Chapter 3 — Proteins and Amino Acids
Key Concepts Quick Recap
| Term | Definition |
|---|
| Amino acid | Has –NH₂ (amino), –COOH (carboxyl), R group, and H on α-carbon |
| Peptide bond | Covalent bond between –COOH of one and –NH₂ of next amino acid (dehydration) |
| Isoelectric point (pI) | pH at which amino acid/protein has zero net charge; minimum solubility |
| α-helix | Secondary structure; H-bonds within same polypeptide chain |
| β-pleated sheet | Secondary structure; H-bonds between adjacent polypeptide chains |
| Denaturation | Loss of 3D structure (but not primary) due to heat, pH, chemicals |
| Coagulation | Irreversible denaturation (e.g. cooking egg white) |
I. Choose the Best Answer (MCQs)
1. The bond that links amino acids in a protein chain is called: (a) Ester bond (b) Glycosidic bond (c) Peptide bond (d) Disulphide bond
Answer(c) Peptide bond Peptide bond: formed between –COOH of one amino acid and –NH₂ of the next with loss of water (condensation). This bond is planar and rigid (partial double bond character).
2. The α-helix is stabilised by: (a) Disulphide bonds (b) Hydrogen bonds within the same polypeptide (c) Ionic bonds (d) Van der Waals forces
Answer(b) Hydrogen bonds within the same polypeptide In α-helix: H-bonds between C=O of residue n and N-H of residue n+4 within the same chain. 3.6 residues per turn. Right-handed helix.
3. Denaturation of proteins destroys: (a) Primary structure (b) Secondary and tertiary structure (c) Amino acid composition (d) Peptide bonds
Answer(b) Secondary and tertiary structure Denaturation: disruption of H-bonds, ionic bonds, disulphide bonds → loss of 3D shape → loss of function. Primary structure (sequence of amino acids) is NOT destroyed. The protein can sometimes refold (renaturation). Example: heat denatures egg white.
4. The quaternary structure of haemoglobin consists of: (a) 1 polypeptide chain (b) 2 polypeptide chains (c) 4 polypeptide chains (2α + 2β) (d) 6 polypeptide chains
Answer(c) 4 polypeptide chains (2α + 2β) Haemoglobin: tetramer of 2α and 2β subunits, each with a haem group containing Fe²⁺. Quaternary structure is stabilised by non-covalent interactions between subunits.
5. An amino acid at its isoelectric point: (a) Has maximum solubility (b) Carries a positive charge (c) Carries no net charge (d) Is completely ionised
Answer(c) Carries no net charge At pI: +NH₃ group is balanced by –COO⁻ group → zwitterion (both charges, but net charge = 0). Minimum solubility and electrophoretic mobility = 0 at pI.
6. The essential amino acid that is also the precursor of serotonin and melatonin is: (a) Phenylalanine (b) Tyrosine (c) Tryptophan (d) Lysine
Answer(c) Tryptophan Tryptophan (essential) → serotonin (5-hydroxytryptamine) → melatonin. Also the precursor of nicotinic acid (Vitamin B3/Niacin). Deficiency leads to Pellagra.
7. Which test is used to detect proteins? (a) Benedict's test (b) Biuret test (c) Iodine test (d) Fehling's test
Answer(b) Biuret test Biuret test: proteins (2+ peptide bonds) + NaOH + CuSO₄ → violet/purple colour (Cu²⁺ complexes with peptide bonds). Positive for: proteins, peptides. Negative for: amino acids (only 1 peptide bond), carbohydrates.
II. Short Answer Questions
Q1. Describe the four levels of protein structure.
Answer
1. Primary structure: Linear sequence of amino acids joined by peptide bonds. Determined by gene (DNA). Example: insulin has 51 amino acids (2 chains: A=21, B=30).
2. Secondary structure: Local folding of polypeptide backbone:
- α-helix: right-handed spiral; H-bonds within chain; 3.6 residues/turn. Example: myoglobin (75% α-helix).
- β-pleated sheet: parallel or antiparallel strands; H-bonds between strands. Example: silk fibroin.
3. Tertiary structure: Overall 3D shape of entire polypeptide. Stabilised by: disulphide bonds (–S–S–, strongest), hydrophobic interactions, hydrogen bonds, ionic bonds. Example: myoglobin, ribonuclease.
4. Quaternary structure: Association of 2+ polypeptide subunits. Stabilised by non-covalent interactions. Example: haemoglobin (2α + 2β subunits), DNA polymerase.
Exam Checklist
- Peptide bond: –COOH + –NH₂ → –CO–NH– + H₂O (condensation).
- 4 levels: primary (sequence) → secondary (α-helix/β-sheet) → tertiary (3D) → quaternary (subunits).
- α-helix: H-bonds within chain; β-sheet: H-bonds between chains.
- Denaturation: secondary/tertiary destroyed; primary intact.
- Isoelectric point: net charge = 0; minimum solubility.
- Biuret test: violet with proteins; Benedict's test: reducing sugars.
- Essential amino acids (10): PVT TIM HaLL — Phe, Val, Trp, Thr, Ile, Met, His, Arg, Leu, Lys.
