# Enzymes

> What enzymes are, the lock and key and induced fit models, and how temperature, pH, substrate concentration and inhibitors change enzyme activity.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/enzymes/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The fruit salad that would not set
Lena makes a jelly with fresh pineapple for a party. She puts it in the fridge. Hours later it is still runny soup. Her friend makes the same jelly with tinned pineapple, and it sets perfectly.

The difference is an enzyme. Fresh pineapple has bromelain, an enzyme that cuts up proteins. Jelly is held together by a protein called gelatin, so the bromelain chops it to pieces. Tinned pineapple has been heated, which destroys the enzyme. The same trick is used in meat tenderisers.

**In short:** Enzymes are proteins that speed up chemical reactions in living things without being used up. Each enzyme has an active site that fits its substrate, and it works best at an optimum temperature and pH: too hot, and it denatures and stops working.

## Starting point: what you already know

- A chemical reaction turns reactants into products.
- Reactions go faster when particles collide more often or with more energy (Year 10 chemistry, rates of reaction).
- A **catalyst** speeds up a reaction without being used up.

## What is an enzyme?

An **enzyme** is a **protein** that acts as a **biological catalyst**. It speeds up a reaction in a living thing and is not used up, so it can be used again and again.

- The substance an enzyme acts on is its **substrate**.
- The part of the enzyme the substrate fits into is the **active site**.
- Most enzymes work on only one substrate. They are **specific**.

Enzymes speed reactions up by lowering the **activation energy**: the energy needed to get the reaction started. Without enzymes, the reactions in your cells would be far too slow to keep you alive.

### Key idea: Two kinds of reaction
- **Breaking down** (catabolic): amylase breaks starch into sugars. Protease breaks proteins into amino acids. Lipase breaks fats into fatty acids and glycerol.
- **Building up** (anabolic): DNA polymerase joins nucleotides into new DNA.

## How an enzyme works

1. The substrate collides with the active site and binds, forming an **enzyme-substrate complex**.
2. The reaction happens.
3. The products are released. The enzyme is unchanged and ready again.

**Lock and key model:** the active site has a fixed shape that matches the substrate, like a key in a lock.

**Induced fit model:** the active site is flexible. It **changes shape slightly** as the substrate binds, gripping it more tightly. This is the better model today because it explains how an enzyme can strain the bonds of its substrate.

## Factors that affect enzyme activity

*Diagram: Graph of enzyme activity against temperature, rising slowly to a peak at about 37 degrees then dropping steeply to zero*
*Activity rises with temperature up to the optimum, then falls quickly as the enzyme denatures.*

**Temperature.** As temperature rises, particles move faster and collide more, so activity rises. Above the **optimum temperature** (about 37 degrees for human enzymes), the bonds holding the enzyme's shape break. The active site changes shape and the substrate no longer fits. The enzyme is **denatured**. This is permanent. At low temperatures enzymes are just slow, not denatured.

**pH.** Each enzyme has an **optimum pH**. Pepsin in the stomach works best at about pH 2. Amylase in saliva works best at about pH 7. Too far from the optimum and the enzyme denatures.

**Substrate concentration.** More substrate means more collisions, so activity rises, until every active site is busy. Then the graph levels off, because the enzyme is **saturated**.

**Enzyme concentration.** More enzyme gives more active sites, so a faster rate, as long as there is plenty of substrate.

### Watch out: Do not say enzymes are "killed"
Enzymes are proteins, not living things. Say they are **denatured**: their shape, and so their active site, has changed.

**Mind map**

```text
Enzymes
  What they are
    Proteins
    Biological catalysts
    Specific to one substrate
  How they work
    Active site
    Enzyme-substrate complex
    Lower activation energy
  Models
    Lock and key
    Induced fit
  Factors
    Temperature: optimum, then denatured
    pH: each has an optimum
    Substrate: levels off when saturated
    Inhibitors slow them
```

### Year 11: Inhibitors and cofactors
An **inhibitor** slows an enzyme down.

- A **competitive inhibitor** looks like the substrate and blocks the **active site**. Adding lots of substrate can outcompete it.
- A **non-competitive inhibitor** binds **somewhere else** on the enzyme and changes the shape of the active site. Adding more substrate does not help.

Many drugs and poisons are inhibitors. Cyanide stops a respiration enzyme in mitochondria.

Some enzymes need a helper to work: a **cofactor** (a metal ion such as zinc or iron) or a **coenzyme** (an organic molecule, often made from vitamins). This is one reason we need vitamins and minerals in our diet.

### Year 12: Enzymes keep the internal environment in balance
In Module 8, **homeostasis** keeps body temperature near 37 degrees and blood pH near 7.4. The main reason is to keep enzymes near their optimum. A fever above about 42 degrees is dangerous because enzymes in the brain start to denature. In Module 6, enzymes are also the tools of biotechnology: restriction enzymes cut DNA and DNA ligase joins it.

## Quick summary

- Enzymes are proteins that act as biological catalysts. They are specific and reusable.
- The substrate binds to the active site to form an enzyme-substrate complex.
- Induced fit: the active site changes shape slightly to grip the substrate.
- Activity rises with temperature to an optimum, then falls as the enzyme denatures.
- Each enzyme has an optimum pH. Activity levels off when all active sites are busy.

## Quiz

Try these questions from the chapter quiz.

1. Enzymes are mostly made of
   - A. fats
   - B. sugars
   - C. proteins
   - D. minerals
   - Answer: C. proteins. Enzymes are proteins.

2. What does an enzyme do to a reaction?
   - A. Speeds it up without being used up
   - B. Slows it down
   - C. Is used up in the reaction
   - D. Only works outside cells
   - Answer: A. Speeds it up without being used up. Enzymes are biological catalysts.

3. The part of the enzyme where the substrate binds is the
   - A. substrate site
   - B. active site
   - C. product
   - D. membrane
   - Answer: B. active site. The active site matches the substrate.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/enzymes/#quiz