1.4 - 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 (rate increases as enzyme concentration increases, but beyond a certain point has no effect as substrate concentration becomes the limiting factor)
  • Substrate concentration (rate increases as substrate concentration increases, but beyond a certain point has no effect as enzyme concentration becomes the limiting factor)
  • Temperature (rate increases up to the optimum temperature, then decreases beyond the optimum)
  • pH (more acidic or alkaline than the optimum pH disrupts enzyme-substrate binding and decreases the rate)
• Inhibitors


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


  • Reversible (bind to the active site through hydrogen bonds and weak ionic interactions, can be competitive or non-competitive)
  • Irreversible (cause disulphide bonds within the protein structure to break, or covalently bind to the active site)
• Competitive inhibitors


  Similar in structure to the substrate molecule, bind to the active site of the enzyme and compete with substrate for the enzyme
• Non-competitive inhibitors


  Bind to an allosteric site on the enzyme, changing the shape of the active site and preventing substrate binding
• Inhibitor drugs


  • Penicillin (inhibits enzyme transpeptidase in bacterial cell wall formation)
  • Ritonavir (inhibits HIV protease in viral assembly and spread)
  • Digitalis (non-competitive inhibitor of enzyme involved in cardiac muscle contraction)