1.6

• Activation energy


  Amount of energy needed to make a reaction happen
• Active site


  The part of the enzyme that the substrate binds to
• Affinity


  The degree to which a substance tends to combine with another
• Catalyst


  A substance that increases the rate of a chemical reaction without being used up
• Enzyme


  A biological catalyst consisting of protein molecules, which catalyses or speeds up the rate of a chemical reaction
• Haemoglobin


  The protein responsible for transporting oxygen in the blood stream
• Metabolic pathways


  Integrated and controlled pathways of enzyme-catalysed reactions within a cell
• Substrate


  A substance on which enzymes act
• Metabolic pathways


  1. Anabolic reactions

  2. Catabolic reactions
• Anabolic reactions build up small molecules and require energy for the reaction to occur
• Catabolic reactions break down large molecules into smaller molecules and release energy as a result of the reaction
• Metabolic pathways


  • Controlled by the presence or absence of particular enzymes
  • Regulated by the rate of reaction of key enzymes
• Induced fit


  The active site changes shape to better fit the substrate after the substrate binds
• In animals, specific metabolic pathways can produce vitamins and haemoglobin. An example of a metabolic pathway is cellular respiration
• Metabolic pathways


  1. Start with a substrate

  2. Finish with an end product
• Enzymes control metabolic pathways
• Metabolic pathways can have reversible steps, irreversible steps and alternative steps
• As substrate concentration increases

  The enzyme reaction increases until all of the active sites are occupied by the substrate
• When all the active sites are occupied, the enzymes are saturated
• The substrate molecule(s) have a high affinity for the active site and the subsequent products have a low affinity allowing them to leave the active site
• The binding of the enzyme to the substrate also lowers the activation energy of the reaction
• Inhibitors


  • Can be used to stop an enzyme from binding to its substrate
  • Can directly control the progress of a metabolic pathway
• Competitive inhibition


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


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


  The end-product in the metabolic pathway reaches a critical concentration and then binds to an enzyme at the start of the pathway, blocking the pathway, and so prevents further synthesis of the end-product