AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Enzymes (A-level only): Enzymes are proteins.
An enzyme is a protein, a chain of amino acids joined by peptide links, folded into a compact globular shape with its polar R groups mostly on the outside so it dissolves in water.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An enzyme is a protein, a chain of amino acids joined by peptide links, folded into a compact globular shape with its polar R groups mostly on the outside so it dissolves in water. Its catalytic power comes entirely from that fold: the tertiary structure creates a small cleft, the active site, whose shape and charge distribution are complementary to one particular substrate. Anything that disturbs the fold destroys the function. Heating or a large change in pH breaks the hydrogen bonds and ionic attractions between R groups, the chain unravels, the active site loses its shape and the enzyme is denatured, even though every peptide link and therefore the whole primary structure is still intact.
What earns the marks
- An enzyme described as a protein, a chain of amino acids joined by peptide links.
- The tertiary structure creating the active site.
- Denaturing described as loss of the three-dimensional shape of the active site.
- Stating that hydrogen bonds and ionic attractions are broken while the peptide links survive.
Common mistakes and how to put them right
Saying that heat breaks the peptide links of an enzyme.
Claiming a denatured enzyme has a different primary structure.
Describing enzymes as inorganic catalysts like the transition metals.
Saying the enzyme is used up during the reaction it catalyses.
Exam tips
- If the question is about denaturing, name the bonds that break and the bonds that do not; that contrast is where the marks sit.
- The tertiary structure is the answer to almost every 'why does the active site work' question.
Try the lesson
The start of the Enzymes (A-level only): Enzymes are proteins. lesson, made from checked teaching notes for this specification point.
Have a go first
In terms of their fundamental chemical structure, to which class of biological macromolecules do enzymes belong?
Listen
An enzyme is a protein, a chain of amino acids joined by peptide links, folded into a compact globular shape with its polar R groups mostly on the outside so it dissolves in water. Its catalytic power comes entirely from that fold: the tertiary structure creates a small cleft, the active site, whose shape and charge distribution are complementary to one particular substrate. Anything that disturbs the fold destroys the function. Heating or a large change in pH breaks the hydrogen bonds and ionic attractions between R groups, the chain unravels, the active site loses its shape and the enzyme is denatured, even though every peptide link and therefore the whole primary structure is still intact. A strong answer earns marks for these points. an enzyme described as a protein, a chain of amino acids joined by peptide links. the tertiary structure creating the active site. denaturing described as loss of the three-dimensional shape of the active site. stating that hydrogen bonds and ionic attractions are broken while the peptide links survive. Exam tip: If the question is about denaturing, name the bonds that break and the bonds that do not; that contrast is where the marks sit.
Key words
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tertiary structure
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active site
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denaturing
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primary structure unchanged
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See every definition
- tertiary structure
- The fold of the chain that creates a small cleft, the active site, whose shape and charge distribution are complementary to one particular substrate.
- active site
- A small cleft created by the tertiary structure, whose shape and charge distribution are complementary to one particular substrate.
- denaturing
- Loss of the three-dimensional shape of the active site when heating or a large change in pH breaks the hydrogen bonds and ionic attractions between R groups.
- primary structure unchanged
- Every peptide link, and therefore the whole primary structure, is still intact even though the chain has unravelled and the enzyme no longer works.
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