number of chains | Notable feature |
|---|---|---|
| Haemoglobin | 4 | Haem group in each chain binds oxygen |
| Insulin | 2 | Chains joined by disulfide bridges |
| Collagen | 3 | Triple helix with great tensile strength |
| Myoglobin | 1 | Tertiary structure only |
Part 4 of 5Building a Protein Step by Step
1
Ribosomes join amino acids in the order set by mRNA, giving the primary structure.
2
Parts of the chain form helices and sheets by hydrogen bonding, giving secondary structure.
3
R-group interactions fold the chain into its tertiary structure.
4
Some proteins join with other polypeptides, giving quaternary structure.
5
The final shape allows a specific function such as catalysis or oxygen transport.
Part 5 of 5Exam Checklist & Tips
2.2.5 – Key Points for the Exam
- Primary structure The sequence of amino acids in a polypeptide.
- Secondary structure Regular folding such as alpha helices and beta sheets.
- Tertiary structure The overall three-dimensional shape of one polypeptide.
- Quaternary structure The combination of several polypeptides in one protein.
- Prosthetic group A non-protein component such as haem that is permanently part of a protein.
Common Mistakes
- Stating that peptide bonds hold the tertiary structure; they hold the primary structure.
- Saying all proteins have quaternary structure; only multi-chain proteins do.
- Giving R-group bonding as the cause of secondary structure.
- Confusing a haem group with an amino acid.
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