Every enzyme has a specific reaction that it catalyses. When an enzyme is catalysing its reaction, the rate at which the reaction is taking place is referred to as the enzyme activity.
If the reaction catalysed by the enzyme is happening at a fast rate, then there is a high enzyme activity.
If the reaction catalysed by the enzyme is happening at a slow rate, then there is a low enzyme activity.
Enzyme activity is affected by many factors, including the temperature at which the reaction is taking place and the pH.
In order to measure an enzyme's activity, we need to measure the rate of the reaction it is catalysing.
We can measure the rate of a reaction by either measuring the rate at which one of the substrates is being used up or measuring the rate at which one the products is being produced.
For example, if one of the products is a gas, we can collect the gas in a gas syringe as it is produced. We can then measure how much gas is produced in a particular amount of time.
To work out the rate of reaction (and therefore the enzyme activity), we simply divide the amount of substrate used up or product produced by the time over which that happened.
Enzyme activity can be calculated by measuring the amount of one of the substrates that is produced or the amount of one of the products that is produced and then dividing this by the time taken.
The graph below shows how enzyme activity changes with temperature:
Graph showing the relationship between enzyme activity and temperature.
At very low temperature, the activity of any enzyme is close to zero. This is because the substrate molecules and the enzymes have very little kinetic energy, which makes it difficult for the reaction to take place.
As the temperature increases, the kinetic energy of the substrates and enzymes increases, which causes the enzyme activity to increase.
However, if the temperature gets too high, the structure of the enzyme starts to change. The high temperature causes the enzyme to become folded differently, which means that the active site no longer fits the substrates and the reaction cannot take place. The enzyme activity falls to zero. An enzyme that has lost its shape in this way is described as denatured.
Every enzyme has an optimum temperature. This is the temperature at which the enzyme activity is highest.
The optimum temperature is different for different enzymes. Many enzymes in the human body have optimum temperatures of around 37°C, since this is the temperature of the human body. There are some bacteria that are adapted to live in very hot environments. Enzymes in these bacteria often have optimum temperatures of 80°C or more.
pH is a measure of how acidic a solution is. The lower the pH, the more acidic the solution is.
The folding of an enzyme is affected by the pH of the solution that it is in.
Every enzyme has an optimum pH - the pH at which its activity is highest. When the pH is close to the optimum, the enzyme is folded correctly, which allows the active site to bind to the substrates, which is why the enzyme activity is high.
If the pH rises above or falls below the optimum pH, the enzyme activity falls towards zero as the enzyme becomes denatured.
Graph showing the relationship between enzyme activity and pH.
The optimum pH is different for different enzymes. For example, digestive enzymes found in the stomachs of animals have low optimum pHs because stomachs are very acidic environments. Whereas digestive enzymes found in animals' intestines have higher optimum pHs, because intestines are not as acidic.
Different enzymes have different optimum pHs. A digestive enzyme from the intestines has a much higher optimum pH than a digestive enzyme from the stomach.
Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.
What is enzyme activity?
How is enzyme activity measured?
What does 'denatured' mean?
How does enzyme activity change with temperature?
How does enzyme activity change with pH?
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