An enzyme is a protein that speeds up one type of reaction by binding the substrate at its active site. The mechanism has five steps, and a full answer mentions the shape match and the release of the products.
This lesson opens metabolism and enzymes. The next lesson is interpreting temperature and pH graphs.
What are the five steps?
Learn these in order and use the exact terms.
- The substrate collides with the active site of the enzyme.
- The substrate fits the active site, forming an enzyme-substrate complex.
- The enzyme lowers the energy needed to start the reaction.
- The substrate is changed into the products.
- The products leave, and the enzyme is free to bind another substrate.
The enzyme itself is not changed. This is why a small amount of enzyme is enough.
Why is an enzyme specific?
The active site has a particular shape, so only a substrate with the matching shape can bind. This is the lock-and-key idea: the active site is the lock and the substrate is the key.
If a molecule of a different shape arrives, it cannot fit, so the enzyme does not act on it. In SPM answers, write “the substrate is complementary to the active site”, not “the substrate is the same as the enzyme”.
Worked example: why amylase cannot digest protein
Question: Amylase digests starch into maltose, but it does not digest protein. Explain why.
State the mechanism first. Amylase has an active site with a shape that matches starch. Starch binds, forms an enzyme-substrate complex, and is broken into maltose.
Then say what happens with protein. Protein molecules have a different shape from starch, so they are not complementary to the active site of amylase. They cannot bind, no complex forms, and no reaction occurs.
The marks come from using the words active site, complementary and enzyme-substrate complex in a chain of reasons.
A second example shows the same idea from the other side. A test tube contains 1 cm³ of amylase and excess starch. After 10 minutes the amylase is recovered and added to fresh starch, and it digests that too. This shows the enzyme was not used up: it was released unchanged after each reaction.
The mistake that costs marks
Some students write “the enzyme reacts with the substrate and gets used up”. The enzyme takes part but comes out unchanged, so it is a catalyst, not a reactant.
| Student wrote | Problem | Better version |
|---|---|---|
| “The enzyme is used up” | An enzyme is reusable | “The enzyme is released unchanged and binds another substrate” |
| “Enzymes give energy” | They lower the activation energy | “The enzyme lowers the energy needed to start the reaction” |
| “The substrate matches the enzyme” | Only the active site matches | “The substrate is complementary to the active site” |
| “Enzymes are alive” | They are proteins | “Enzymes are proteins made by living cells” |
Check yourself
Catalase breaks down hydrogen peroxide into water and oxygen. A student adds a few drops of catalase to a large volume of hydrogen peroxide and the reaction continues for several minutes. Explain why a small amount of catalase is enough.
Answer
Catalase is not used up. Each molecule binds a hydrogen peroxide molecule at its active site, converts it to products, and releases them unchanged.
The free catalase molecule then binds another hydrogen peroxide molecule, so a small amount can break down a large amount of substrate over time.
What to study next
Enzyme shape changes with temperature and pH, so next read interpreting temperature and pH graphs. Practise reading data with the biological data interpretation trainer.
If you want a teacher to question your mechanism answers sentence by sentence, see online one-to-one Biology tuition or the one-hour trial class (from RM50).