Rate Calculations for Enzyme Activity
- Rate calculations are important in determining how fast an enzyme is working (i.e. the rate of reaction)
- To perform a rate calculation, use the following formula:
Rate = Change ÷ Time
- ‘Change‘ refers to the change in the substance being measured
- This could be the amount of substrate used up in the reaction or the amount of product formed
- ‘Time‘ refers to the time taken for that change to occur
- Another way to view the equation is as follows:
Rate = Amount of substrate used or product formed ÷ Time
Worked Example
Amylase catalyses the breakdown of starch into maltose. 15 grams of starch were added to a solution containing amylase. It took 2 hours for all the starch to be broken down. Calculate the rate of reaction.
Step One: Write out the equation for calculating the rate of enzyme activity
Rate = Change ÷ Time
(In this case, Rate = Amount of substrate used ÷ Time)
Step Two: Substitute in the known values and calculate the rate
Rate = 15 g ÷ 2 hours
Rate = 7.5 g / hr or 7.5 g hr⁻¹
- In the example above, the ‘change’ was the amount of substrate (starch) that is used up in the reaction
- In the example below, the ‘change’ is the amount of product that is formed in the reaction
Worked Example
The enzyme catalase catalyses the breakdown of hydrogen peroxide into water and oxygen. In one experiment, a student found that 45 cm³ of oxygen was released in 5 minutes. Calculate the rate of reaction.
Step One: Write out the equation for calculating the rate of enzyme activity
Rate = Change ÷ Time
(In this case, Rate = Amount of product formed ÷ Time)
Step Two: Substitute in the known values and calculate the rate
Rate = 45 cm³ ÷ 5 minutes
Rate = 9 cm³ / min or 9 cm³ min⁻¹
- Alternatively, you may not be told how much something has changed during a reaction (i.e. how much of a substrate has been used up or how much of a product has been formed)
- Instead, you may only be told the time taken for the reaction to occur
- In this case, you can still calculate the rate of reaction by using the following (slightly different) formula:
Rate = 1 ÷ Time
Worked Example
A student adds a set volume of starch solution to a set volume of amylase solution at a range of different pH values. At each pH, the student times how long it takes for the amylase to break down all of the starch. At pH 6 the time taken for amylase to break down all of the starch was 50 seconds. Calculate the rate of reaction at pH 6.
Step One: Write out the equation for calculating the rate of enzyme activity
Rate = 1 ÷ Time
Step Two: Substitute in the known values and calculate the rate
Rate = 1 ÷ 50 seconds
Rate = 0.02 s⁻¹
Exam Tip
The units for the calculation above are in s⁻¹ because rate is given per unit time. In an exam, you could be asked to plot the reaction rates (from an enzyme catalysed reaction) on a graph. However, using the equation ‘Rate = 1 ÷ Time’ often gives small numbers that are difficult to plot on a graph. In these cases, you can also use the equation:
Rate = 1000 ÷ Time
This equation give you bigger numbers that are easier to plot on a graph. So, for the calculation in the worked example above, you would get:
Rate = 1000 ÷ 50 seconds
Rate = 20 s⁻¹
