Role of Enzymes

Living things carry out thousands of chemical reactions — building large molecules and breaking them down. All these reactions together are called metabolism, and on their own most of them would happen far too slowly to keep a cell alive.

An enzyme solves this. An enzyme is a biological catalyst:

• A catalyst is a substance that speeds up a chemical reaction.
• It is not used up in the reaction, so it can be used again and again.
• "Biological" means it is made by living cells. Enzymes are proteins.

Because the enzyme is released unchanged, even a small amount of enzyme can convert a large amount of substrate over time.

Exam tip. The phrase markers look for is "speeds up the reaction without being used up". Saying an enzyme "makes the reaction happen" is not enough — the reaction could happen anyway; the enzyme makes it happen fast enough to be useful.

Every enzyme molecule has a special pocket called the active site. This is the part of the enzyme where the reaction takes place.

The molecule the enzyme works on is called the substrate.

• The active site has a specific shape that matches the shape of its substrate.
• This is why enzymes are specific: one enzyme works on one substrate (or one type of substrate) only.
• A substrate of the wrong shape cannot fit the active site, so that enzyme cannot speed up its reaction.

Everyday example. The enzyme amylase breaks down starch (its substrate) — but it cannot break down a protein, because a protein does not fit amylase's active site. A different enzyme is needed for each different reaction.

Exam tip. "Specific" is a keyword — it means the enzyme and substrate must have complementary shapes. Do not confuse "specific" with "fast" or "strong".

Scientists explain enzyme specificity using the lock-and-key model:

• The active site is the lock.
• The substrate is the key.
• Only the correctly shaped key (substrate) fits the lock (active site).

When the substrate fits into the active site, they join to form an enzyme–substrate complex. The reaction then takes place and the substrate is changed into product(s), which leave the active site. The enzyme is unchanged and is free to work on the next substrate molecule.

Exam tip. Always include the middle stage — the enzyme–substrate complex. Many students jump straight from substrate to product and lose a mark.

It helps to learn the enzyme reaction as a short sequence you can write out:

1. Substrate binds. The substrate collides with and fits into the enzyme's active site (because their shapes are complementary).
2. Enzyme–substrate complex forms. While joined, the enzyme catalyses the reaction.
3. Products form and leave. The substrate is changed into one or more products, which no longer fit the active site and so are released.
4. Enzyme is reused. The enzyme is unchanged and free to bind another substrate molecule.

You can summarise it as:

substrate  +  enzyme  →  enzyme–substrate complex  →  products  +  enzyme

Two types of reaction enzymes control:

• Breaking down large molecules into small ones (e.g. starch → maltose by amylase).
• Building up small molecules into large ones (e.g. joining glucose molecules together).

Exam tip. If a question gives marks for a "sequence", write each stage in order. The keyword chain substrate → active site → enzyme–substrate complex → products → enzyme unchanged scores reliably.

Without enzymes, the reactions a cell needs — digestion, respiration, building new molecules — would be far too slow to keep the organism alive.

• Enzymes let these reactions happen quickly at the relatively low temperatures of living things (around body temperature), instead of needing the high heat a lab catalyst might.
• Because each enzyme is specific, a cell can control which reactions happen simply by making the right enzymes.
• Because enzymes are not used up, cells only need small amounts of each one.

Link to digestion (later spec points). Digestive enzymes such as amylase, protease and lipase are real examples of biological catalysts breaking large food molecules into small, soluble ones — but for this subtopic you only need the general role described above.

Exam tip. Keep this subtopic general. Detailed effects of temperature and pH on enzyme rate belong to other spec points — here, focus on what an enzyme is and how the lock-and-key mechanism works.