Enzymes
Enzymes are biological catalysts, usually proteins, that lower activation energy. The exam tests active-site models, rate calculations, the effects of temperature, pH, substrate and inhibitor concentration, and immobilised enzymes.
Models and rates
- Lock-and-key describes a rigid complementary active site; induced fit describes a conformational change that tightens around the substrate.
- Rate is fastest at optimum temperature and pH. Above optimum, hydrogen and ionic bonds break, the active site loses shape and the enzyme denatures.
- Increasing substrate concentration increases rate until all active sites are saturated. Increasing enzyme concentration increases rate only if substrate is not limiting.
Inhibition
Competitive inhibitors bind the active site and are overcome by more substrate. Non-competitive inhibitors bind elsewhere and change active-site shape; extra substrate does not reverse this.
Worked example
A reaction produces 12 cm3 of gas in 2 minutes, then 30 cm3 in 10 minutes. The initial rate is 12 / 2 = 6 cm3 per minute. The average rate over 10 minutes is 30 / 10 = 3 cm3 per minute, showing the rate falls as substrate is used up.
Traps
Denaturation is not the same as inhibition. Do not say enzymes are used up. pH changes affect charge and shape, not just temperature. Immobilised enzymes are reused and easier to separate but may have lower activity.