CCEA A Level Biology 1.2 Enzymes Exam Questions

About CCEA A Level Biology 1.2 Enzymes

Enzymes are section 1.2 of Unit AS 1 and are a favourite essay topic on the CCEA paper. You need to explain how enzymes lower activation energy through enzyme-substrate complexes, compare the lock and key and induced fit models, and explain the effects of temperature, pH, substrate and enzyme concentration and inhibitors. CCEA adds applied content: elastase as a biomarker of respiratory infection, active site-directed therapeutic inhibitors and immobilised enzymes in biosensors. This links back to protein structure in 1.1 and forward to respiration and photosynthesis. Examiners often ask you to interpret rate graphs and to explain practical design.

What you need to know

• Structure Enzymes are globular proteins with a specific active site whose shape is set by tertiary structure. Cofactors are non-protein substances needed for activity; coenzymes are organic cofactors.
• How enzymes work They lower activation energy by forming enzyme-substrate complexes. Lock and key says the active site is a rigid complementary shape; induced fit says the active site changes shape slightly as the substrate binds, putting strain on bonds.
• Factors affecting rate Rate rises with temperature until bonds holding tertiary structure break and the enzyme denatures. Each enzyme has an optimum pH. Increasing substrate concentration raises rate until all active sites are occupied; increasing enzyme concentration raises rate while substrate is in excess.
• Inhibition Competitive inhibitors resemble the substrate and bind the active site; raising substrate concentration reduces their effect. Non-competitive inhibitors bind elsewhere, change the active site shape, and are not overcome by more substrate.
• Biomarkers and therapeutic inhibitors Elastase released by white blood cells during respiratory infection hydrolyses elastin, and its presence in blood, urine or sputum acts as a biomarker. Active site-directed inhibitors are designed to block enzymes involved in disease.
• Immobilised enzymes Enzymes are trapped in gels or beads or bonded to fibres or plastic. They are more stable, allow continuous flow and leave the product enzyme-free, but activity may be reduced. Used in biosensors such as glucose test strips and pregnancy tests.
• Practicals Investigate the effect of temperature, pH, substrate and enzyme concentration on rate, and follow the breakdown of starch by amylase using a colorimeter.

Common mistakes in 1.2 Enzymes

• Saying enzymes are killed at high temperature. Enzymes are not alive; they are denatured when the active site loses its shape.
• Saying the enzyme and substrate have the same shape. The active site is complementary to the substrate.
• Explaining a plateau at high substrate concentration by saying the substrate runs out. At high substrate concentration, all active sites are occupied, so enzyme concentration is the limiting factor.
• Stating that non-competitive inhibitors bind to the active site. They bind at an allosteric site, which changes the active site shape.
• Forgetting the drawback of immobilisation. Reduced activity due to restricted access of substrate to the active site should be balanced against the benefits.

Try three 1.2 Enzymes exam-style questions