GCSE · Biology · Edexcel · Spec 1BI0

Mechanism of enzyme action

A tiny worker made by living cells does the same job again and again and never wears out. The catch: it only takes jobs of exactly the right shape.

Biology · Enzyme action

One enzyme, one turn — and then round again

2 stages are missing. Use the rest of the loop to work out which goes where.

Stage 1: Free enzyme with an empty active site
Unchanged and waiting for a substrate whose shape fits.
Stage 2: Substrate binds to the active site
The substrate is the molecule the enzyme acts on.
Stage 4: The reaction takes place
The substrate is broken down into smaller products, or smaller molecules are joined together.

…and stage 5 leads back to stage 1.

Biology · The parts

The lock, the key and the fit

View: Key not yet in the lock. Showing 2 layers: Enzyme, Substrate approaching

View

Explore

Try both views, then tap the parts.

The substrate's shape is complementary to the active site, so it is able to fit into it.

Switch between the two views, then tap a part to see what it does.

Biology · Why enzymes are specific

Would it fit?

For each molecule, decide whether it could be the substrate of enzyme E.

Still to sort

Fits the active site (0)

Its shape is complementary to the bowl.

Where the line is: What decides it is shape: the active site is complementary to the shape of its own substrate.

Does not fit (0)

Its shape is not complementary to the bowl.

Where the line is: A molecule that cannot fit cannot bind, so no enzyme-substrate complex forms.

5 of 5 still to sort.

Enzyme E has a round, bowl-shaped active site. These shapes are made up to practise the idea — real active sites are not this simple.

Biology · Spot the slip

Which idea is closest to yours?

One enzyme molecule has just helped a substrate molecule turn into products.

Which of these is closest to what you think happens to the enzyme?

The enzyme is used up in the reaction, so every substrate molecule needs a fresh enzyme.The reaction changes the enzyme for good, so it can only do the job once.An enzyme will catalyse any substrate that bumps into its active site.The enzyme comes out unchanged and can go again, but only for a substrate that fits.

How sure are you?

I'm sureI think soI'm guessing

The enzyme is used up in the reaction, so every substrate molecule needs a fresh enzyme.

This is a common misconception

A very common slip. An enzyme is a catalyst, and a catalyst is not used up. Look back at the loop: when the products are released, the same enzyme is free to bind another substrate molecule.

The reaction changes the enzyme for good, so it can only do the job once.

This is a common misconception

Catalysts are not permanently changed either. The enzyme that comes out of a turn is unchanged, so one enzyme molecule can catalyse the same reaction many times.

An enzyme will catalyse any substrate that bumps into its active site.

This is a common misconception

Only a substrate whose shape is complementary to the active site can bind. That is why each enzyme is specific: it acts on one particular substrate, or a very small group of similar ones.

The enzyme comes out unchanged and can go again, but only for a substrate that fits.

This matches the evidence

Exactly. Unchanged every turn, and fussy about shape. Those two ideas are the whole mechanism.

An enzyme is not used up: it comes out of every turn unchanged. The shape of its active site decides which substrate it will take.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The shape of the active site decides which substrate the enzyme takes.

What you need to know

An enzyme is a biological catalyst: a protein made by living cells that speeds up a reaction without being used up or permanently changed.
The substrate is the molecule an enzyme acts on. It fits into the enzyme's active site, a region with a particular shape.
In one turn: the substrate binds, an enzyme-substrate complex forms, the reaction takes place, the products are released, and the unchanged enzyme is free to bind another substrate.
Lock and key model: the substrate (the key) fits the active site (the lock) because their shapes are complementary, so each enzyme is specific.

The big picture

An enzyme is a biological catalyst. Its active site has a particular shape that is complementary to its substrate. In one turn the substrate binds, an enzyme-substrate complex forms, the reaction takes place and the products are released, leaving the unchanged enzyme free to bind another substrate. The lock and key model explains why each enzyme is specific.

Key points

1Enzymes are catalysts, so they are not used up or permanently changed. One enzyme molecule can catalyse the same reaction many times.

2The shape of the active site is complementary to the shape of the substrate.

3The reaction at the active site either breaks the substrate down into smaller products or joins smaller molecules together.

4Each enzyme is specific: it catalyses the reaction of one particular substrate, or a very small group of similar substrates.

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Worked example

Problem

One enzyme molecule is in a cell. Three substrate molecules arrive one after another, and all three have a shape that is complementary to its active site. Can that one enzyme molecule catalyse all three reactions?

⚠ Watch out

Believing the enzyme is used up. The substrate is what is turned into products; the enzyme comes out unchanged. The second slip is thinking any substrate will do, when only a substrate with a complementary shape binds.

🧠

Memory hook

Picture a lock that never wears out. The right key slides in, something happens, and the key leaves as something new. (Here the picture bends: a real key would not change, but the substrate does.) The lock is then just as ready as before.

✓

Check yourself

Explain in two sentences why the enzyme at the end of a turn is the same as at the start, and why it ignores a molecule of the wrong shape.

Flashcards

(9)

What is an enzyme?

A biological catalyst: a protein made by living cells that speeds up chemical reactions without being used up or permanently changed.

Why can one enzyme molecule catalyse the same reaction many times?

Because it is not used up or permanently changed. It comes out of each turn unchanged.

What is the substrate?

The molecule the enzyme acts on. It fits into the enzyme's active site.

What is the active site?

The region of the enzyme with a particular shape, into which the substrate fits.

What is an enzyme-substrate complex?

What forms when the substrate binds to the active site: enzyme and substrate joined together.

What can the reaction at the active site do to the substrate?

Break it down into smaller products, or join smaller molecules together.

What happens right after the products are released?

The unchanged enzyme is free to bind another substrate molecule.

What does the lock and key model say?

The substrate is the key and the active site is the lock. The key fits the lock because their shapes are complementary (they match).

What does it mean to say an enzyme is specific?

It can only catalyse the reaction of one particular substrate, or a very small group of similar substrates.

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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One enzyme, one turn — and then round again

Would it fit?

Which idea is closest to yours?

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