1.6

• Enzymes

  • Biological catalysts which speed up the rate of reaction
  • Specific to one substrate
  • Made of proteins
  • Have an active site with a complementary shape to the substrate
  • Have optimum temperature and pH
  • Denatured at high temperatures
• Active site

  The location on an enzyme's surface where substrate molecules bind and the chemical reaction takes place
• Metabolic pathways

  • Integrated and controlled pathways of enzyme-catalysed reactions within a cell
  • Can have reversible steps, irreversible steps and alternative routes
• Anabolic reactions

  Build up large molecules from small molecules and require energy
• Catabolic reactions

  Break down large molecules into smaller molecules and release energy
• Metabolic pathways are controlled by

  • The presence or absence of particular enzymes
  • The regulation of the rate of reaction of key enzymes
• Enzymes lower the activation energy

  For a reaction
• Substrate molecules

  Have a high affinity for the active site
• Induced fit

  The active site changes shape to better fit the substrate after the substrate binds
• Induced fit

  Ensures the active site comes in very close contact with the substrate molecules, increasing the chance of the chemical reaction taking place
• Product(s)

  Have low affinity allowing them to leave the active site
• Activation energy

  The energy required to start a reaction
• Substrate X is normally converted to Product Y

  If X is removed and Y is in high concentration the pathway will be reversed
• Substrate concentration

  • As it increases, the rate of enzyme reaction increases then remains constant
  • When the graph is increasing, substrate concentration is the limiting factor
  • As substrate concentration increases, more substrate molecules can bind to active sites and the rate of reaction increases
  • When the graph remains constant, the substrate concentration is high enough to allow all available active sites on enzymes to be occupied
  • Adding more substrate makes no difference to the reaction rate, enzyme concentration is now a limiting factor
• Inhibitors

  Chemicals which slow down or stop an enzyme controlled reaction
• Competitive inhibitors

  • Bind at the active site preventing the substrate from binding
  • Competitive inhibition can be reversed by increasing substrate concentration
• Non-competitive inhibitors

  • Bind away from the active site but change the shape of the active site preventing the substrate from binding
  • Non-competitive inhibition cannot be reversed by increasing substrate concentration
• Feedback inhibition

  • Occurs when the end-product in the metabolic pathway reaches a critical concentration
  • The end-product then inhibits an earlier enzyme, blocking the pathway, and so prevents further synthesis of the end-product
• Enzyme action

  • Enzymes lower the activation energy for a reaction
  • Substrate molecules have a high affinity for the active site
  • Induced fit occurs when the active site changes shape to better fit the substrate after the substrate binds
  • Induced fit ensures the active site comes in very close contact with the substrate molecules, increasing the chance of the chemical reaction taking place
  • The product(s) have low affinity allowing them to leave the active site
• Enzyme inhibition

  • Metabolic pathways can be controlled through competitive, non-competitive and feedback inhibition of enzymes
  • Competitive inhibitors bind at the active site preventing the substrate from binding
  • Competitive inhibition can be reversed by increasing substrate concentration
  • Non-competitive inhibitors bind away from the active site but change the shape of the active site preventing the substrate from binding
  • Non-competitive inhibition cannot be reversed by increasing substrate concentration
  • Feedback inhibition occurs when the end-product in the metabolic pathway reaches a critical concentration
  • The end-product then inhibits an earlier enzyme, blocking the pathway, and so prevents further synthesis of the end-product