Factors that affect enzymes
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of enzyme structure, the lock and key model, and the role of enzymes as biological catalysts.
What is an enzyme and what is its function?
An enzyme is a biological catalyst that speeds up chemical reactions in living organisms.
What is meant by the 'lock and key model'?
The active site is specific and complementary to the substrate.
What type of molecule are enzymes made from?
Protein
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the factors that affect enzymes, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Measuring the Rate of an Enzyme-Controlled Reaction
The rate of an enzyme-controlled reaction refers to how quickly the enzyme converts substrate into product.
How is rate measured?
-
You can calculate the rate of reaction by measuring:
-
The amount of product formed over time or amount of substrate used over time (e.g. volume of gas produced, disappearance of substrate).
-
The time taken for a reaction to complete (e.g. colour change from iodine-starch test).
Formula for rate :
Rate of reaction = Change in amount ÷ Time taken
For example:
-
If 30 cm³ of gas is produced in 10 seconds:
→ Rate = 30 ÷ 10
Rate = 3 cm³/s
Rate from a graph
The rate of an enzyme can also be calculated from a graph.
The gradient of the line is the rate of the reaction. The steeper the gradient the greater the rate of reaction.
Take a look at the study note about calculating the gradient of a graph in the skills section.
Enzymes and Their Conditions
Enzymes work best under specific conditions. Their activity can be affected by:
-
Temperature
-
pH
-
Substrate concentration
-
Enzyme concentration
Temperature
-
As temperature increases, particles gain kinetic energy, leading to more frequent successful collisions between enzyme and substrate. This leads to the formation of more enzyme-substrate complexes.
-
This increases the rate of reaction up to the optimum temperature.
-
Beyond the optimum temperature, the enzyme's structure denatures (bonds break), changing the shape of the active site so it can no longer bind to the substrate.
-
Enzyme activity falls sharply.
pH
-
Enzymes have an optimum pH at which they function best (e.g. protease enzymes in the stomach = pH 2, amylase enzymes in the small intestine = pH 7.5).
-
A pH that is too high or too low breaks bonds in the enzyme, denaturing it.
-
The active site changes shape, and enzyme-substrate complexes no longer form.
-
Some enzymes can tolerate a wider range of pH values.
Graph showing three different enzymes with different optimum pHs. Pepsin works best at an acidic pH of 2.5, this is a protease enzyme in the stomach.
Substrate Concentration:
-
More substrate means more chances of collision with enzyme active sites and so a faster rate of reaction.
-
But eventually, all active sites become occupied, and the rate plateaus (remains constant) - the enzyme is working at its maximum rate.
Enzyme Concentration:
-
More enzymes means more active sites available so a faster reaction (if there’s enough substrate available).
-
If substrate is limited, increasing enzyme concentration further has no effect on the rate of reaction.
Key Terms
-
Denature – permanent change in the shape of an enzyme’s active site.
-
Optimum – the best condition for enzyme activity, when reaction rate is at its maximum.
No answer provided.
Exam Tips
- Don’t say ‘the enzyme is killed’- remember, enzymes are proteins and are not living things. Explain that an enzyme is denatured and the active site can no longer bind to the substrate.
- Remember, low temperatures do not denature enzymes - they just slow down the rate of a reaction. Only temperatures above the optimum temperature will denature enzymes.
No answer provided.
Practice Question
Explain why enzyme activity decreases at temperatures above 40°C. (3 marks)
- High temperatures break bonds in the enzyme’s structure.
- This changes the shape of the active site and the enzyme is denatured.
- The substrate no longer binds to the active site, so no enzyme-substrate complexes can form.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!