IB Biology HLEnzymes & MetabolismPaper 1 & 2~13 min read
How Enzymes Work
Nothing steers a substrate towards an enzyme. Everything is moving at random, and a reaction only happens when a substrate happens to hit the active site the right way round. Once it does, the enzyme takes over.
📚 What you need to know
Substrates reach the active site by random collision, driven by their kinetic energy. More kinetic energy means more collisions and more product.
A collision only works if it happens at the right orientation and speed; otherwise the molecules just bounce apart.
When a substrate binds, a temporary enzyme–substrate complex forms. Products form inside it, then leave, and the enzyme is reused.
The old lock-and-key model said the active site was already a perfect match for the substrate.
The induced-fit model replaced it: the active site (and sometimes the substrate) changes shape slightly as the substrate enters. These are conformational changes.
Induced fit gives an ideal binding arrangement and maximises the enzyme’s ability to catalyse the reaction. It is the model we use today.
Enzymes can be immobilised — attached to an inert support, trapped in a matrix such as alginate gel, or held behind a partially permeable membrane.
Denaturation happens at high temperature or extreme pH: bonds holding the 3D shape break, the active site changes shape, and the substrate can no longer bind.
Getting to the active site
Substrate molecules are not delivered to the enzyme. They move because they have kinetic energy, and that movement is random. Every so often a substrate bumps into an enzyme’s active site.
Two things decide whether that bump does anything:
Orientation. The substrate has to arrive the right way round. If it is facing the wrong way, the shapes do not line up and the molecules simply bounce off each other. This is an unsuccessful collision.
Speed. The collision needs enough energy behind it for bonds to be made or broken.
Give the molecules more kinetic energy and they move faster, collide more often, and form more enzyme–substrate complexes — so more product appears per second. That single chain of reasoning is the backbone of almost every rate question in this topic.
Write it as a chain and you will pick up every mark: more kinetic energy → faster movement → more frequent successful collisions → more enzyme–substrate complexes → faster rate. Examiners are looking for the links, not just the ends.
Two-substrate reactions. Some enzymes need two substrate molecules to arrive at the active site at the same time. Both have to collide correctly at once, which is exactly why holding them in position matters so much.
The enzyme–substrate complex
When a substrate does bind successfully, the structure that forms is called an enzyme–substrate complex. It is temporary. Inside it, the chemical structure of the substrate changes: bonds are strained, broken or formed, and the substrate becomes the product. The substrate at this moment is said to be in a transitional state.
The products no longer fit the active site properly, so they detach and move away. The active site is left empty, and the enzyme — completely unchanged — is ready for the next substrate.
Nothing is consumed except the substrate. That is the whole reason a cell only needs a small quantity of each enzyme.
Lock and key, then induced fit
The first explanation of all this was the lock-and-key model. It said the active site was already shaped as a precise complement to the substrate, so the substrate slid in like a key into a lock. Simple, and roughly right.
It was replaced by the induced-fit model, which is very similar but adds one crucial idea: the enzyme and substrate interact. As the substrate enters, the active site changes shape slightly to close around it. Sometimes the substrate changes shape a little too. These small shape changes are called conformational changes.
Why induced fit matters
The active site moulds around the substrate, giving an ideal binding arrangement. This strains the substrate’s bonds and maximises the enzyme’s ability to catalyse the reaction.
Both models explain specificity. Only induced fit explains why binding itself helps the reaction along.
🧠
A handshake, not a lock
A lock does not move when you put the key in. A hand closes around whatever you put in it. Enzymes shake hands.
Immobilised enzymes
In industry, enzymes are often fixed in place rather than being stirred loose into the mixture. This is called immobilisation, and sometimes it is the large substrate molecule that is immobilised instead.
Method
What it looks like
Attachment to an inert substance
Enzymes bonded to a surface such as glass beads
Entrapment in a matrix
Enzymes held inside a gel, for example alginate beads
Entrapment behind a partially permeable membrane
Enzymes sealed in a compartment that substrate can enter but the enzyme cannot leave
Why bother?
The product is not contaminated with enzyme, so there is no separation or filtering step afterwards.
The enzyme can be reused many times, which matters because enzymes are expensive.
Immobilised enzymes tolerate changes in temperature and pH better — immobilisation tends to make them more stable.
Substrate can be exposed to higher enzyme concentrations than in solution, increasing throughput.
