AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: explain why a stereospecific active site can…
Full specification wording
Enzymes (A-level only): Students should be able to: explain why a stereospecific active site can only bond to one enantiomeric form of a substrate or drug.
Enantiomers are non-superimposable mirror images, which arise when a carbon atom carries four different groups.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 5 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Enantiomers are non-superimposable mirror images, which arise when a carbon atom carries four different groups. They have identical bonds and identical bond angles, so they differ only in the arrangement in space, like a left and a right hand. The active site of an enzyme is built from amino acids, which are themselves chiral, so the site has a definite handedness too. A substrate binds only when several groups meet matching points in the site at the same time, and a mirror image cannot bring all of them into position at once: line up two and the third points the wrong way. So one enantiomer binds and reacts and the other does not, which is why a racemic drug is at best half active.
What earns the marks
- Enantiomers defined as non-superimposable mirror images arising from a carbon with four different groups.
- The active site itself being chiral because it is built from chiral amino acids.
- Only one enantiomer being able to align its groups with the binding points of the site.
- The consequence, that the other enantiomer is not catalysed or is inactive as a drug.
Common mistakes and how to put them right
Saying the two enantiomers have different bond lengths or bond angles.
Claiming enantiomers have different molecular formulae.
Saying both enantiomers fit but react at different rates, with no reason given.
Forgetting that half of a racemic mixture is the form that does not work.
Exam tips
- Use the word 'non-superimposable'; 'different shape' on its own is not enough for the definition mark.
- Say why the mismatch matters: three or more points of contact have to line up at once, and a mirror image cannot manage it.
Try the lesson
The start of the Enzymes (A-level only): Students should be able to: explain why a stereospecific active site can only bond to one enantiomeric form of a substrate or drug. lesson, made from checked teaching notes for this specification point.
Have a go first
An enzyme has a stereospecific active site. Which statement explains why this active site bonds to only one enantiomeric form of a substrate?
Listen
Enantiomers are non-superimposable mirror images, which arise when a carbon atom carries four different groups. They have identical bonds and identical bond angles, so they differ only in the arrangement in space, like a left and a right hand. The active site of an enzyme is built from amino acids, which are themselves chiral, so the site has a definite handedness too. A substrate binds only when several groups meet matching points in the site at the same time, and a mirror image cannot bring all of them into position at once: line up two and the third points the wrong way. So one enantiomer binds and reacts and the other does not, which is why a racemic drug is at best half active. A strong answer earns marks for these points. enantiomers defined as non-superimposable mirror images arising from a carbon with four different groups. the active site itself being chiral because it is built from chiral amino acids. only one enantiomer being able to align its groups with the binding points of the site. the consequence, that the other enantiomer is not catalysed or is inactive as a drug. Exam tip: Use the word 'non-superimposable'; 'different shape' on its own is not enough for the definition mark.
Key words
Say what each word means before you turn the card.
enantiomers
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chiral active site
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racemic drugs
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See every definition
- enantiomers
- Non-superimposable mirror images, which arise when a carbon atom carries four different groups.
- chiral active site
- A site with a definite handedness because it is built from amino acids, which are themselves chiral.
- racemic drugs
- A drug supplied as a racemic mixture, which is at best half active because only one enantiomer binds and reacts.
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