Enzyme Action

• • Enzymes have an active site where specific substrates bind forming an enzyme-substrate complex
  • The active site of an enzyme has a specific shape to fit a specific substrate
  • Extremes of heat or pH can change the shape of the active site, preventing substrate binding – this is called denaturation
  • Substrates collide with the enzymes active site and this must happen at the correct orientation and speed in order for a reaction to occur
• • The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
  • The shape of the active site (and therefore the specificity of the enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme:
  • Proteins are formed from chains of amino acids held together by peptide bonds
  • The order of amino acids determines the shape of an enzyme
  • If the order is altered, the resulting three-dimensional shape changes
• • An enzyme-substrate complex forms when an enzyme and its substrate join together
  • The enzyme-substrate complex is only formed temporarily, before the enzyme catalyses the reaction and the product(s) are released
• • Enzyme reactions can either be catabolic or anabolic
  • Catabolic reactions involve the breakdown of complex molecules into simpler products, which happens when a single substrate is drawn into the active site and broken apart into two or more distinct molecules
  • Examples of catabolic reactions include cellular respiration and hydrolysis reactions
• • Anabolic reactions involve the building of more complex molecules from simpler ones by drawing two or more substrates into the active site, forming bonds between them and releasing a single product
  • Examples of anabolic reactions include protein synthesis and photosynthesis
• Activation energy


  The amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed
• All chemical reactions are associated with energy changes
• For a reaction to proceed there must be enough activation energy
• Enzymes


  They work by lowering the activation energy of a reaction
• Enzymes


  • They speed up chemical reactions because they influence the stability of bonds in the reactants
  • They destabilise bonds in the substrate making it more reactive
• How enzymes work


  1. Lowering the activation energy of a reaction

  2. Providing an alternative energy pathway