Enzymes 1.4

• Enzymes


  Specific as their active site is only complementary to one substrate
• Enzymes


  • Tertiary structure proteins with a very specific 3D shape
  • Have an active site held together by peptide, hydrogen, ionic and disulphide bonds
• Lock and key theory

  Substrate fits exactly into the active site of the enzyme forming an enzyme-substrate complex, the reaction occurs and the products are released, the enzyme remains unchanged
• Enzymes


  • Intracellular (work inside cells)
  • Extracellular (secreted from cells for use outside of the cell)
• At low temperatures


  Low kinetic energy, few successful collisions where the substrate enters the active site
• As temperature increases


  Kinetic energy increases, more collisions and enzyme-substrate complexes formed per unit time, leading to increased product
• If temperature continues to increase

  Kinetic energy increases to a point where bonds holding the 3D tertiary structure of the active site are weakened, the active site loses its shape, the enzyme is denatured
• At low substrate concentrations


  Substrate concentration is the limiting factor for the rate of reaction
• As substrate concentration increases

  Rate of reaction increases until enzyme concentration becomes the limiting factor
• Competitive inhibitors


  Complementary in shape to the active site, prevent formation of enzyme-substrate complexes by blocking the active site, do not bind permanently
• Non-competitive inhibitors


  Bind to the enzyme away from the active site, alter the shape of the active site so no enzyme-substrate complexes can be formed, some bind reversibly, others bind irreversibly
• Induced fit theory


  Active site and substrate are not fully complementary in shape, reactive groups align and the substrate forces its way into the active site, both areas change structure slightly, the bonds in the substrate weaken and the reaction occurs at a lower activation energy
• Enzymes are catalysts, which means they lower the activation energy of reactions but remain unchanged in the reaction
• Small changes from the optimum pH

  Make small reversible changes in the enzyme molecule reducing its efficiency
• Large changes in pH


  Disrupt ionic and hydrogen bonds in the enzyme causing permanent changes to the shape of the active site, preventing the formation of enzyme-substrate complexes, denaturing the enzyme
• Immobilised enzymes


  • Increased stability so will denature at higher temperature and can be used efficiently over a wider range of pH
  • Products uncontaminated with enzyme
  • Enzymes easily added and removed, therefore giving control over reactions, alternatively recovered for re-use
• Uses of immobilised enzymes

  • Biosensors
  • Creating lactose-free milk
• As competitive inhibitor concentration increases


  Effect of inhibitor decreases as substrate collides more often with active site
• Increasing substrate concentration


  Increases rate of reaction but some of the enzyme is always affected, so the rate is always lower than it would be without the inhibitor
• Increasing enzyme concentration


  Increases rate of reaction as more active sites are available for reactions
• Metabolism


  1. Anabolic reactions (building up molecules)

  2. Catabolic reactions (breaking down molecules)
• Enzymes catalyse anabolic and catabolic reactions in metabolism
• With enzyme


  Activation energy is lower than without enzyme
• With enzyme


  Overall energy released during the reaction is the same as without enzyme