EnzymesOxford AQA IGCSE Biology: Revision notes

Enzymes are biological catalysts that speed up chemical reactions without being used up in the process. They work by lowering the activation energy required for reactions to occur, allowing reactions to happen at body temperature that would otherwise require much higher temperatures.

Enzymes are proteins with a specific three-dimensional shape. Each enzyme has an active site – a region with a unique shape that is complementary to the substrate it acts upon. The substrate is the substance that the enzyme acts on, and the product is what is formed after the reaction.

The basic reaction can be summarised as:

• Enzyme + Substrate → Enzyme–Substrate Complex → Enzyme + Product

Lock and Key Model (basic explanation) The lock and key model is the simpler explanation of enzyme action. It describes the active site as a rigid, fixed shape that is complementary to the substrate. The substrate fits into the active site like a key fits into a lock. Once the substrate is positioned correctly, the enzyme catalyses the reaction, and the product is released.

Induced Fit Model (higher tier understanding) The induced fit model is a more accurate explanation of how enzymes actually work. It suggests that the active site is not completely rigid; instead, it changes shape slightly when the substrate approaches and binds to it. This induced change in shape:

• Allows the active site to fit the substrate more precisely
• Puts strain on the substrate bonds, lowering the activation energy even further
• Improves enzyme efficiency and specificity

After the product is released, the active site returns to its original shape, ready to catalyse another reaction.

Optimum Temperature Enzymes have an optimum temperature – the temperature at which they show maximum activity. For human enzymes, this is typically around 37°C (body temperature).

Effect of temperature changes:

• Below optimum: At lower temperatures, enzyme and substrate molecules move more slowly. Fewer collisions occur between enzyme and substrate molecules, so the rate of reaction is slower.

• At optimum: Maximum molecular movement and collision frequency; the enzyme is most active.

• Above optimum: The enzyme becomes denatured. The heat energy causes bonds within the enzyme's protein structure to break. The active site changes shape permanently and becomes complementary to the substrate no longer. The enzyme can no longer bind substrate or catalyse reactions. This change is irreversible – the enzyme cannot refold correctly even if cooled.

Enzyme Denaturation Denaturation is a permanent change to the three-dimensional structure of the enzyme protein. Once denatured:

• The active site is permanently altered
• The enzyme cannot function
• The enzyme cannot be 'repaired' by cooling

This is different from reduced activity at lower temperatures – that is reversible, but denaturation is not.

Optimum pH Enzymes have an optimum pH – the pH at which they show maximum activity. Different enzymes have different optimum pH values:

• Pepsin (stomach enzyme): optimum pH ≈ 2 (acidic)
• Trypsin (small intestine enzyme): optimum pH ≈ 8 (alkaline)
• Amylase (saliva): optimum pH ≈ 7 (neutral)

Effect of pH changes:

• At optimum pH: Enzyme is fully active; the active site is correctly shaped.

• Away from optimum pH: As pH changes, hydrogen ions and hydroxide ions in the solution affect the ionic and hydrogen bonds that hold the enzyme protein in its three-dimensional shape. The active site becomes less complementary to the substrate, and enzyme activity decreases.

• Extreme pH (very high or very low): The enzyme becomes denatured. Bonds break, the active site changes shape permanently, and the enzyme loses all function. This denaturation at extreme pH is irreversible.

Key difference from temperature: Both temperature and extreme pH can cause denaturation, but at moderate pH changes (away from optimum but not extreme), enzyme activity is reversibly reduced. The enzyme regains activity if pH is restored to optimum. However, at extreme pH, denaturation is permanent.

Effect of Substrate Concentration

At a fixed enzyme concentration, increasing substrate concentration increases the rate of reaction – but only up to a point.

• At low substrate concentration: Few substrate molecules are available. Many enzyme active sites are empty and unoccupied. Increasing substrate concentration increases the number of enzyme–substrate complexes formed, so the reaction rate increases.

• At high substrate concentration: All enzyme active sites are occupied (the enzyme is saturated). Every enzyme molecule is working as fast as possible. Adding more substrate has no effect on the rate – it remains constant at the maximum rate (Vmax). The enzyme is the limiting factor, not the substrate.

The rate of reaction increases with substrate concentration until the enzyme becomes saturated, then it plateaus.

Effect of Enzyme Concentration

At a fixed substrate concentration that is in excess:

• Increasing enzyme concentration directly increases the rate of reaction.
• More enzyme molecules mean more active sites available to bind substrate.
• Doubling the enzyme concentration doubles the rate of reaction (assuming substrate is not limiting).
• This relationship is linear as long as substrate concentration remains high enough.

Summary of Limiting Factors:

• Low substrate, normal enzyme: substrate is limiting (add more substrate to increase rate)
• High substrate, low enzyme: enzyme is limiting (add more enzyme to increase rate)

Investigating enzyme activity involves measuring how the rate of reaction changes under different conditions. Common experiments use:

Example 1: Amylase and Starch Amylase is an enzyme that breaks down starch into simpler sugars (maltose).

1. Add amylase enzyme to a starch solution.
2. At timed intervals (e.g. every 10 seconds), remove a small sample and add iodine solution.
3. Starch turns blue-black with iodine; as starch is broken down, the blue colour fades.
4. Measure the time taken for the blue colour to disappear (this indicates all starch has been broken down).
5. Repeat at different temperatures or pH values.

Example 2: Catalase and Hydrogen Peroxide Catalase breaks down hydrogen peroxide (H₂O₂) into water and oxygen gas: 2H₂O₂ → 2H₂O + O₂↑

1. Add catalase enzyme (from liver or potato) to hydrogen peroxide solution.
2. Measure the rate of oxygen gas production:
   • By collecting gas in a gas syringe over time
   • By timing how long the foam (from oxygen bubbles) takes to rise a fixed height
   • By measuring the volume of gas produced in a given time
3. Repeat at different temperatures or pH values.

Investigating Temperature:

• Use a water bath to set the temperature.
• Incubate enzyme and substrate at the temperature, then mix and measure reaction rate.
• Test temperatures from 10°C to 70°C in 10°C intervals.
• Plot a graph of rate vs. temperature to find the optimum.

Investigating pH:

• Prepare buffer solutions of different pH values (pH 2, 4, 6, 7, 8, 10, 12).
• Add enzyme to each buffer and measure reaction rate.
• Plot a graph of rate vs. pH to identify the optimum pH.

Key Practical Points:

• Control variables: Keep all conditions constant except the one being tested (temperature, pH, substrate concentration, or enzyme concentration).
• Repeat measurements: Take multiple readings to ensure reliability.
• Quantify the rate: Use measurable changes (time to colour change, gas volume, product formed per unit time).