Enzyme Action and Kinetics
Enzymes are globular proteins that lower activation energy by forming enzyme–substrate complexes at a specific active site. Induced fit: the active site changes shape slightly as the substrate binds. Rate rises with temperature (more collisions) up to an optimum, then falls as bonds break and the enzyme denatures. Away from the optimum pH, ionic and hydrogen bonds change, altering the active site. Rate increases with substrate concentration until all active sites are occupied (Vmax). Km is the substrate concentration at ½Vmax: a low Km means high affinity. Competitive inhibitors bind the active site (overcome by more substrate; Vmax unchanged, Km increases). Non-competitive inhibitors bind elsewhere, changing the active site (Vmax decreases). Immobilised enzymes can be reused and give product free of enzyme.
Learning objectives
- Explain how enzymes lower activation energy.
- Explain effects of temperature, pH and concentration.
- Compare competitive and non-competitive inhibition.
- Use Vmax and Km.
Why does adding more substrate eventually stop increasing the rate?
Core idea
Your body digests a meal at 37 °C in hours. Without enzymes, it would take years.
Enzymes lower activation energy by binding substrates at a complementary active site. Rate depends on how often enzyme–substrate complexes form.
Where the inhibitor binds decides its effect.
Vmax = 10, Km = 2 mmol dm⁻³. Rate at [S] = 6?
- Goal
- v.
- Strategy
- Substitute.
- Equation.
- Three-quarters of Vmax.
Summary
- Induced fit.
- Temperature, pH, concentration.
- Vmax, Km; inhibitors.
- Active site
- Region of an enzyme where the substrate binds.
- Km
- Substrate concentration giving half the maximum rate.
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