What are enzymes?

Enzymes are biological catalysts. In simple terms, they speed up chemical reactions.

Living organisms have chemical reactions happening in them all the time. One way enzymes are used is to break down large molecules. For example, enzymes called carbohydrases convert carbohydrates into simple sugars.

This type of reaction needs to be carefully controlled. Reactions outside the body are often sped up by increasing the temperature, but that wouldn't be very helpful in your body, as all the chemical reactions would speed up, and our bodies need to be at an optimum temperature to work properly. So, living organisms use enzymes to control the speed of chemical reactions.

What do enzymes look like?

Enzymes have an active site which is shaped to fit the molecule ( substrate ) that needs to change in the reaction. This is called the lock and key mechanism. If the substrate doesn't fit into the active site, the reaction won't be catalysed.

A slightly adjusted model is the induced fit model, where the active site changes slightly when the substrate binds to it.

Examples of Enzymes

Carbohydrases convert carbohydrates into simple sugars. Amylase is an example of a carbohydrase. It breaks down starch into maltose.

Lipases convert lipids into glycerol and fatty acids.

Proteases convert proteins into amino acids.

Enzymes are also used to join molecules together. For example, proteins are made by joining amino acids together.

What conditions do enzymes need to work?

Enzymes need the right conditions to work properly.

Enzymes and temperature

Enzymes have an optimum temperature at which they are the most active, but if the temperature gets too high, the enzymes stop working. This is because bonds holding the enzyme in shape break, which changes the shape of the active site so the substrate no longer fits. When an enzyme changes shape like this, it's said to be denatured.

Enzymes and pH

As with temperature, enzymes also have an optimum pH. If the pH is too high or too low, it alters the bonds in the enzyme and, consequently, the shape of the active site.

Enzymes and substrate concentration

The higher the substrate concentration, the faster the reaction, as the likelihood of an enzyme and substrate making contact is greater the more substrate there is.

There is a maximum substrate concentration, though, as once all enzymes are in use, adding more substrate makes no difference to the rate of reaction.

Fun facts about enzymes

1. The optimum temperature for most human enzymes is body temperature.
2. Stomach enzymes work best at a low pH, as the stomach is acidic.
3. Enzymes are very efficient and can catalyse a reaction millions of times faster than it would happen without an enzyme!
4. Enzymes are not used up. They are catalysts, which means they speed up a reaction without being changed or used up.
5. Most enzymes are proteins, but there are riboenzymes made of RNA.
6. Luciferase is an enzyme involved in bioluminescence in fireflies.
7. Biological washing powder contains enzymes which break down stains on your clothes!

Easy science experiments to learn about enzymes

Make a model of the digestive system to learn about digestive enzymes!

Enzymes in fruits such as pineapple, kiwi and papaya break down proteins in jelly, stopping it from setting! This is a great visual way to see enzymes in action!

Find out how to stop polyphenol oxidase from turning fruit brown with a simple kitchen hack.

Use play dough to make a model of an enzyme and substrate.

Last Updated on September 10, 2025 by Emma Vanstone