Explain the enzyme-substrate complex and product release
Trace the Extended reaction sequence through temporary binding at the active site.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall protein building blocks and the distinction between a molecule and a living cell.
- Use the supplied observations, quantities and units rather than assuming an optimum for every enzyme.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does S denote in this stated reaction sequence?
What you will learn
- Trace the Extended reaction sequence through temporary binding at the active site.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Follow the named stages
In the Extended model, the substrate binds at the complementary active site and forms an enzyme-substrate complex. This is a temporary association during the reaction, not a new living cell or a permanent use-up of the enzyme.
The enzyme catalyses conversion of the bound substrate into product or products. Those products leave the active site, and the enzyme is available for another suitable substrate. Name substrate and product separately rather than treating them as unchanged labels.
Use a symbolic sequence
A useful schematic is E + S → ES → E + P for a stated one-product reaction. E denotes enzyme, S substrate, ES the enzyme-substrate complex and P product. A different supplied reaction may release more than one product.
The drawing is a molecular model, not proof of the actual time taken by each stage. Product release frees the active site; saying only that the substrate “touches the enzyme” omits complex formation, conversion and release.
Sources for this lesson
Pause and explain
What frees the active site after conversion?
Worked example
Explain E + S → ES → E + P using the required Extended terms.
Show the worked solution
- S binds at E’s active site to form the enzyme-substrate complex ES.
- The enzyme catalyses conversion of substrate to product.
- P leaves and E is available again.
Answer A temporary enzyme-substrate complex forms at the active site, product is formed and released, and the enzyme is unchanged overall.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Name ES in the supplied reaction sequence.
Give me a hint
Use the temporary-binding term.
Compare my reasoning
- E and S are associated at the active site.
The enzyme-substrate complex.
Look for these in your work
- Use the complete term.
- Identify active-site binding.
Put these events in order: product release, substrate binding, substrate conversion.
Give me a hint
Start with the reactant arriving.
Compare my reasoning
- Substrate binding forms the complex.
- Conversion produces product.
- Product is released.
Substrate binding → conversion → product release.
Look for these in your work
- Keep all three events.
- Do not consume E as product.
A student explains enzyme action only as “substrate touches enzyme”. What should an Extended explanation add?
Give me a hint
Trace binding, chemical change and exit.
Compare my reasoning
- Name the active site and temporary enzyme-substrate complex.
- Describe conversion into product.
- Describe release and enzyme availability.
Add complex formation at the active site, substrate-to-product conversion and product release with enzyme reuse.
Look for these in your work
- Use all required terms.
- Explain the sequence rather than contact alone.
Common mistakes
- ES is not enzyme plus a whole cell.
- The complex is temporary, not permanent catalytic consumption.
What can you explain now?
What frees the active site after conversion?
Compare with the explanation
Product release.
The enzyme becomes available again.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain E, S, ES and P without looking at the symbolic sequence.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.