Factors affecting enzyme action

Physical factors affect enzyme activity.

Temperature

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 not true of the enzymes in all organisms.

How temperature affects enzyme action

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.

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 comprised of 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 no longer fit. The rate of reaction will be affected, or the reaction will stop.

Diagram showing how high teperatures alter enzyme structures

A graph to show the effect of temperature on enzyme activity:

Y 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.

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 be. Again, the shape of the enzyme, along with its active site, will change.

Extremes of pH also denature enzymes. The changes are usually, though not always, permanent.

Enzymes work inside and outside cells, for instance in the digestive system where cell pH is kept at 7.0pH to 7.4pH. 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

A graph to show the effect of pH on an enzyme's activity:

Graph showing that as the pH increases so does the rate of enzyme activity
Question

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

Pancreatic protease (trypsin).

Effects of concentration

Substrate concentration

Enzymes will work best if there is plenty of substrate available. As the concentration of the substrate increases, so does the enzyme activity. This means that more substrate can be broken down by the enzymes if there is more substrate available.

This does not mean that the enzyme activity does not increase without end. This is because the enzyme can't work any faster even though there is plenty of substrate available. So when the amount of available substrate exceeds the amount of enzymes, then no more substrate can be broken down. The enzyme concentration is the limiting factor slowing the reaction.

Enzyme concentration

As the concentration of the enzyme is increased, the enzyme activity also increases. This means that more substrate will be broken down if more enzyme is added.

Again, this increase in enzyme activity does not occur forever. So when the amount of available enzyme exceeds the amount of substrate then no more substrate can be broken down. The substrate concentration is the limiting factor slowing the reaction.

Graph showing that as the substrate concentration increases, so does the rate of reaction