Biology (9700)
Temperature: Rate increases (more kinetic energy, more collisions) until **optimum** (~37°C for human enzymes). Above optimum → **denaturation** (active site shape changes permanently). pH: Each enzyme has an optimum pH. Extreme pH denatures (disrupts ionic/hydrogen bonds). Substrate concentration: Rate increases then plateaus (all active sites occupied = **Vmax**). Enzyme concentration: More enzyme = faster rate (if excess substrate).
Competitive: Inhibitor has similar shape to substrate, competes for active site. Can be overcome by increasing substrate concentration. Non-competitive: Inhibitor binds to allosteric site, changes active site shape. Cannot be overcome by more substrate.
Topic 3 of 4Cambridge A Levels
Enzymes
Enzyme kinetics, inhibition, and factors affecting activity
Enzymes are biological catalysts — globular proteins that lower activation energy. The active site has a specific 3D shape complementary to the substrate.
Lock and key model: Substrate fits exactly into active site. **Induced fit model** (more accurate): active site changes shape slightly to fit substrate more tightly → enzyme-substrate complex → products released.
Factors affecting rate:
Inhibition:
Key Points to Remember
- 1Induced fit: active site moulds around substrate
- 2Denaturation above optimum temperature (shape change)
- 3Vmax reached when all active sites occupied
- 4Competitive inhibitors compete for active site; non-competitive bind elsewhere
Pakistan Example
Enzyme Denaturation — Why Cooking Kills Nutrients
When Pakistani cooks boil daal or vegetables for too long, enzymes in the food denature — their active sites lose shape above 60-70°C, destroying vitamin-processing enzymes. This is why nutritionists recommend shorter cooking times. Conversely, the proteases in papaya (papain) are used as meat tenderisers in Lahori BBQ.