CIE A Level Biology Topic 3: Enzymes Exam Questions

About CIE A Level Biology Topic 3: Enzymes

Enzymes in the Cambridge 9700 syllabus builds directly on protein structure from Topic 2. You need to explain how enzymes lower activation energy by forming enzyme-substrate complexes, compare the lock and key and induced fit models, and describe how temperature, pH, enzyme concentration and substrate concentration affect rate. The topic also covers competitive and non-competitive inhibition, the meaning of Vmax and the Michaelis-Menten constant (Km) as a measure of affinity, and the use of immobilised enzymes. It is a favourite for practical-based questions, so expect data on initial rates, graph sketching and evaluation of experimental methods in Papers 2, 3 and 5.

What you need to know

• Mode of action Enzymes are globular proteins that lower activation energy. The substrate binds to the complementary active site to form an enzyme-substrate complex, then an enzyme-product complex. Enzymes can act intracellularly or extracellularly.
• Models Lock and key: the active site is a fixed complementary shape. Induced fit: the active site changes shape slightly as the substrate binds, putting strain on bonds.
• Factors affecting rate Rate increases with temperature until the optimum, then falls as hydrogen and ionic bonds break and the active site changes shape (denaturation). pH changes alter ionic and hydrogen bonding in the active site.
• Inhibition Competitive inhibitors resemble the substrate and bind to the active site; increasing substrate concentration overcomes them. Non-competitive inhibitors bind elsewhere, change the active site shape and lower Vmax.
• Vmax and Km Vmax is the maximum rate when all active sites are occupied. Km is the substrate concentration at half Vmax; a low Km means high affinity. Competitive inhibitors raise Km but Vmax is unchanged.
• Immobilised enzymes Enzymes trapped in alginate beads are easily separated from product, can be reused and are often more stable at extreme temperature and pH.

Common mistakes in Topic 3: Enzymes

• Saying enzymes are killed at high temperature; enzymes are not alive, they are denatured.
• Describing the active site and substrate as having the same shape; they are complementary.
• Stating that a competitive inhibitor reduces Vmax; with enough substrate the same Vmax is reached, but Km increases.
• Using the overall rate instead of the initial rate; the initial rate is measured from the tangent at time zero, before substrate concentration falls.
• Explaining the drop above the optimum as fewer collisions; the drop is due to denaturation and loss of the active site shape.

Try three Topic 3: Enzymes exam-style questions