4.2.4Additional lessons
Protein Structure and Mutations — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 33 of 101
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 5Levels of Protein Structure
A protein works because of its shape, and shape depends on the amino acid sequence.
Primary
- Primary This is the linear sequence of amino acids held by peptide bonds, and it is fixed by the gene.
Secondary
- Secondary Hydrogen bonds between backbone groups fold the chain into alpha helices and beta pleated sheets.
Tertiary
- Tertiary Interactions between R groups, including ionic bonds, disulfide bridges and hydrophobic effects, fold the chain into a specific 3D shape.
Quaternary
- Quaternary Some proteins contain several polypeptides joined together, such as haemoglobin with four chains.
| Level | Bonds involved |
|---|---|
| Primary | Peptide bonds |
| Secondary | Hydrogen bonds |
| Tertiary | Ionic, disulfide, hydrogen, hydrophobic interactions |
| Quaternary | Same interactions between separate chains |
Part 2 of 5From Mutation to Altered Protein
1
A point mutation changes one base in the DNA, for example GAG becoming GTG in a gene.
2
The mRNA codon changes accordingly, so GAG becomes GUG and a different amino acid may be inserted.
3
The new R group may have different charge or polarity, which changes bonding with nearby parts of the chain.
4
The tertiary structure may alter, and so the active site or binding region may no longer work properly.
Part 3 of 5Sickle Cell Example
Sickle Cell Example
The change
- The change A single base substitution in the gene for the beta chain of haemoglobin replaces glutamic acid with valine at position 6.
Effect on structure
- Effect on structure Valine is hydrophobic and causes haemoglobin molecules to stick together in low oxygen conditions.
Effect on cells
- Effect on cells Red blood cells become sickle shaped, block small blood vessels and carry less oxygen.
Inheritance
- Inheritance People with one copy of the allele are carriers and show some resistance to malaria, which helps explain its frequency.
Part 4 of 5Worked Example
Question
- In sickle cell, the sixth codon changes from GAG to GUG.
- Identify the type of mutation and its effect.
- Compare the codonsonly the middle base changes (A to U), so one base is substituted.
- GAG codes for glutamic acid and GUG codes for valine.
- So a single amino acid differs, which is a missense mutation.
Answer Base substitution (missense) replacing glutamic acid with valine.
Part 5 of 5Exam Checklist & Tips
4.2.4 – Key Points for the Exam
- Primary structure The sequence of amino acids in a polypeptide.
- Tertiary structure The overall 3D folding of a single polypeptide.
- Point mutation A change affecting a single base in DNA.
- Substitution A mutation in which one base is replaced by another.
- Hydrophobic Tending to avoid water and cluster in the interior of a folded protein.
Common Mistakes
- Saying that a mutation always changes the protein; silent mutations do not alter the amino acid.
- Stating that peptide bonds hold tertiary structure; they form the primary structure.
- Writing that a protein changes shape because of a different gene in the same cell rather than because of its own amino acid changes.
- Forgetting to link the changed amino acid to altered bonding and then to altered function.
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