Enzymes: Temperature & pH (Cambridge O Level Biology)

The effect of temperature

• The specific shape of an enzyme is determined by the amino acids that make the enzyme
• The three-dimensional shape of an enzyme is especially important around the active site area; this ensures that the enzyme’s substrate will fit into the active site enabling the reaction to proceed
• Enzymes work fastest at their ‘optimum temperature’ – in the human body, the optimum temperature is around 37°C
• Heating to high temperatures (beyond the optimum) will start to break the bonds that hold the enzyme together – the enzyme will start to distort and lose its shape – this reduces the effectiveness of substrate binding to the active site reducing the activity of the enzyme
• Eventually, the shape of the active site is lost completely and the enzyme is described as being ‘denatured’
• Substrates cannot fit into denatured enzymes as the specific shape of their active site has been lost

Enzyme denaturation diagram

Denaturation is largely irreversible – once enzymes are denatured they cannot regain their proper shape and activity will stop

• Increasing temperature from 0°C to the optimum increases the activity of enzymes as the more energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction
• This means that low temperatures do not denature enzymes, but at lower temperatures with less kinetic energy both enzymes and their substrates collide at a lower rate

The effect of temperature on enzyme activity diagram

 

This graph shows the effect of temperature on the rate of activity of an enzyme

The effect of pH

• The optimum pH for most enzymes is 7 but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
• If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be destroyed
• This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity
• Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

Enzyme denaturing due to pH changes diagram

If pH is increased or decreased away from the optimum, then the shape of the enzyme is altered

The effect of pH on enzyme activity diagram

This graph shows the effect of pH on the rate of activity of an enzyme from the duodenum