Enzymes

• Enzymes


  Globular proteins which increase the rate of reaction by lowering the activation energy of the reaction they catalyse
• Active site


  The area of the enzyme where the reaction with the substrate takes place
• Enzymes


  • Specific to substrates they bind to, meaning that only one type of substrate fits into the active site of the enzyme
  • When the enzyme and substrate form a complex, the structure of the enzyme is altered so that the active site of the enzyme fits around the substrate (induced fit model)
  • The lock and key model of enzyme activity is based on the idea that the substrate fits into the enzyme the way a key fits into the lock due to the complementarity in shape between the two structures
• Types of enzymes


  • Intracellular (catalyse reactions inside of cells, e.g. ATP synthase, DNA helicase, DNA polymerase, lysosome)
  • Extracellular (digestive enzymes secreted by the cell)
• Immobilised enzymes


  Used in industrial processes to make the enzyme more stable and reusable
• Factors affecting the rate of enzyme-controlled reactions

  • Enzyme concentration
  • Substrate concentration
  • Temperature
  • pH
• Enzyme concentration


  The rate of reaction increases as enzyme concentration increases, but increasing the enzyme concentration beyond a certain point has no effect on the rate of reaction as substrate concentration becomes the limiting factor
• Substrate concentration


  As concentration of substrate increases, rate of reaction increases as more enzyme-substrate complexes are formed, but beyond a certain point the rate of reaction no longer increases as enzyme concentration becomes the limiting factor
• Temperature


  Rate of reaction increases up to the optimum temperature which is the temperature enzymes work best at, rate of reaction decreases beyond the optimum temperature
• pH


  In the case where the pH is more acidic than the optimum pH, H+ ions disrupt the enzyme-substrate binding and decrease the rate of reaction. In the case where the pH is more alkaline than the optimum, the OH- ions disrupt the binding, also leading to a decrease in product formation
• Inhibitors


  Substances which slow down or stop a reaction by affecting the binding of substrate to the enzymes
• Types of inhibitors


  • Reversible
  • Irreversible
• Irreversible inhibitors


  Heavy metal ions (e.g. mercury, silver) which cause disulphide bonds within the protein structure to break, affecting protein activity, and cyanide which covalently binds to the active site, preventing the binding of the substrate
• Reversible inhibitors


  Bind to the active site through hydrogen bonds and weak ionic interactions, therefore they do not bind permanently
• Types of reversible inhibitors


  • Competitive
  • Non-competitive
• Competitive inhibitors


  Similar in structure to the substrate molecule, they bind to the active site of the enzyme, decreasing its activity as they compete with substrate for the enzyme. Increasing the substrate reverses the effect of competitive inhibitors by outcompeting them
• Non-competitive inhibitors


  Bind at another site on the enzyme known as the allosteric site, changing the shape of the active site and preventing the binding of the substrate. Increasing the concentration of substrate has no effect on non-competitive inhibition
• Inhibitor drugs


  • Penicillin (inhibits enzyme transpeptidase in bacterial cell wall formation)
  • Ritonavir (inhibits HIV protease, responsible for assembly of new viral particles and spread of infection)
  • Digitalis (non-competitive inhibitor which binds to an enzyme involved in restricting contraction of the cardiac muscle)