Explain active sites and enzyme–substrate complexes
Substrate binding at an active site forms an enzyme–substrate complex and provides a route with lower activation energy.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons teach enzyme concepts and experimental reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Recognise that proteins have specific three-dimensional shapes and that reaction rates can be compared using controlled measurements.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does a substrate bind to an enzyme?
What you will learn
- Describe the active site and enzyme–substrate complex
- Explain how enzymes lower activation energy without being consumed
Binding occurs at an active site
The active site is the region of an enzyme where a substrate binds. The substrate has a shape and chemical properties that allow a temporary interaction with this site.
When the substrate binds, an enzyme–substrate complex forms. The substrate is converted into product or products, which leave the active site. The enzyme is available to catalyse another cycle.
A lower-energy reaction route
The enzyme–substrate interaction helps reactions proceed through a route with lower activation energy. More substrate molecules can then reach the reaction state in a given time, increasing reaction rate.
The enzyme does not change the identities of the overall reactants and products or the total energy change of the reaction. It changes how quickly the reaction proceeds.
Pause and explain
What happens to activation energy when an enzyme catalyses a reaction?
Worked example
Place the stages of an enzyme reaction in order and explain the energy effect.
Show the worked solution
- Substrate binds at the active site.
- An enzyme–substrate complex forms; product is made and leaves.
- The enzyme provides a route with lower activation energy.
Answer Substrate binds at the active site to form an enzyme–substrate complex. The reaction proceeds by a route with lower activation energy, products leave, and the enzyme can catalyse another cycle.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how an enzyme and substrate form a complex.
Give me a hint
Name the binding region and the temporary state.
Compare my reasoning
- The substrate binds to the enzyme’s active site.
- This produces an enzyme–substrate complex.
The substrate binds at the enzyme’s active site, forming a temporary enzyme–substrate complex.
Look for these in your work
- Active site named
- Complex described
Explain how a lower activation energy can increase the rate of a reaction.
Give me a hint
Relate the barrier to the number of successful reactions per time.
Compare my reasoning
- A lower barrier makes reaction easier under the same conditions.
- More substrate molecules can react per unit time.
With a lower activation-energy barrier, a greater proportion of substrate molecules can react in a given time under the same conditions, so the reaction rate increases.
Look for these in your work
- Barrier linked to reaction
- Rate explained per time
A student says that an enzyme changes the products because it lowers activation energy. Correct the explanation.
Give me a hint
Separate reaction pathway from overall reaction outcome.
Compare my reasoning
- The enzyme changes the rate, not the overall identities of reactants and products.
- It provides a lower-activation-energy route and is not used up overall.
The enzyme increases the rate by providing a route with lower activation energy. It does not change the overall reactants or products and is not used up overall.
Look for these in your work
- Rate distinguished from products
- Catalytic role accurately stated
Common mistakes
- Describing the active site as a separate enzyme
- Saying the enzyme supplies the reaction energy or changes the products
- Treating the enzyme–substrate complex as a permanent bond
What can you explain now?
Where does a substrate bind to an enzyme?
Compare with the explanation
At the active site
The active site binds substrate during catalysis.
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Make it stick
What binds at an active site, what complex forms, and how does an enzyme affect activation energy?
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