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 = the change in the substance being measured
- E.g. the amount of substrate used up in the reaction or the amount of product formed
- Time = the time taken for that change to occur
- Change = the change in the substance being measured
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 1: write out the equation for calculating the rate of enzyme activity
rate = change ÷ time
In this case: rate = amount of substrate used ÷ time
Step 2: 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
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 1: write out the equation for calculating the rate of enzyme activity
rate = change ÷ time
In this case: rate = amount of product formed ÷ time
Step 2: substitute in the known values and calculate the rate
rate = 45 cm³ ÷ 5 minutes
rate = 9 cm³ / min or 9 cm³ min⁻¹
- In some situations you may not be told how much something has changed during a reaction; 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 1: write out the equation for calculating the rate of enzyme activity
rate = 1 ÷ time
Step 2: substitute in the known values and calculate the rate
rate = 1 ÷ 50 seconds
rate = 0.02 s⁻¹
The units for the calculation above are in s⁻¹ because rate is given per unit time.
Examiner Tip
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
