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Enzymes, explained simply
The proteins that make the chemistry of life fast enough to keep you alive.
Enzymes are biological catalysts that speed up reactions without being used up. Most are proteins, though some RNA molecules are catalysts too. In a protein enzyme, the active site binds its substrate. Excessive heat or extreme pH can change its shape and reduce activity.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
How an enzyme works
- 01
Substrate binds
Active siteThe substrate fits into the active site, forming an enzyme–substrate complex. Each enzyme is specific to one substrate (or a very small group).
- 02
Reaction happens
The enzyme lowers the activation energy, so the substrate is converted into products much faster than it would be on its own.
- 03
Products leave
The products are released. The enzyme is unchanged and can be used again.
Enzyme is not used up
Lock and key vs induced fit
In the lock and key model, the active site is a fixed shape and only the matching substrate fits, like a key in a lock. This is the model GCSE uses.
The more accurate induced fit model, used at A-level and in AP, says the active site changes shape slightly as the substrate binds, giving a tighter fit that helps the reaction along.
Try it: change the temperature
Compare fresh enzyme samples at different temperatures in this illustrative model, with an optimum of 37°C. The percentages are not measured data or predictions for every human enzyme. Moving the slider back represents a fresh sample, not recovery of a heat-denatured one.
Relative rate: 100%
What affects enzyme activity
| Factor | Effect | Why |
|---|---|---|
| Temperature | Rate rises to an optimum, then falls sharply | Warmer means more collisions. Too hot breaks the bonds holding the enzyme's shape, so it denatures. |
| pH | Each enzyme has an optimum pH | Extreme pH changes the enzyme's shape. Pepsin works best in acid (about pH 2), while most enzymes in cells prefer near neutral. |
| Substrate concentration | Rate rises, then levels off | At first more substrate means more collisions. Then all the active sites are busy, so more substrate can't help. |
| Enzyme concentration | Rate rises (if there's plenty of substrate) | More enzyme means more active sites available. |
Key terms
- Catalyst
- Something that speeds up a reaction without being used up.
- Active site
- The part of the enzyme where the substrate binds.
- Substrate
- The molecule an enzyme acts on.
- Denature
- To disrupt a protein enzyme's folded structure, reducing or removing its activity. This is often irreversible, but not always.
- Optimum
- The temperature or pH where an enzyme works fastest.
- Activation energy
- The energy needed to start a reaction. Enzymes lower it.
Common mistakes
Enzymes aren't alive, so they can't be "killed". They're proteins. Heat denatures them (changes their shape).
Enzymes aren't used up. They come out of the reaction unchanged and can work again.
Cold slows enzymes, it doesn't denature them. Warm them up again and they work again. Denaturing by heat is usually permanent.
Enzymes are specific. The active site only fits its own substrate, so each enzyme catalyses one reaction.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
Why does an enzyme stop working at a high temperature?
How to get there
- 1
High temperature gives particles more energy and breaks the bonds that hold the enzyme's shape.
- 2
The active site changes shape, so the substrate no longer fits.
- 3
Denaturing often cannot be reversed, so enzyme activity stays low. The uncatalysed reaction may still happen much more slowly.
Check yourself
What is the part of an enzyme where the substrate binds called?
The substrate binds to the enzyme's active site.
What can happen if a protein enzyme is heated enough to denature it?
Too much heat breaks bonds that hold the enzyme's shape, so the active site no longer fits the substrate.
A graph of rate against substrate concentration levels off. Why?
At high substrate concentration every enzyme is working at its maximum rate, so adding more substrate makes no difference.
FAQ
What is an enzyme?
A biological catalyst that speeds up a reaction without being used up. Most enzymes are proteins; catalytic RNA molecules are called ribozymes.
What is the difference between competitive and non-competitive inhibition?
In the standard model, a reversible competitive inhibitor competes with substrate for the active site; extra substrate can reduce its effect. A pure non-competitive inhibitor lowers the maximum reaction rate without changing substrate affinity, so extra substrate cannot restore that maximum rate.
Why do human enzymes work best at about 37°C?
Human enzymes have evolved to work best near body temperature. The optimum varies between enzymes, and enzymes from heat-loving bacteria work at much higher temperatures.
What is the difference between denaturing and slowing down?
Cold slows an enzyme because there are fewer collisions, but its shape is unchanged. Heat or extreme pH denatures it by changing its shape. See also how proteins are made.
Sources and further reading
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