Practical - Investigating Enzymatic Reactions
Measuring Product Formation: This often involves collecting a gas (e.g., oxygen from hydrogen peroxide breakdown by catalase) using a gas syringe or measuring a color change (e.g., using a colorimeter to track the appearance of a colored product).
Measuring Substrate Disappearance: A common technique is the use of iodine to test for the presence of starch; as amylase breaks down starch, the blue-black color of the iodine solution fades to orange-brown over time.
Controlling Variables: To ensure a fair test, factors like enzyme concentration, substrate concentration, and volume must be kept constant while the independent variable (e.g., pH or temperature) is varied.
Calculating Rate: The rate is typically calculated as the inverse of time () or by determining the initial gradient of a graph showing product concentration against time ().
| Feature | Temperature Investigation | pH Investigation |
|---|---|---|
| Control Method | Thermostatically controlled water bath | Use of buffer solutions |
| Kinetic Effect | Increases collision frequency and energy | Affects ionization of active site residues |
| High-End Effect | Rapid drop due to thermal denaturation | Gradual or rapid drop due to charge disruption |
| Equilibration | Reagents must reach temp before mixing | Buffers act immediately upon mixing |
Initial Rate vs. Average Rate: The initial rate is the most accurate measure of enzyme activity because it occurs when substrate concentration is not yet a limiting factor and product inhibition has not occurred.
Inactivation vs. Denaturation: Inactivation is often reversible (e.g., at very low temperatures where molecules lack kinetic energy), whereas denaturation is usually an irreversible change in protein structure.
Identify the Independent Variable: Always clarify what is being changed (e.g., the temperature of the water bath) versus what is being measured (e.g., the volume of gas produced).
Explain the 'Why' of Controls: If asked why a boiled enzyme is used as a control, explain that it proves the reaction is specifically catalyzed by the functional enzyme and not by spontaneous chemical breakdown.
Precision in Graphing: When plotting rates, ensure the axes are correctly labeled with units (e.g., for ). Use the linear portion of the curve (the first 30-60 seconds) to calculate the initial rate.
Check for Reliability: Mention the importance of repeats and calculating a mean to identify anomalies and improve the reliability of the findings.
'Enzymes are killed': Avoid saying enzymes are 'killed' by heat or pH; they are not living organisms. Use the term denatured to describe the loss of structural integrity.
Confusing Time and Rate: A common error is plotting 'time taken' on the y-axis and describing it as 'rate.' Remember that a shorter time indicates a higher rate; they are inversely proportional.
Ignoring Equilibration: In temperature experiments, failing to allow the enzyme and substrate to reach the target temperature separately before mixing leads to inaccurate data as the reaction starts at an undefined temperature.