In the last article we looked at how enzymes work as biological catalysts. However, there are various factors affecting enzyme activity, and consequently the rate of the reactions they catalyse. Today we will look at the four main factors that influence the rate of enzyme activity.
Substrate concentration
This one makes quite good sense. If there is a higher substrate concentration, collisions between the enzyme and substrate are more likely, so more active sites are occupied (more enzyme-substrate complexes form) and the reaction happens more quickly.
However this is only true up until all of the available active sites are occupied – after that point, you would have to add more enzyme in order to speed up the reaction any further. You could say that enzyme concentration has become the limiting factor. This is why a graph of substrate concentration vs rate of reaction plateaus (levels off) at the saturation point – all the active sites are saturated with substrate.
Don’t forget that unless the substrate is constantly replaced, substrate concentration will gradually decrease over time during a reaction because it is being converted to product.
Enzyme concentration
This graph is a very similar shape to the previous one, and for similar reasons. If more enzymes are available, collisions between the enzyme and substrate are more likely, more active sites are available to be occupied, so more enzyme-substrate complexes can form.
If substrate is completely unlimited, the graph would continue in a straight line. However there may come a point where there the amount of substrate is limiting the rate of reaction – there simply isn’t enough substrate to fill all of the many available active sites. In this case, the graph plateaus.
Temperature
Enzymes have an optimum temperature at which they work at their fastest rate. In the human body the optimum temperature for most enzymes would be about 37°C because that is normal body temperature. Either side of that temperature, the rate is slower.
Below the optimum temperature, there is a gradual increase in the rate of reaction as more heat is provided. The heat energy is transferred to kinetic energy. If the enzyme and substrate have more kinetic energy they move around more quickly, and are more likely to collide with each other with enough energy to form the enzyme-substrate complex. So more reactions can occur.
Above the optimum temperature, an enzyme can become denatured (no longer functional). This is because too much kinetic energy can break bonds holding the protein in shape e.g hydrogen bonds. If the active site changes shape due to broken bonds, the substrate can no longer bind and the reaction stops completely.
pH
Similar to temperature, enzymes have an optimum pH at which they work at their fastest rate. In the human body this is normally around pH 7, but a notable exception is pepsin. We looked at pepsin in the digestion article – it digests protein in the stomach, and the stomach is a very acidic pH 2, so pepsin works best at that pH.
Above or below the optimum pH, the enzyme becomes denatured. Remember from chemistry that at acidic pH there are lots of H+ ions, and at alkaline pH there are lots of OH– ions. Too many of either of these can disrupt the ionic bonds in the protein, meaning the active site changes shape and the substrate can no longer bind.
Summary
Don’t forget that all of these factors affecting enzyme activity interact to determine the rate of an enzyme-controlled reaction.
- Increasing substrate or enzyme concentration increases the rate of the reaction up until the other one becomes the limiting factor.
- Increasing temperature increases the rate of the reaction due to providing more kinetic energy, until the enzyme becomes denatured.
- Enzymes have an optimum pH – either side of that, the enzyme becomes denatured.
