# Proteins
Proteins are the most versatile biological molecules, functioning as enzymes, structural components, antibodies, hormones, and transport molecules. Their diverse functions arise from their complex 3D structures.
1. Amino Acids
General structure: central carbon bonded to:
- Amino group (—NH₂)
- Carboxyl group (—COOH)
- Hydrogen atom (—H)
- R group (variable — determines which amino acid)
20 different amino acids; R group varies.
2. Peptide Bonds
Condensation reaction between amino group and carboxyl group:
Peptide bond formed between C=O and N—H. Hydrolysis breaks peptide bonds (adds water).
3. Levels of Protein Structure
Primary Structure
- Sequence of amino acids in the polypeptide chain
- Determined by DNA/gene sequence
- Held by peptide bonds
Secondary Structure
- Polypeptide folds into:
- α-helix: coiled; held by H-bonds between C=O and N—H
- β-pleated sheet: flat; held by H-bonds between adjacent chains
Tertiary Structure
- Further folding of secondary structure into 3D shape
- Held by: H-bonds, ionic bonds, disulfide bridges (S—S), hydrophobic interactions
- Determines the protein's function
Quaternary Structure
- Two or more polypeptide chains (subunits) working together
- May include non-protein groups (prosthetic groups, e.g. haem in haemoglobin)
- Examples: haemoglobin (4 subunits), insulin (2 chains)
4. Globular vs Fibrous Proteins
| Feature | Globular | Fibrous |
|---|---|---|
| Shape | Spherical/compact | Long, thin |
| Solubility | Soluble | Insoluble |
| Function | Metabolic (enzymes, antibodies) | Structural |
| Examples | Haemoglobin, insulin, enzymes | Collagen, keratin |
5. Enzymes
Biological catalysts:
- Active site complementary to substrate shape
- Induced fit model: active site changes shape slightly upon substrate binding
- Enzyme-substrate complex → products
- Lowers activation energy
Factors Affecting Enzyme Activity
- Temperature (optimum ≈ 37°C; denaturation above)
- pH (each enzyme has optimum)
- Substrate concentration (rate plateaus at Vmax)
- Enzyme concentration
- Inhibitors: competitive (blocks active site) and non-competitive (changes shape)
6. Testing: Biuret Test
- Add Biuret reagent (NaOH + CuSO₄) to sample
- Positive: blue → purple/lilac (peptide bonds present)
- Negative: stays blue
7. Practice Questions
- Describe the four levels of protein structure.
- Explain the difference between the lock-and-key and induced fit models.
- How do competitive and non-competitive inhibitors differ?
- Why does changing pH affect enzyme activity?
- Compare globular and fibrous proteins with examples.
Want to check your answers and get step-by-step solutions?
Summary
- Amino acids: NH₂—CHR—COOH; joined by peptide bonds (condensation)
- 4 levels: primary (sequence) → secondary (helix/sheet) → tertiary (3D shape) → quaternary (subunits)
- Globular (soluble, metabolic) vs fibrous (insoluble, structural)
- Enzymes: induced fit model; affected by T, pH, substrate conc., inhibitors
- Biuret test: blue → purple = protein present