Conditions can be controlled carefully, keeping the enzyme close to its optimum.
They are used in food processing, pharmaceuticals, manufacturing and environmental management.
Denaturation
An enzyme’s 3D shape is held together by weak bonds between the R-groups of its amino acids — hydrogen bonds and similar interactions. High temperature makes those bonds vibrate more until they break. Extreme pH interferes with the same bonding.
Here is the chain, and every link is worth writing out:
🧩 What denaturation actually does
Heat or extreme pH breaks the weak bonds holding the protein’s shape (peptide bonds stay intact).
The 3D shape of the protein changes.
Because the protein has moved, the shape of the active site changes too.
The substrate can no longer bind, so no enzyme–substrate complexes form.
The reaction that was being catalysed stops. The enzyme often becomes insoluble and forms a precipitate.
Denaturation is a shape problem, not a broken chain. The amino acids are all still joined; they are just in the wrong arrangement.
Human numbers worth knowing. Body temperature is about 37°C, so temperatures above roughly 40°C already start to denature human enzymes, and very few work above 50°C. That is why a high fever is dangerous.
Never say a denatured enzyme has been “killed” or “destroyed”. It was never alive, and it has not been destroyed — it has been misfolded. Examiners specifically penalise those two words.
Worked examples
WE 1
Describe the induced-fit model
Describe the induced-fit model of enzyme action and explain how it differs from the lock-and-key model. (4 marks)
Point 1: what lock and key claimed
The lock-and-key model said the active site was already precisely complementary to the substrate, which fitted in like a key in a lock.
Point 2: what induced fit adds
In induced fit the enzyme and substrate interact, and the active site changes shape slightly as the substrate enters.
Point 3: the correct term
These shape changes are called conformational changes, and sometimes the substrate changes shape as well.
Point 4: why it matters
This gives an ideal binding arrangement, which maximises the enzyme’s ability to catalyse the reaction.
Same specificity, but the site moulds itself around the substrateuse the phrase “conformational change” — it is usually a marking point
WE 2
Explain denaturation at high temperature
Explain why the rate of an enzyme-catalysed reaction falls to zero when the temperature is raised well above the optimum. (4 marks)
Point 1: the bonds
Increased vibration breaks the weak bonds between the R-groups that hold the enzyme in its folded shape.
Point 2: the shape
The 3D shape of the protein changes, which changes the shape of the active site.
Point 3: the binding
The substrate is no longer complementary to the active site, so it cannot bind and no enzyme–substrate complexes form.
Point 4: the consequence
The reaction is no longer catalysed and the rate falls to zero; the change is permanent.
Bonds break → shape changes → no binding → no reactiondo not write “killed” or “destroyed”, and do not say peptide bonds break
WE 3
Justify immobilising an enzyme
A company uses an enzyme to produce a food ingredient. Suggest two advantages of immobilising the enzyme rather than mixing it into solution. (2 marks)
Advantage 1
The enzyme does not end up in the product, so the product is uncontaminated and no separation step is needed.
Advantage 2
The enzyme can be recovered and reused many times, which is cheaper because enzymes are expensive.
Cleaner product, and the same enzyme used again and againgreater stability at varying temperature and pH is also accepted
💡 Exam tips
Learn the collision chain by heart: kinetic energy → collision frequency → successful collisions → E–S complexes → rate.
Say enzyme–substrate complex, not “the enzyme joins the substrate”.
Induced fit is the model we currently use — say so if asked which is accepted.
Denaturation questions want bonds → shape → active site → no binding, in that order.
State clearly that peptide bonds are not broken during denaturation. It is a quick extra mark.
For immobilisation, learn all three methods and at least three advantages.
⚠ Common mistakes
Saying enzymes “attract” substrates. They do not. Substrates arrive by random movement.
Saying the enzyme is destroyed or killed. It is denatured.
Claiming denaturation breaks the primary structure. The amino acid sequence is unaffected.
Describing lock and key as the current model. Induced fit replaced it.
Forgetting that products must leave. If they stayed in the active site, the enzyme could not be reused.
Saying low temperature denatures enzymes. Cold slows enzymes down; it does not usually denature them, and the effect is reversible.
Up next: Enzyme Activity (Skills). You now know why temperature and pH matter. The next page turns that into the three sketch graphs you are expected to draw, plus the practicals behind them.
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