The big idea: Enzymes are globular proteins that act as biological catalysts — they speed up the chemical reactions of life without being used up.
Every enzyme has a special pocket on its surface called the active site. The molecule the enzyme acts on — the substrate — binds into this active site.
When the substrate is bound, the two together form an enzyme-substrate complex. The reaction then takes place, the products are released, and the enzyme is left unchanged, ready to be used again.
An enzyme at work: the substrate binds the active site, forming an enzyme-substrate complex (induced fit); the reaction occurs and the products are released, leaving the enzyme unchanged and able to be reused.
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- Enzyme
- A globular protein that acts as a biological catalyst, speeding up a specific reaction without being used up.
- Catalyst
- A substance that speeds up a reaction but is not changed or used up by it, so it can be reused.
- Substrate
- The reactant molecule (or molecules) that an enzyme acts on.
- Active site
- The specific region on the surface of an enzyme where the substrate binds; its shape is complementary to the substrate.
- Enzyme-substrate complex
- The temporary structure formed when a substrate is bound to an enzyme's active site, just before the reaction occurs.
- Product
- The new molecule (or molecules) formed by the reaction, which is then released from the active site.
The active site is the key part: Only a small part of a large enzyme — the active site — actually touches the substrate.
This is why an enzyme can be a big protein yet still be choosy about exactly which molecule it works on: everything depends on the shape of the active site.
Each enzyme works on only one substrate (or one type of reaction). This is called specificity.
Specificity comes from shape: the active site is complementary to the substrate, so only that substrate fits — like one particular key fitting one particular lock.
Why enzymes are specific: The active site has a particular 3-D shape.
Only a substrate whose shape is complementary to the active site can bind to it.
A substrate of the wrong shape simply does not fit, so the enzyme cannot act on it. That is why each enzyme catalyses only one reaction — its specificity.
The active site is complementary in shape to one substrate, so only that substrate fits — this is the source of an enzyme's specificity.
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Induced fit — the modern model: The active site is not a rigid, perfectly pre-shaped hole.
Instead, as the substrate binds, the active site changes shape slightly to mould more closely around it. This is called induced fit (the substrate 'induces' a better fit).
This snug moulding puts a strain on the substrate's bonds, which helps the reaction happen more easily — so induced fit explains both specificity and how enzymes speed reactions up.
Lock-and-key (older idea)
- Active site is a rigid, exact match
- Substrate slots in unchanged
- A simple picture of specificity
- Now superseded by induced fit
Induced fit (modern model)
- Active site is slightly flexible
- Active site moulds around the substrate
- Strains the substrate's bonds → reaction easier
- The accepted model — name this one
| Feature | Lock-and-key (older model) | Induced fit (modern model) |
|---|---|---|
| Shape of the active site before binding | Already a perfect, rigid match for the substrate | A close but not exact match — slightly flexible |
| What happens as the substrate binds | The substrate simply slots in; nothing moves | The active site changes shape slightly to mould around the substrate |
| Why specificity still holds | Only a substrate of the right shape fits the rigid site | Only a substrate of a roughly complementary shape can induce the right fit |
| Which model the IB uses | A useful simple picture, but now superseded | The accepted model — name THIS one if asked |
A memory hook: Lock-and-key = a stiff lock that never moves. Induced fit = a glove that adjusts around the hand once it goes in.
If an exam asks which model the diagram shows and the active site changes shape on binding, the answer is induced fit.
What every enzyme has in common: However different they look, all enzymes share the same core features: they are globular proteins, they are biological catalysts, each has an active site, each is specific, and each is left unchanged so it can be reused.
A 'feature shared by all enzymes' question is asking for one of these.
| Feature shared by every enzyme | What it means |
|---|---|
| They are globular proteins | Folded into a rounded 3-D shape, with a specific surface pocket |
| They are biological catalysts | They speed up a reaction without being used up |
| They have an active site | A specific region whose shape is complementary to one substrate |
| They are specific | Each enzyme catalyses only one reaction / one substrate |
| They are unchanged and reusable | The enzyme is released intact and can catalyse the same reaction again |
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How this is tested: On Paper 1A a multiple-choice question often shows a diagram of an enzyme and substrate and asks you to identify the model of binding — if the active site changes shape as the substrate binds, the answer is induced fit, not lock-and-key.
Another Paper 1A format asks for a feature shared by all enzymes — choose a true general statement (globular protein, has an active site, specific, a catalyst, unchanged/reusable).
On Paper 2 an Explain question can ask why enzymes are specific — the marks are for linking the complementary shape of the active site and substrate to induced fit.
IB-style question — explain why enzymes are specific
Enzymes are described as specific. Explain how the structure of an enzyme makes it specific to its substrate. [4]
How to score all four marks
- Name the active site. Each enzyme has a region called the active site, where the substrate binds.
- Link shape to binding. The active site has a specific 3-D shape that is complementary to the shape of one particular substrate.
- Explain the consequence. Only a substrate of the correct, complementary shape can fit and bind, forming an enzyme-substrate complex; a substrate of the wrong shape cannot bind.
- Add induced fit. As the substrate binds, the active site changes shape slightly to mould around it (induced fit), so only the right substrate produces the proper fit. (Award 1 mark per distinct point, up to 4.)
Final answer
Each enzyme has an active site of a specific shape that is complementary to one substrate; only that substrate fits and binds (forming an enzyme-substrate complex), and induced fit moulds the site around it — so the enzyme acts on only one substrate.
✓ Why this scores full marks: Each sentence is a separate scoring point — active site, complementary shape, only the right substrate fits, induced fit.
An 'Explain [4]' needs the cause-and-effect chain spelled out, not just the word 'specific' repeated.
An enzyme at work: the substrate binds the active site, forming an enzyme-substrate complex (induced fit); the reaction occurs and the products are released, leaving the enzyme unchanged and able to be reused.
Interactive diagram
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Reading a data question: Membrane and enzyme topics are a Paper-1B / Paper-2 data favourite. A table might show one enzyme tested against several different substrates, with a product formed for only one of them.
The reasoning to write is always the same: the enzyme catalyses only the substrate whose shape is complementary to its active site, so the others give no product.