Factors affecting enzyme action
Physical factors affect enzyme activity.
Temperature
At low temperatures, the number of successful collisions between the A protein which catalyses or speeds up a chemical reaction. and A substance on which enzymes act. is reduced because their The movement of molecules resulting from their kinetic energy. 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 To change the shape of an enzyme's active site, for example because of high temperatures or extremes of pH. Denatured enzymes no longer work..
Enzymes therefore work best at a particular temperature.
Organic compound made up of amino acid molecules. One of the three main food groups, proteins are needed by the body for cell growth and repair. are chains of The building blocks that make up a protein molecule. 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.
A graph to show the effect of temperature on enzyme activity:
The effect of pH
Enzymes are also sensitive to Scale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline.. 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 Property of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative 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 Organ system involved in breaking food down so that it can be absorbed into the bloodstream. 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 The best or most appropriate - for instance, the conditions under which an enzyme works best (eg temperature and pH). 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:
| Enzyme | Optimum pH |
| Salivary amylase | 6.8 |
| Stomach protease (pepsin) | 1.5–2.0 |
| Pancreatic protease (trypsin) | 7.5–8.0 |
| Enzyme | Salivary amylase |
|---|---|
| Optimum pH | 6.8 |
| Enzyme | Stomach protease (pepsin) |
|---|---|
| Optimum pH | 1.5–2.0 |
| Enzyme | Pancreatic protease (trypsin) |
|---|---|
| Optimum pH | 7.5–8.0 |
A graph to show the effect of pH on an enzyme's 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 A substance on which enzymes act. 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.