Vmax & the Michaelis-Menten Constant (CIE A Level Biology)

• 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 (V_max) and 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), 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 V_max (maximum rate of reaction at saturating substrate concentrations) and the K_m, which is the substrate concentration at ½V_max (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 V_max, ½V_max and K_m values shown