MS1- metabolic pathways

• 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 pathways of enzyme-catalysed reactions within a cell.
• Metabolic pathways are integrated and controlled pathways of enzyme-catalysed reactions within a cell.
• Metabolic pathways can have reversible steps, irreversible steps and alternative routes
• Reactions within a pathway can be anabolic or catabolic
• Anabolic reactions build up large molecules from small molecules and require energy
• Catabolic reactions break down large molecules into smaller molecules and release energy
• protein pores, protein pumps and enzymes are embedded in membranes
• Enzymes are vital proteins involved in metabolic pathways
• Some enzymes can be found embedded within the cell membrane
• protein pumps- for example the sodium potassium pump that pumps sodium out of the cell and potassium into the cell.
• Protein pores allow ions of a particular size to pass through the membrane
• 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
• Enzymes are to bind to their substrate because they have an active site
• An induced fit occurs where the active site of the enzymes changes shape slightly to better fit the substrate after the substrate binds
• The substrate molecules have a high affinity (attraction) for the active site
• The substrate orientates to fit the active site and is held firmly in place to allow the reaction to take place
• The products have a low affinity for the active site allowing them to leave the active site after the reaction has taken place.
• The binding of an enzyme to its substrate 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
• When the product concentration increases the reaction is slowed, stopped or even reversed
• Some metabolic pathways are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction
• Different chemicals can influence enzyme activity. Inhibitors can be used to stop an enzyme from binding to its substrate
• As a result, inhibitors can directly control the progress of a metabolic pathway
• There are three types of inhibition:

  • competitive inhibition
  • non-competitive inhibition
  • feedback inhibition
• 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
• The benefits of the action of a feedback inhibitor are:

  • prevents overproduction of enzymes
  • prevents wastage of energy/amino acids involved in enzyme production