AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The action of enzymes as catalysts, including the concept of a…
Full specification wording
Enzymes (A-level only): The action of enzymes as catalysts, including the concept of a stereospecific active site that binds to a substrate molecule.
The active site is a small cleft in the folded protein whose shape and pattern of charges are complementary to one particular substrate.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The active site is a small cleft in the folded protein whose shape and pattern of charges are complementary to one particular substrate. The substrate binds there, forming an enzyme-substrate complex, and is held in exactly the orientation the reaction needs, with the bonds that must break already strained. That gives the reaction an alternative route with a lower activation energy, so at any given temperature a far larger fraction of collisions has enough energy to react and the rate rises sharply. The enzyme is unchanged at the end and immediately binds another substrate molecule. Because the amino acids that form the site are themselves chiral, the site is chiral, so it fits only one enantiomer of a substrate.
What earns the marks
- The active site having a shape complementary to the substrate.
- The substrate binding to give an enzyme-substrate complex.
- Catalysis described as providing an alternative route of lower activation energy.
- The enzyme being unchanged at the end so it can act again.
Common mistakes and how to put them right
Saying a catalyst lowers the activation energy of the original route rather than offering a different one.
Claiming the enzyme shifts the position of the equilibrium as well as speeding the reaction up.
Describing the substrate as permanently bonded into the active site.
Saying the active site accepts any molecule of roughly the right size.
Exam tips
- The phrase examiners want is 'alternative route of lower activation energy'; write it out in full.
- Remember the stereochemical point: an enzyme does not work equally well on both optical isomers of its substrate.
Try the lesson
The start of the Enzymes (A-level only): The action of enzymes as catalysts, including the concept of a stereospecific active site that binds to a substrate molecule. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the role of an enzyme in a reaction?
Listen
The active site is a small cleft in the folded protein whose shape and pattern of charges are complementary to one particular substrate. The substrate binds there, forming an enzyme-substrate complex, and is held in exactly the orientation the reaction needs, with the bonds that must break already strained. That gives the reaction an alternative route with a lower activation energy, so at any given temperature a far larger fraction of collisions has enough energy to react and the rate rises sharply. The enzyme is unchanged at the end and immediately binds another substrate molecule. Because the amino acids that form the site are themselves chiral, the site is chiral, so it fits only one enantiomer of a substrate. A strong answer earns marks for these points. the active site having a shape complementary to the substrate. the substrate binding to give an enzyme-substrate complex. catalysis described as providing an alternative route of lower activation energy. the enzyme being unchanged at the end so it can act again. Exam tip: The phrase examiners want is 'alternative route of lower activation energy'; write it out in full.
Key words
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active site
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enzyme-substrate complex
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lower activation energy
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stereospecificity
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See every definition
- active site
- A small cleft in the folded protein whose shape and pattern of charges are complementary to one particular substrate.
- enzyme-substrate complex
- Formed when the substrate binds to the active site, holding it in exactly the orientation the reaction needs.
- lower activation energy
- Catalysis provides an alternative route with a lower activation energy, so a far larger fraction of collisions has enough energy to react.
- stereospecificity
- Because the amino acids that form the site are themselves chiral, the site is chiral, so it fits only one enantiomer of a substrate.
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