2.2.1Unit 2: Cells, Development, Biodiversity and Conservation
Amino Acids, Proteins and Protein Structure — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 26 Unit quiz
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 5Amino Acids and Peptide Bonds
Proteins are polymers whose monomers are amino acids.
General structure
- General structure Every amino acid has a central carbon bonded to an amine group, a carboxyl group, a hydrogen atom and an R group.
R group
- R group This variable side chain differs between the 20 amino acids and decides whether each one is polar, non-polar, acidic or basic.
Condensation
- Condensation A hydroxyl from one carboxyl group and a hydrogen from another amine group leave as water when two amino acids join.
Peptide bond
- Peptide bond The covalent C–N bond formed between the carboxyl carbon of one amino acid and the amine nitrogen of the next.
Dipeptide and polypeptide
- Dipeptide and polypeptide Two amino acids make a dipeptide, while many amino acids joined in a chain make a polypeptide.
Hydrolysis
- Hydrolysis Adding water breaks peptide bonds and returns a polypeptide to its separate amino acids, as happens in digestion.
Part 2 of 5Four Levels of Structure
1
Primary structure is the exact sequence of amino acids in the polypeptide, held together by peptide bonds, and it is set by the gene.
2
Secondary structure is local folding into an α-helix or β-pleated sheet, held in shape by hydrogen bonds between the backbone groups.
3
Tertiary structure is the further folding of the whole chain into a three-dimensional shape, stabilised by interactions between R groups.
4
These R group interactions are hydrogen bonds, ionic bonds, disulfide bridges between cysteine residues, and hydrophobic interactions.
5
Quaternary structure exists only when a protein has two or more polypeptide chains, which may also include a non-protein prosthetic group.
Part 3 of 5Structure and Function
Structure and Function
Globular proteins
- Globular proteins Compact and roughly spherical with hydrophobic R groups inside,
- which makes them soluble and suited to metabolic roles such as enzymes.
Haemoglobin
- Haemoglobin A globular protein of four polypeptide chains, each with an iron-containing haem group that binds oxygen.
Fibrous proteins
- Fibrous proteins Long strands with repetitive sequences that are insoluble and strong, giving structural support in tissues.
Collagen
- Collagen Three polypeptides wound into a rope shape with many cross-links, giving great tensile strength in tendons and skin.
| Feature | Globular | Fibrous |
|---|---|---|
| Shape | Compact, spherical | Long, rope-like |
| Solubility | Usually soluble | Usually insoluble |
| Typical role | Enzymes, transport, hormones | Support and strength |
| Example | Haemoglobin | Collagen, keratin |
Part 4 of 5Worked Example
Question
- A single polypeptide is made of 150 amino acids.
- State the number of peptide bonds and the number of water molecules released when it forms.
- Each peptide bond joins two neighbouring amino acids, so a chain of n amino acids has n − 1 bonds.
- Number of peptide bonds = 150 − 1 = 149.
- Each condensation reaction releases one water molecule, so water released = 149.
Answer 149 peptide bonds are formed and 149 water molecules are released.
Part 5 of 5Exam Checklist & Tips
2.2.1 – Key Points for the Exam
- Amino acid The monomer of proteins, with an amine group, carboxyl group, hydrogen and variable R group on one carbon.
- Peptide bond The covalent bond joining two amino acids, formed by a condensation reaction.
- Primary structure The sequence of amino acids in a polypeptide chain.
- Tertiary structure The overall three-dimensional folding of a single polypeptide held by R group interactions.
- Quaternary structure The arrangement of two or more polypeptide chains in one protein.
Common Mistakes
- Describing the peptide bond as hydrogen bonding;
- it is a covalent bond and hydrogen bonds hold the secondary structure.
- Saying the R group takes part in the peptide bond, when only the amine and carboxyl groups do.
- Writing that all proteins have a quaternary structure, when many have only one polypeptide.
- Mixing up primary structure, which is a sequence, with secondary structure, which is a fold.
Last updated:
\n