9.1.2

Factors Affecting Enzyme Activity

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Investigating Rates of Enzyme-Controlled Reactions

Changes to the tertiary structure of an enzyme through changing the pH or temperature will affect how fast reactions are catalysed.

Temperature

Temperature

  • Increasing the temperature will increase the kinetic energy of the molecules.
  • This increases the chance of a collision between the enzyme and substrate and so more collisions are likely in a set period of time. In other words, the rate of reaction is faster.
  • Increasing the temperature by 10oC will approximately double the rate of reaction for most enzyme-controlled reactions.
pH

pH

  • Changing the pH changes the number of hydroxide ions and hydrogen ions (OH− and H+) surrounding the enzyme.
  • These interact with the charges on the enzyme’s amino acids, affecting hydrogen bonding and ionic bonding, so resulting in changes to the tertiary structure.
Denatured enzymes

Denatured enzymes

  • Increasing or decreasing the temperature or pH outside of an optimal range can affect chemical bonds within the active site and the enzyme will not work as well.
    • At extreme temperatures and pH values, the enzyme's structure may be changed. This is called a denatured enzyme.

Enzyme and Substrate Concentration

Reaction rate is influenced by the relative enzyme and substrate concentrations.

Enzyme concentration

Enzyme concentration

  • Increasing the concentration of enzyme in a solution means there are more enzyme molecules available to catalyse the substrate in a given amount of time
Substrate concentration

Substrate concentration

  • Increasing the concentration of the substrate increases the numbers of substrate molecules that can form enzyme-substrate (ES) complexes at any one time.
  • This increases the initial rate of reaction but when all the enzyme molecules are engaged in ES complexes the rate cannot increase any further.
  • The rate will then plateau because the enzyme is said to be saturated.

Inhibition of Enzyme Activity

Reaction rate is influenced by the presence of competitive and non-competitive inhibitors.

Function of competitive inhibitors

Function of competitive inhibitors

  • Inhibitors are chemicals that slow down the rate or stop the reaction altogether.
  • Enzyme-substrate complexes cannot be formed or are formed at a much lower rate.
Structure of competitive inhibitors

Structure of competitive inhibitors

  • Competitive inhibitors are similar in shape to the usual substrate and affect the active site directly, blocking access for the formation of ES complexes.
  • Increasing the substrate concentration can compensate for the effects of a competitive inhibitor as there is no permanent damage to the shape of the active site.
    • Malonate ions are similar in shape to succinate ions and act as a competitive inhibitor of succinate dehydrogenase, an important enzyme in the Krebs cycle.
Function of non-competitive inhibitors

Function of non-competitive inhibitors

  • Some non-competitive inhibitors have reversible effect but others are irreversible and denature the enzyme.
    • E.g. Lead denatures a number of enzymes required to synthesise haemoglobin.
Structure of non-competitive inhibitors

Structure of non-competitive inhibitors

  • Non-competitive inhibitors affect another part of the enzyme molecule causing a change to the shape of the active site.
  • The active site is no longer complementary to the substrate molecules.
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Practice questions on Factors Affecting Enzyme Activity

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Which of the following statements about temperature and pH are true?True / false
  2. 2
    What is the effect of increasing temperature on enzyme function?Multiple choice
  3. 3
    What is the impact of a 10°C increase in temperature on the rate of reaction, assuming that the new temperature is still in the optimum range of the enzyme?Multiple choice
  4. 4
    Which of the following statements about enzyme and substrate concentration are true?True / false
  5. 5
    What happens to the rate of reaction when the enzymes become saturated?Multiple choice
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Enzymes

Enzyme-Controlled Reactions