• 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??Vmax?(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

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