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 is not true of the enzymes in all organisms.

At higher temperatures, more collisions occur so the rate of reaction increases.

However, extremely high temperatures disrupt the shape of the active site, which will reduce its activity, or prevent it from working. The enzyme is now denatured.

Enzymes therefore work best at their optimum temperature.

Denaturing in details

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 will no longer fit. The rate of reaction will be slowed, 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 previously 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 permanent once the enzyme molecule has been denatured.

Enzymes work outside cells, for instance in the digestive system and also inside cells 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 small intestine is produced by the secretion of sodium hydrogen carbonate.

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 enzyme 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).