Most of the food we eat is complex carbohydrates, proteins and lipids. These must be broken down to be absorbed into the body.

The chemical reactions required to break them down would be too slow without enzymes.

Enzymes are biological catalysts– they speed up chemical reactions.

Enzymes are required for most of the chemical reactions that occur in organisms. These reactions occur in the breakdown of chemical molecules, which we see in the digestive system.

Enzymes are also involved in the building up of chemical molecules elsewhere in the body.

Enzymes are proteins that have a complex 3D shape. Each enzyme has a region called an active site.

The substrate – the molecule or molecules taking part in the chemical reaction – fits into the active site. Once bound to the active site, the chemical reaction takes place .

Diagram showing how enzymes work in the body

In an organism, the active site of each enzyme is a different shape. It is a perfect match to the shape of the substrate molecule, or molecules. This is essential to the enzyme being able to work. One enzyme is therefore specific to one substrate's chemical reaction, or type of chemical reaction.

This theory for the way in which enzymes work is called the lock and key theory.

Factors affecting enzyme action

Physical factors affect enzyme activity.


At low temperatures, the number of successful collisions between the enzyme and substrate is reduced because their molecular movement decreases. The reaction is slow.

The human body is maintained at 37°C as this is the temperature at which the enzymes in our body work best. This is not true of the enzymes in all organisms.

Higher temperatures disrupt the shape of the active site, which will reduce its activity, or prevent it from working. The enzyme will have been denatured.

Enzymes therefore work best at a particular temperature.

Enzyme action

Proteins are chains of amino acids joined end to end. This chain is not straight – it twists and folds as different amino acids in the chain are attracted to, or repel each other.

Each enzyme is made from proteins made of these twisting and folding amino acids, and therefore the enzyme has a unique shape. This structure is held together by weak forces between the amino acid molecules in the chain.

High temperatures will break these forces. The enzyme, including its active site, will change shape and the substrate will no longer fit. The rate of reaction will be affected, or the reaction will stop.

Diagram showing how high teperatures alter enzyme structuresY axis: enzyme activity. X axis: temperature, centigrade.  Plotted line climbs slowly until about half way on x axis. Climbs steeply to optimum temperature then falls steeply to 0.A graph showing the effect of temperature on enzyme activity

The effect of pH

Enzymes are also sensitive to pH. Changing the pH of its surroundings will also change the shape of the active site of an enzyme.

Many amino acids in an enzyme molecule carry a charge. Within the enzyme molecule, positively and negatively charged amino acids will attract. This contributes to the folding of the enzyme molecule, its shape, and the shape of the active site.

Changing the pH will affect the charges on the amino acid molecules. Amino acids that attracted each other may no longer. Again, the shape of the enzyme, along with its active site, will change.

Extremes of pH also denature enzymes. The changes are usually permanent.

Enzymes work inside and outside cells, for instance in the digestive system where cell pH is kept at 7.0 to 7.4. Cellular enzymes will work best within this pH range. Different parts of the digestive system produce different enzymes. These have different optimum pHs. The optimum pH in the stomach is produced by the secretion of hydrochloric acid.

The optimum pH in the duodenum is produced by the secretion of sodium hydrogencarbonate.

The following table gives examples of how some of the enzymes in the digestive system have different optimum pHs:

EnzymeOptimum pH
Salivary amylase6.8
Stomach protease (pepsin)1.5 - 2.0
Pancreatic protease (trypsin)7.5 - 8.0
Graph showing that as the pH increases so does the rate of enzyme activityA graph showing the effect of pH on enzyme activity

Suggest an enzyme that would produce a trend as shown in the graph above.

Pancreatic protease (trypsin).