3.1.2

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.

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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.
  • The temperature coefficient (Q10) measures the change in the rate of reaction when temperature increases by 10oC. Q10 is usually around 2.
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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.
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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.

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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
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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.
    • This is known as Vmax.
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KM

  • KM is known as the Michaelis-Menten constant.
  • We can calculate KM by looking at the concentration of substrate at Vmax/2.
    • So, we can think of Km as the substrate concentration at which the rate is half of the maximum rate of that enzyme.
  • We use Km as a measure of enzyme-substrate affinity.
    • The application of enzyme activity in this way is known as Michaelis-Menten kinetics.
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Interpreting KM

  • A smaller KM means an enzyme has a high affinity for a substrate.
    • This is because a smaller concentration of substrate is required to reach Vmax/2.

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