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• Enzyme


  Type of protein
• Enzymes


  • Usually globular and water-soluble
  • Has tertiary structure with hydrophilic R groups on the outside and hydrophobic R groups on the inside
• How an enzyme lowers activation energy

  1. Holding the substrate molecules close together

  2. Placing a physical strain on the bonds so that they are weaker and are broken more easily
• Intracellular enzymes


  Enzymes that act inside cells
• Extracellular enzymes


  Enzymes that act outside cells
• Induced fit model of enzymes

  • The substrate interacts with the R groups of amino acids at the active site of the enzyme
  • The shape of the active site changes to create a better fit or stronger binding to the substrate
• Lock and key hypothesis

  The substrate is complementary in shape to the enzyme's active site and binds to it
• Investigating the progress of enzymatic reactions

  1. Analyse the amount of product of the reaction is present

  2. Analyse a colour change indicating the removal of substrate or presence of products
• Monitoring enzymatic reactions using colorimetry

  1. Use a colorimeter to build a calibration curve (measure absorbance of known concentrations of starch in the presence of iodine solution at equal increments of concentration)

  2. Measure samples of your reaction solution at equal time intervals and use your calibration curve to estimate concentration of starch
• Factors affecting enzymatic reactions

  • Temperature
  • Enzyme concentration
  • Substrate concentration
  • Inhibitor concentration
  • pH - using a buffer solution
• Effect of temperature on enzyme activity

  • As temperature increases, the rate of activity increases as enzyme and substrate molecules gain kinetic energy, so the number of successful collisions to form enzyme-substrate (ES) complexes increase
  • It reaches a maximum at the optimum temperature
  • Beyond that temperature, enzymes become denatured and fewer ES complexes are formed, so the activity decreases
• Effect of pH on enzyme activity

  • Enzyme activity is highest at the optimum pH
  • As pH increases or decreases from its optimum, it will lead to a partial or permanent denaturation of the enzyme so the activity decreases
• Function & importance of a buffer solution in enzymatic reactions

  • Buffer solutions maintain pH by minimising changes to pH in a system
  • Important as the products of a reaction may affect a system's pH, which in turn can affect reaction rates
• Effect of substrate concentration on enzyme activity

  • When substrate concentration increases, enzyme activity increases as substrate concentration is limiting
  • Beyond a certain substrate concentration, enzyme activity plateaus because enzyme concentration is limiting. All enzyme active sites are occupied. Vmax is reached
• Mode of action of a competitive inhibitor

  • A molecule that is similar in shape to the substrate, so is also complementary to the active site and binds to it, blocking ES complex formation
  • Increasing substrate concentration decreases the effect of inhibition, and inhibition is reversible
• Mode of action of a non-competitive inhibitor

  • Binds to an allosteric site of the enzyme and disrupts the tertiary structure of the enzyme, changing the shape of the active site so ES complexes cannot be formed
  • Increasing substrate concentration has no effect on the degree of inhibition
• Irreversible inhibitors


  • Permanently prevent formation of ES complexes
  • Heavy metal ions e.g. mercury, silver cause disulphide bonds in tertiary structure to break
  • Bind to enzymes by strong (covalent) bonds e.g. cyanide binds to cytochrome c
• Reversible inhibitors


  • May be competitive or non-competitive
  • Bind to enzyme temporarily e.g. by H-bonds or a few ionic bonds
  • ES complexes can form after the inhibitor is released
• Michaelis-Menten equation


  • Used to calculate the maximum rate of reaction (Vmax) by relating the velocity of enzyme reactions (V) to concentration of a substrate [S]
  • Vmax represents the maximum rate of reaction achieved by the system at maximum substrate concentration
  • Km (½ Vmax) can be used to tell us the affinity of an enzyme to its substrate - as the lower the concentration of substrate required to achieve ½ Vmax indicates a greater affinity
• How an enzyme can be immobilised

  1. They are immobilised by attaching them to an insoluble, inert material e.g. calcium alginate

  2. This forms a gel capsule around them thus holding them in place during the reaction
• Advantages of immobilising enzymes

  • Enables enzymes to be easily separated from the products and thus reused
  • Immobilised enzymes are encapsulated and therefore not fully exposed to denaturing conditions, making them more able to withstand changes to pH & temperature in solution
• Differences between reactions of free enzymes and immobilised enzymes

  Immobilisation allows for increased resistance of enzymes to denaturing conditions, so reactions will proceed to a larger range than if free enzymes were involved