CIE AS Biology (9700) exams from 2022

Revision Notes

3.2.6 Vmax & the Michaelis-Menten Constant

Vmax & the Michaelis-Menten Constant

  • Substrate concentration affects the rate of catalysis in an enzyme-substrate reaction
  • When the substrate concentration is fixed (and enzyme concentration is kept constant) the initial rate of reaction is fastest and as active sites become engaged, the reaction rate falls
  • The Michaelis-Menten model describes the kinetics of such enzyme catalysed reactions
  • In this model, two values are used to describe an enzyme catalysed reaction, the maximal rate or maximal velocity (Vmax) and the Michaelis-Menten constant (Km)
  • These values are derived from the reaction rate at different substrate concentrations
  • The maximum rate of reaction (Vmax) is used to derive the Michaelis–Menten constant (Km), which is used to compare the affinity of different enzymes for their substrates

Michaelis-Menten enzyme kinetics

  • The Michaelis-Menten model is used to investigate the kinetics of enzyme catalysed reactions (enzyme kinetics is an area in biochemistry that studies how different variables affect reaction rates)
  • The rate of reaction is measured at different substrate concentrations, producing a graph like the one below
  • The two important values deduced are the Vmax (maximum rate of reaction at saturating substrate concentrations) and the Km, which is the substrate concentration at ½Vmax (also known as the Michaelis-Menten constant)
    • The Michaelis-Menten constant is the substrate concentration at which the enzyme works at half its maximum rate
    • At this point, half of the active sites of the enzyme are occupied by substrate molecules
    • The higher the affinity of the enzyme for the substrate, the lower the substrate concentration needed for this to occur
    • This is why the Michaelis-Menten constant is a measure of the affinity of an enzyme for its substrate
  • There is an inverse relationship between the Km and the affinity of an enzyme for its substrate
  • An enzyme with a high Km has a low affinity for its substrate and an enzyme with a low Km has a high affinity for its substrate

Exam Tip

Make sure you can identify the Vmax and the Km on a graph!

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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