GCSE · Biology · Edexcel · Spec 1BI0

Enzyme denaturation

Bend a lock so its hole changes shape and the key won’t go in, even though the key is fine. Two very different things can do that to an enzyme.

Build the web

Two ways to break one lock

Too much heat and a pH far from the optimum both end with an enzyme that can no longer catalyse its reaction. Build the web: pick a factor, pick what it acts on, and add the arrow. Then check your web against the lesson’s.

  1. Temperature above the optimum
  2. pH far from the optimum
  3. Shape-holding bonds are disrupted
  4. Active site changes shape
  5. Substrate no longer fits
  6. No enzyme–substrate complex forms
  7. Enzyme can no longer catalyse the reaction (outcome)
Add a link

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Look at the lock

The same enzyme, before and after
substrateenzyme

View: Working enzyme. Showing 1 layer: Working enzyme

View

Explore

Flip the view and watch what happens to the pocket.

Shape intact, substrate fits, enzyme–substrate complex forms.

Switch between the two views, then tap the dashed regions to see what each one is doing.

Side by side

Working enzymevsDenatured enzyme

Same enzyme, two states. Start with the row people get wrong most often.

Focus

Is the enzyme molecule still there?

Working enzyme

Yes, folded and held in shape by its bonds

Denatured enzyme

Yes, but its shape has changed

The insight

This is the one people trip over. Denaturation does not make the enzyme disappear. The molecule is still there, it is the shape that has gone wrong.

Active site

Working enzyme

A specific shape, complementary to the substrate

Denatured enzyme

A changed shape

Substrate

Working enzyme

Fits into the active site

Denatured enzyme

No longer fits

Enzyme–substrate complex

Working enzyme

Can form

Denatured enzyme

Cannot form

The reaction

Working enzyme

Catalysed by the enzyme

Denatured enzyme

No longer catalysed by the enzyme

Check what you really think

Can you undo it?

Enzymes, temperature, pH and the shape of the active site.

Which of these is closest to what you think right now?
How sure are you?

Your turn to explain

Write it in your own words

Explain why the rate of an enzyme-controlled reaction falls quickly when the temperature rises above the enzyme’s optimum temperature. [5 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How too much heat or the wrong pH ruins an enzyme’s active site — and why it stays ruined

What you need to know

  • An enzyme’s active site has a specific shape that is complementary to its substrate. That shape depends on how the enzyme’s protein chain is folded and held in place by bonds within the molecule.
  • Denaturation is a change in the shape of the active site so that the substrate no longer fits. A denatured enzyme cannot form enzyme–substrate complexes, so it can no longer catalyse its reaction.
  • Temperatures above the optimum denature an enzyme: the extra energy makes the molecule vibrate more, which breaks the bonds holding its shape.
  • A pH far from the optimum, too acidic or too alkaline, also denatures an enzyme because it disrupts the bonds that hold the enzyme in shape.
  • Denaturation is permanent. Low temperatures do not denature enzymes; they only slow reactions because molecules move more slowly and collide less often.

The big picture

An enzyme works because its active site has a specific shape that matches its substrate, and that shape depends on the enzyme’s folded protein chain being held in place by bonds. Temperatures above the optimum, or a pH far from the optimum, disrupt those bonds. The active site changes shape, the substrate no longer fits, no enzyme–substrate complex can form and the enzyme can no longer catalyse its reaction. This is denaturation, and it is permanent. Low temperatures do not denature enzymes; they only slow reactions down.

Key points

1Shape is the whole job: the substrate fits the active site because their shapes are complementary.
2Bonds inside the folded protein chain hold the active site in its shape.
3Heat (above the optimum) and pH (far from the optimum) both end up disrupting those bonds.
4Changed active site, no fit, no enzyme–substrate complex, no catalysis.
5Cooling or restoring the pH does not bring the shape back. Cold only slows a reaction.

Worked example

Problem

Explain why an enzyme stops catalysing its reaction when it is in a solution with a pH far from its optimum.

⚠ Watch out

Jumping straight from ‘it’s too hot’ (or ‘the pH is wrong’) to ‘so the enzyme stops working’. The explanation lives in the middle of the chain: say what happens to the bonds, then to the active site, then to the fit of the substrate.

🧠

Memory hook

Bonds break, site bends, key won’t fit. And a bent lock stays bent. (Where the lock-and-key picture breaks down: a real active site is held in shape by bonds inside a folded chain, which is why disrupting those bonds is what changes it.)

✓

Check yourself

An enzyme has been denatured by heat, then cooled right down. Does its active site return to its original shape? (No: denaturation is permanent.)

Flashcards

(9)
What gives an enzyme’s active site its shape?
The way the enzyme’s protein chain is folded and held in place by bonds within the molecule.
What does it mean to say the active site is complementary to the substrate?
It has a specific shape that the substrate fits into.
What is denaturation?
A change in the shape of an enzyme’s active site so that the substrate no longer fits.
What is an enzyme–substrate complex?
What forms when the substrate fits into the active site. A denatured enzyme cannot form one.
What does extra heat energy do to an enzyme molecule above its optimum temperature?
It makes the molecule vibrate more, which breaks the bonds holding its shape.
How can pH denature an enzyme?
A pH far from the optimum (too acidic or too alkaline) disrupts the bonds that hold the enzyme in shape, altering the active site.
Does cooling a denatured enzyme, or restoring its optimum pH, bring it back?
No. Denaturation is permanent, and neither restores the active site’s shape.
Do low temperatures denature enzymes?
No. They only slow reactions down, because molecules move more slowly and collide less often.
Put in order: active site changes shape, bonds disrupted, substrate no longer fits, no complex forms.
Bonds disrupted, then the active site changes shape, then the substrate no longer fits, then no enzyme–substrate complex forms.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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