Factors affecting enzyme action

Physical factors affect enzyme activity.

Temperature

At low temperatures, the number of collisions between the enzyme and substrate are reduced because molecules have low kinetic energy. Fewer collisions result in fewer successful collisions in a given time and the reaction is slow.

As the temperature and therefore kinetic energy of the molecules increases, the enzymes and substrates will collide more often which results in an increased chance of a successful collision between the enzymes active site and the substrate. This will result in more enzyme/substrate complexes forming in a given time and the rate of the reaction is increased.

Very high temperatures disrupt the shape of the active site, which will reduce its activity by preventing the formation of enzyme/substrate complexes. The enzyme will have been denatured.

Enzymes therefore work best at a particular temperature called the optimum temperature.

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 do so. Again, the shape of the enzyme, along with its active site, will change. This means the enzyme is denatured.

Small fluctuations from the optimum can inactivate enzymes - this is not permanent. Extremes of pH denature enzymes - these changes are 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 pH2 which is produced by the secretion of hydrochloric acid.

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:

Y axis: enzyme activity. X axis: pH. Plotted line starts at origin, rises at an increasing rate to the Optimum pH and falls at reducing rate to 0.  Working range is area between line and x-axis.
Question

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

Pancreatic protease (trypsin).

Denaturing

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 or extreme pH will break these forces. The enzyme, including its active site, will change shape and the substrate no longer fit. The rate of reaction will slow more and more as the enzymes denature.

Diagram showing how high teperatures alter enzyme structures