Metabolic Pathways

• Metabolism refers to all of the chemical reactions that take place inside living cells.
• Unicellular and multicellular organisms must control their metabolism in order to survive.
• Metabolic pathways can be described as a series of chemical reactions that start with a substrate and finish with an end product.
• Metabolic pathways are integrated and controlled enzyme-catalysed reactions within a cell.
• Metabolic pathways can have reversible steps, irreversible steps and alternative routes.
• Reactions within metabolic pathways can be anabolic or catabolic.
• Anabolic reactions build up large molecules from small molecules and require energy.
• An example of an anabolic reaction is photosynthesis, where plants make glucose molecules from different raw materials.
• Catabolic reactions break down large molecules into smaller molecules and release energy.
• An example of a catabolic reaction is the process of food digestion, where different enzymes break down food particles so they can be absorbed by the small intestine.
• Membrane Proteins

  • Enzymes are vital proteins involved in metabolic pathways. 
  • Some enzymes can be found embedded within the cell membrane. 
  • Other proteins found embedded within the membrane act as:
  • pumps which pump ions into and out of cells (by active transport).
  • pores which allow ions of a particular size to pass through the membrane (by passive transport).
• All metabolic pathways have to be regulated and controlled to stop the build-up of an end product that isn’t needed.
• The cell can control a metabolic pathway by the presence or absence of a particular enzyme.
• The cell can also regulate the rate of reaction of key enzymes.
• Induced fit occurs when the active site of an enzyme changes shape to better fit the substrate after the substrate binds.
• The active site of an enzyme has a high affinity to the substrate as well as being specific to the substrate, orientating the reactants into the correct positions for the reaction to take place.
• As the products are made, they are no longer specific to the active site (they have a low affinity to the enzyme) and so are released.
• Some metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction.
• The binding of the enzyme to its substrate also lowers the activation energy of the reaction (amount of energy needed to make a reaction happen).
• If an enzyme is present, the amount of energy needed to make a product is lowered.
• The rate of enzyme reaction can be affected by substrate concentration.
• As the substrate concentration increases, the enzyme reaction increases until all of the active sites are occupied by the substrate.
• When all active sites are occupied, the enzyme is saturated.
• At this saturation point, adding more substrate makes no difference to the reaction rate.
• Inhibitors can be used to stop an enzyme from binding to its substrate, directly controlling the progress of a metabolic pathway.
• There are three types of inhibition: competitive inhibition, non-competitive inhibition, and feedback inhibition.
• Competitive inhibitors bind at the active site preventing the substrate from binding.
• Non-competitive inhibitors bind away from the active site but change the shape of the active site preventing the substrate from binding.
• Competitive inhibition can be reversed by increasing substrate concentration.
• 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, inhibiting an earlier enzyme, blocking the pathway, and preventing further synthesis of the end-product.