CIE A Level Biology (9700) 2019-2021

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

Graph showing the Michaelis-Menten model, downloadable AS & A Level Biology revision notes

A graph showing the effect of substrate concentration on initial reaction rate, with Vmax, ½Vmax and Km values shown

Exam Tip

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


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.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Go to Top