Effect of temperature, substrate concentration and pH on reaction rate

The rate of an enzyme-catalysed reaction is calculated by measuring the rate at which a substrate is used up or by the rate at which a product is formed.

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\text{Rate of reaction} = \frac{\text{amount of substrate used or amount of product formed}}{\text{time taken }}

Temperature

As with many chemical reactions, the rate of an enzyme-catalysed reaction increases as the temperature increases. However, at high temperatures the rate decreases again because the enzyme becomes denatured and can no longer function. This is shown in the graph below.

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.

As the temperature increases so does the rate of enzyme activity. An optimum activity is reached at the enzyme's optimum temperature. A continued increase in temperature results in a sharp decrease in activity as the enzyme's active site changes shape. It is now denatured.

Effects of pH

Changes in pH also alter the shape of an enzyme’s active site. Each enzyme work bests at a specific pH value. The optimum pH for an enzyme depends on where it normally works. For example, enzymes in the small intestine have an optimum pH of about 7.5, but stomach enzymes have an optimum pH of about 2.

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

In the graph above, as the pH increases so does the rate of enzyme activity. An optimum activity is reached at the enzyme’s optimum pH, pH 8 in this example. A continued increase in pH results in a sharp decrease in activity as the enzyme’s active site changes shape. It is now denatured.

Substrate concentration

Enzymes will work best if there is plenty of substrate. As the concentration of the substrate increases, so does the rate of enzyme activity. However, the rate of enzyme activity does not increase forever. This is because a point will be reached when the enzymes become saturated and no more substrates can fit at any one time even though there is plenty of substrate available.

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

As the substrate concentration increases so does the rate of enzyme activity. An optimum rate is reached at the enzyme’s optimum substrate concentration. A continued increase in substrate concentration results in the same activity as there are not enough enzyme molecules available to break down the excess substrate molecules.