The aim of this practical is to investigate the rate of enzyme-controlled reactions. The details of the practical itself vary depending on what the enzyme, substrate, and product are. Let’s use catalase as an example.

Catalase is an intracellular enzyme which catalyses the conversion of toxic hydrogen peroxide (the substrate) into oxygen and water (the products). The rate of reaction can be calculated by plotting a graph of the volume of oxygen produced over time. To measure the volume of oxygen produced, the reaction can be set up in a test tube with a bung and delivery tube connected to either a gas syringe or an upturned measuring cylinder. Watch this video for a demonstration of both techniques (yes, I know it’s a GCSE chemistry video, but the techniques are the same). We also need to know how the volume changes over time, so take a reading at set time intervals.

When a graph has been plotted, the rate of reaction can be calculated for any point on the graph by drawing a tangent to the curve at that point, than calculating the gradient of the tangent. To calculate the initial rate of reaction, a tangent can be drawn at 0 on the graph. The initial rate of reaction will always be the highest rate because there is the highest concentration of substrate at that point. As soon as the reaction begins, the substrate starts being converted to product so the substrate concentration decreases over time. This is why the rate slows down over time, and the reaction will eventually stop when all the substrate has been converted to product.

Another way to measure the rate of reaction would be to measure the amount of substrate used up over time (e.g. measure a change in mass), so in an exam question be careful to look for how the rate has been measured.

### Control variables

Remember that enzyme activity can be affected by a number of different factors. In any investigation, it is really important to control any variables which may affect your results. For example, if you were investigating the rate of an enzyme-controlled reaction over time, you would want to make sure all of these variables were controlled:

• Volume and concentration of the substrate solution
• Volume and concentration of the enzyme solution
• pH of the solutions
• Temperature of the solutions

However, always be careful to understand what the aim of the investigation is. For example, if the aim is to look at the effect of temperature on the rate of reaction, you would not want to control the temperature – instead, temperature would be the independent variable (the one you change).

### Summary

• The rate of an enzyme-controlled reaction can be measured by recording the volume of product produced or substrate used up over time.
• Finding the gradient of a tangent to the curve will give you the rate of reaction at that point.
• All factors that could affect enzyme activity must be controlled to make sure the experiment is valid.