Vmax & the Michaelis-Menten Constant (Cambridge International AS Biology)

Spec point — explain that 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

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
  • As substrate is used up (converted to product), the substrate concentration falls, so there are fewer enzyme-substrate collisions and 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 (V_max)
  • The Michaelis-Menten constant(K_m)

• These values are derived from the reaction rate at different substrate concentrations

• The maximum rate of reaction (V_max) is used to derive the Michaelis–Menten constant (K_m)

  • This is then 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 V_max (maximum rate of reaction at saturating substrate concentrations)

  • The K_m, which is the substrate concentration at ½V_max(this is 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 K_m****and the affinity of an enzyme for its substrate

• An enzyme with a high K_m** has a low affinity for its substrate and an enzyme with a low K_m** has a high affinity for its substrate

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

Exam Hint Make sure you can identify the V_max and the K_m on a graph like the one above.