2.2.3Additional lessons
Protein Structure: Effect of pH and Temperature — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 19 of 142
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 4Levels of Structure
Primary
- Primary The sequence of amino acids in the polypeptide, held by peptide bonds and determined by the gene.
Secondary
- Secondary Regular folding such as the alpha helix and beta pleated sheet, held by hydrogen bonds between backbone groups.
Tertiary
- Tertiary The overall three-dimensional shape of one polypeptide, held by interactions between R groups such as ionic bonds, disulfide bridges and hydrophobic interactions.
Quaternary
- Quaternary The arrangement of two or more polypeptides into one protein, as in haemoglobin, which has four.
Conformation and function
- Conformation and function The precise shape decides how the protein binds other molecules, for example the active site of an enzyme.
Haemoglobin
- Haemoglobin Haemoglobin shows quaternary structure, because four polypeptide chains each hold a haem group that binds oxygen.
Part 2 of 4Denaturation
1
Heat gives the atoms in a protein more kinetic energy, so they vibrate and break weak bonds such as hydrogen bonds.
2
Changes in pH alter the charge on R groups, which disrupts ionic bonds and hydrogen bonds that hold the shape.
3
The tertiary structure unfolds and the protein loses its specific conformation, although the primary structure is intact.
4
The altered shape means that an enzyme active site or a binding site no longer fits its substrate or ligand.
5
Denaturation is usually irreversible, as when the albumin in egg white turns from clear to solid white on cooking.
Part 3 of 4Effects and Exceptions
Effects and Exceptions
Optimum conditions
- Optimum conditions Each protein works best within a narrow range of temperature and pH that suits its normal environment.
Thermophiles
- Thermophiles Thermus aquaticus lives in hot springs at up to about 80 °C and has proteins that stay folded at temperatures that would denature most others.
Extreme pH
- Extreme pH Stomach enzymes work at low pH, whereas most intracellular enzymes function near neutral pH.
Spider silk
- Spider silk Silk proteins keep their properties over a wide temperature range, unlike most proteins.
| Factor | What it disrupts | Result |
|---|---|---|
| High temperature | Hydrogen bonds and other weak interactions | Loss of tertiary and secondary structure |
| Extreme pH | Ionic and hydrogen bonds | Change in charge and loss of shape |
Part 4 of 4Exam Checklist & Tips
2.2.3 – Key Points for the Exam
- Primary structure The sequence of amino acids in a polypeptide.
- Tertiary structure The three-dimensional folding of a single polypeptide.
- Denaturation Permanent change in the three-dimensional structure of a protein that destroys its function.
- Disulfide bridge A covalent bond between two cysteine R groups that stabilises tertiary structure.
Common Mistakes
- Saying heat breaks peptide bonds; denaturation affects weaker bonds and leaves the primary structure intact.
- Writing that enzymes are killed; they are not alive, so they are denatured.
- Assuming all proteins have the same optimum pH or temperature.
- Describing the secondary structure as held by R group interactions, which is tertiary structure.
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