Factors Affecting Enzyme Activity

This lesson covers:

  1. What happens when an enzyme is denatured
  2. The effects of temperature and pH on enzyme-catalysed reactions
  3. The effects of enzyme and substrate concentration on enzyme-catalysed reactions

Enzyme denaturation

Changes in temperature or pH can affect the rate of enzyme-catalysed reactions.

Diagram showing enzyme denaturation where changes in temperature or pH cause the active site to change shape, preventing substrate binding.

Drastic temperature increases or changes to the pH causes bonds to break, changing the enzyme's tertiary structure.


This causes the active site to change shape so that the substrate no longer fits. This means that enzyme-substrate complexes cannot be formed and the enzyme is denatured. 

Different factors affect the rate of enzyme-controlled reactions

You need to be able to describe and explain how the four factors affect enzyme reactions.


These factors are:

  1. Temperature 
  2. pH
  3. Substrate concentration 
  4. Enzyme concentration

Temperature

All enzymes have an optimum temperature, but these can vary. The graph below shows how temperature affects the rate of a specific enzyme-catalysed reaction.

Graph showing the effect of temperature on the rate of an enzyme-catalysed reaction.

Explanation

  1. The molecules have more kinetic energy, causing more collisions and enzyme-substrate complexes.
  2. The optimum temperature is the temperature this enzyme works fastest at. 
  3. Too much kinetic energy causes the active site to change shape and the enzyme denatures.

Description

  1. As temperature increases, the rate of reaction increases. 
  2. The maximum rate is reached at the optimum temperature. 
  3. As temperature increases past the optimum, the rate of reaction decreases until the reaction stops.

Temperature coefficient (Q10)

The temperature coefficient (Q10) is a value that shows how much the rate of reaction changes when the temperature is increased by 10°C.


It can be calculated using this formula: 

Q10​=R1​R2​​

Where:

  • R2 = The rate of reaction at the higher temperature (+10°C).
  • R1 = The rate of reaction at the lower temperature.

pH

All enzymes have an optimum pH, but these can vary. The graph below shows how pH affects the rate of a specific enzyme-catalysed reaction.

Graph showing how pH affects the rate of a specific enzyme-catalysed reaction with an optimum pH.

Explanation

  1. In acidic conditions, H+ ions break ionic/hydrogen bonds and denature enzymes.
  2. The optimum pH is the pH the enzyme works fastest at.
  3. In alkaline conditions, OH- ions break ionic bonds or hydrogen bonds and denature enzymes.

Description

  1. Below the optimum pH, the rate of reaction is low or zero.
  2. The maximum rate of reaction is reached at the optimum pH.
  3. Above the optimum pH, the rate of reaction is low or zero. 

Substrate concentration

The graph below shows how substrate concentration affects the rate of an enzyme-catalysed reaction.

Graph showing the effect of substrate concentration on the rate of an enzyme-catalysed reaction.

Explanation

  1. There are more substrate molecules to form more  enzyme-substrate complexes.
  2. This is the saturation point, which is when all active sites are occupied by a substrate and enzyme concentration becomes the limiting factor. 

Description

  1. As the substrate concentration increases, the rate of reaction increases. 
  2. As the substrate concentration increases further, the rate of reaction plateaus (levels off).

Enzyme concentration

The graph below shows how enzyme concentration affects the rate of an enzyme-catalysed reaction.

Graph showing the effect of enzyme concentration on the rate of an enzyme-catalysed reaction.

Explanation

  1. There are more enzyme molecules to form more enzyme-substrate complexes.
  2. All substrate molecules available are being acted upon and substrate concentration becomes the limiting factor.

Description

  1. As the enzyme concentration increases, the rate of reaction increases. 
  2. As the enzyme concentration increases further, the rate of reaction plateaus (levels off).

Worked example - Calculating Q10

Given that at 20°C, the rate of reaction is 5 products produced per minute, and at 30°C, the rate of reaction is 10 products produced per minute, calculate Q10.


Step 1: Equation

Q10​=R1​R2​​


Step 2: Substitution and correct evaluation

Q10​=510​=2