How do enzymes speed up reactions?

They lower the activation energy of the reaction they catalyse.

What decides the shape of an enzyme's active site?

Its tertiary structure, which depends on the sequence of amino acids (primary structure).

What is an enzyme-substrate complex?

The temporary structure formed when the substrate is bound to the active site.

What does it mean to say an enzyme is specific?

Its active site is complementary to only one substrate, or a few similar ones, so it catalyses only that reaction.

Describe the lock and key model.

The active site is a rigid shape exactly complementary to the substrate.

Describe the induced-fit model.

The active site changes shape slightly as the substrate binds, moulding around it.

How does induced fit lower activation energy?

The change in shape puts strain on bonds in the substrate, so less energy is needed for the reaction.

Why was the lock and key model modified?

New evidence showed that enzymes are flexible and can change shape when the substrate binds.

What is an intracellular enzyme? Give an example.

One that works inside the cell that made it, for example catalase.

What is an extracellular enzyme? Give an example.

One that is secreted and works outside the cell that made it, for example trypsin.

Why can a mutation stop an enzyme working?

It may change the primary and so the tertiary structure, altering the active site so the substrate is no longer complementary.

Is an enzyme used up in a reaction?

No. It is unchanged at the end and can be used again.

Enzyme actionAQA A-Level Biology: Flashcards

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What is activation energy?

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What is activation energy?: The minimum energy needed for a reaction to start.
How do enzymes speed up reactions?: They lower the activation energy of the reaction they catalyse.
What decides the shape of an enzyme's active site?: Its tertiary structure, which depends on the sequence of amino acids (primary structure).
What is an enzyme-substrate complex?: The temporary structure formed when the substrate is bound to the active site.
What does it mean to say an enzyme is specific?: Its active site is complementary to only one substrate, or a few similar ones, so it catalyses only that reaction.
Describe the lock and key model.: The active site is a rigid shape exactly complementary to the substrate.
Describe the induced-fit model.: The active site changes shape slightly as the substrate binds, moulding around it.
How does induced fit lower activation energy?: The change in shape puts strain on bonds in the substrate, so less energy is needed for the reaction.
Why was the lock and key model modified?: New evidence showed that enzymes are flexible and can change shape when the substrate binds.
Who proposed the induced-fit model, and when?: Koshland, in 1958.
What is an intracellular enzyme? Give an example.: One that works inside the cell that made it, for example catalase.
What is an extracellular enzyme? Give an example.: One that is secreted and works outside the cell that made it, for example trypsin.
Why can a mutation stop an enzyme working?: It may change the primary and so the tertiary structure, altering the active site so the substrate is no longer complementary.
Is an enzyme used up in a reaction?: No. It is unchanged at the end and can be used again.

Exam questions on Enzyme action

Catalase, found inside liver cells, catalyses the breakdown of hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen. Trypsin is made by the pancreas and secreted into the small intestine, where it catalyses the hydrolysis of proteins to shorter polypeptides.
Explain why trypsin does not catalyse the breakdown of hydrogen peroxide.2 marks
In 1894 Emil Fischer proposed that an enzyme and its substrate fit together like a lock and key. In 1958 Daniel Koshland proposed a modified model after it was found that some enzymes can act on substrates of slightly different shapes and that the shape of an enzyme can change when its substrate binds.
Explain how the induced-fit model accounts for an enzyme lowering the activation energy of a reaction.2 marks
Lactase is an enzyme made by cells lining the small intestine. It is anchored in the cell surface membrane with its active site facing the gut lumen, where it hydrolyses the disaccharide lactose to glucose and galactose. Some adults make too little lactase, and can take a tablet containing lactase from a fungus with dairy foods.
A mutation in a different person replaces one amino acid in the active site of lactase with an amino acid with a different R group. The enzyme no longer hydrolyses lactose. Explain why.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).