Chapter 4 — Enzymes
Key Concepts Quick Recap
| Term | Definition |
|---|
| Enzyme | Biological catalyst (mostly protein); speeds up reactions without being consumed |
| Active site | Region on enzyme where substrate binds; specific shape (lock-and-key) |
| Substrate | Molecule that enzyme acts on |
| Product | Result of enzyme-catalysed reaction |
| Coenzyme | Organic non-protein cofactor (vitamins B1, B2, B3, B12) |
| Cofactor | Non-protein component needed for enzyme activity (metal ions, coenzymes) |
| Km | Substrate concentration at half Vmax; measure of affinity |
| Vmax | Maximum reaction rate when enzyme is saturated with substrate |
I. Choose the Best Answer (MCQs)
1. Enzymes are mostly: (a) Carbohydrates (b) Lipids (c) Proteins (d) Nucleic acids
Answer(c) Proteins Almost all enzymes are proteins (globular proteins). Exception: Ribozymes (catalytic RNA) — e.g., RNase P, ribosome (peptidyl transferase activity is due to rRNA).
2. The "lock and key" model of enzyme-substrate interaction was proposed by: (a) Michaelis and Menten (b) Emil Fischer (c) Daniel Koshland (d) Pauling
Answer(b) Emil Fischer Emil Fischer (1894): lock-and-key model — enzyme (lock) has a rigid active site that fits exactly the complementary substrate (key). Koshland (1958): induced fit model — active site is flexible, changes shape when substrate binds.
3. The unit of enzyme activity is: (a) Katal (b) Mole (c) Dalton (d) Joule
Answer(a) Katal 1 Katal = amount of enzyme that catalyses conversion of 1 mole of substrate per second. Older unit: International Unit (IU) = 1 μmol substrate converted per minute.
4. Competitive inhibitors: (a) Decrease Km and decrease Vmax (b) Increase Km but Vmax unchanged (c) Decrease Vmax but Km unchanged (d) Both Km and Vmax unchanged
Answer(b) Increase Km but Vmax unchanged Competitive: inhibitor at active site → higher [S] needed to reach ½Vmax → Km ↑. But if [S] is very high → all inhibitor displaced → Vmax unchanged.
5. An enzyme without its cofactor is called: (a) Holoenzyme (b) Apoenzyme (c) Proenzyme (d) Coenzyme
Answer(b) Apoenzyme
- Apoenzyme: protein part of enzyme alone (inactive without cofactor).
- Cofactor: non-protein component (metal ion or coenzyme).
- Holoenzyme: Apoenzyme + Cofactor = fully active enzyme.
6. The Q₁₀ temperature coefficient for enzymes is approximately: (a) 1 (b) 2 (c) 5 (d) 10
Answer(b) 2 Q₁₀ ≈ 2 for enzyme reactions: for every 10°C rise in temperature, reaction rate approximately doubles. But above optimum temperature, enzyme denatures → rate falls sharply.
7. Allosteric enzymes are regulated by: (a) Competitive inhibitors at active site (b) Molecules binding at non-active (allosteric) sites (c) Changes in pH only (d) Temperature only
Answer(b) Molecules binding at non-active (allosteric) sites Allosteric regulation: regulator molecule (activator or inhibitor) binds at allosteric site (not active site) → conformational change → alters enzyme activity. Important in metabolic regulation (feedback inhibition).
II. Short Answer Questions
Q1. Explain the Michaelis-Menten equation.
Answer
The Michaelis-Menten equation describes the rate of enzyme-catalysed reactions:
v = Vmax[S] / (Km + [S])
where:
- v = reaction velocity at substrate concentration [S]
- Vmax = maximum velocity (enzyme saturated with substrate)
- Km = Michaelis constant = [S] at v = Vmax/2
Interpretation:
- When [S] << Km: v ≈ Vmax[S]/Km → rate proportional to [S] (first order kinetics).
- When [S] >> Km: v ≈ Vmax → rate independent of [S] (zero order kinetics).
- When [S] = Km: v = Vmax/2.
Significance of Km: Enzyme with LOW Km has HIGH affinity (reaches Vmax/2 at low substrate concentration).
Q2. What are the factors affecting enzyme activity?
Answer
1. Temperature: Activity increases with temperature (up to optimum, ~37°C in humans). Above optimum → denaturation → activity falls sharply.
2. pH: Each enzyme has an optimum pH. Pepsin: pH 1.5-2; trypsin: pH 7.5-8; salivary amylase: pH 7. Outside optimum → ionisation of active site residues changes → activity decreases.
3. Substrate concentration: Activity increases with [S] until Vmax (saturation).
4. Enzyme concentration: Activity proportional to [enzyme] (if substrate not limiting).
5. Inhibitors: Competitive (increase apparent Km) or non-competitive (decrease Vmax) inhibitors reduce activity.
6. Cofactors: Metal ions (Mg²⁺, Zn²⁺, Fe²⁺) and coenzymes (NAD, FAD) essential for some enzymes.
Exam Checklist
- Enzymes are proteins (mostly); ribozymes = catalytic RNA.
- Lock-and-key (Fischer); Induced fit (Koshland).
- Km = [S] at ½Vmax; low Km = high affinity.
- Apoenzyme + cofactor = holoenzyme.
- Competitive inhibition: Km ↑, Vmax same. Non-competitive: Km same, Vmax ↓.
- Optimum temperature for human enzymes: ~37°C; pH varies by enzyme.
- Michaelis-Menten: v = Vmax[S]/(Km+[S]).
