unit 2 - ka1: metabolic pathways

• 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 metabolic pathways 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, pumps and enzymes are embedded in membranes
• protein pores - larger molecules depend on certain protein molecules to let them move across the membrane into or out of the cell. these protein molecules contain pores. they provide channels for specific substances to diffuse across the membrane.
• protein pumps - other protein molecules present in the cell membrane and act as carrier molecules which recognise specific ions and transport them across the membrane. these molecules used in active transport are often called pumps. +

  A protein pump requires energy therefor factors such as temperature, availability of oxygen and glucose concentration will affect the rate of active transport.
• enzymes in the membrane - some protein molecules embedded in the phospholipid membrane are enzymes which organise the sequence of reactions essential to the cell.
• metabolic pathways are controlled by the presence or absence of particular enzymes and the regulation of the rate of reaction of key enzymes.
• induced fit and the role of the active site of an enzyme in affecting activation energy and the affinity of the substrate and products for the active site.
• induced fit occurs when the active site changes shape to better fit the substrate after the substrate binds.
• the active site has a high affinity to the substrate as well as being specific to the substrate. this orientates the reactants into the correct positions for the reaction to take place. the products have a low affinity to the active site, allowing them to leave the active site.
• 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 the active sites are occupied, the enzyme is saturated. at the saturation point, adding more substrate does not affect the reaction rate.
• most 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.
• Competitive inhibitors bind at the active site preventing the substrate from binding. Competitive inhibition can be reversed by increasing substrate concentration.
• Control of metabolic pathways through competitive, non-competitive and feedback inhibition of enzymes.
• 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.