enzymes

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  • Enzymes
    • Definition
      • Enzyme are biological catalysts as they control and increase the rate of reactions that take place in living organisms
        • All catalysts change the rate of chemical reactions but are not themselves changed by the reactions - therefore, during enzyme driven reactions the enzyme is not used up - as a result enzymes can catalyse the same reaction many times
    • Many hundreds of chemical reactions are taking place in the cells of living organisms all the time
      • In order for an organism to function these chemical reactions have to take place very rapidly and in a controlled manner
    • Collision Theory
      • for a reaction to take place between reacting particles it is necessary that they collide
        • the temp of the human body is approx 37
          • at lower temps there is less kinetic energy and as a result particles move more slowly and there will be fewer collisions
            • therefore, at lower temps there will be fewer collisions between an enzyme and its substrate which leads to a lower rate of reaction
          • if the human body temp falls below 35.5 this can be life-threatening as enzyme-driven reactions slow down
    • Substrate
      • the molecule that the enzyme binds to and reacts with
    • Structure
      • complex in which protein molecules (made from chains of amino acids) that make them up are folded around each other
      • an enzyme will therefore have a 3d shape and it is the shape which is crucial for an enzyme to function or catalyse a specific reaction
        • if it is altered, the enzyme cannot bind to its substrate and so cannot function
          • enzymes are substrate specific, each enzyme catalyses a specific reaction
      • enzymes contain an active site: this is where the substrate of the reaction binds to the enzyme
    • How do they work
      • the substrate (the reactant) fits into the active site of an enzyme forming an enzyme-substrate complex
        • lock and key - the idea of the substrate fitting into the enzyme
          • only the correct substrate can fit into the active site of an enzyme, once the correct substrate is attached the reaction then takes place rapidly and the products are released from the enzyme
    • Conditions which affect enzyme reactions
      • Effect of pH on enzyme activity
        • enzymes are protein molecules
          • the shape of these molecules is determined by the amino acids that make them up and by the attractive forces between different regions of the protein molecule
            • change in pH affects these forces and can change the overall shape of the enzyme denaturing the enzyme and altering the shape of the active site
              • no enzyme-substrate complexes will form and the enzyme will no longer function
        • pH of the human body is kept fairly constant at approx 7.2-7.4
          • stomach pH 1-2
          • duodenum pH 8.3
      • Effect of enzyme concentration on enzyme activity
        • the more enzymes there are the faster the enzyme reaction
          • this is because there are more active sites for substrates to attach or bind to
            • the rate of reaction will eventually fall as the substrate is used up
        • if the substrate concentration is maintained at a high level and other conditions such as pH and temperature are kept constant, the rate of reaction is proportional to the enzyme conc
          • the enzyme conc is increases, so will be the rate of the enzyme driven reaction
        • as the substrate concentration increases the rate of an enzyme reaction increases
          • if the substrate conc increases while the enzyme concentration remains constant, eventually every enzyme's active site will be filled with substrate
            • at this point an enzyme is working at its maximum rate
        • if temp is kept the same, increase the rate of reaction if there is a high substrate conc by increasing the enzyme conc
      • Effect of temp on enzyme activity
        • the rate of a chemical reaction will increase with increasing temperature
          • increase in temp will cause the reacting particles to move faster, and increase their kinetic energy
            • same for an enzyme driven reaction - enzyme and substrate will collide more often and with more energy as temp increases thereby increasing the rate of reaction
              • if the temp is lowered the enzyme will become less active but it will resume activity once the temp is raised to the normal working temperature - enzymes do not become denatured at low temps
        • enzymes function best at an optimum temp
          • increase in temp above the optimum temp changes the shape of the protein molecule as in increase temp affects the bonds holding the protein structure together
            • if these bonds are broken the enzyme can no longer function as this changes the shape of the enzyme, the shape of the active site
              • enzyme is said to be denatured and no more enzyme-substrate complexes can form and as a result will reduce the rate of reaction
        • Denaturation definition:
          • the irreversible change to the shape of the active site that inhibits enzyme action
            • human body about 41 degrees
            • substrate no longer fits into the active site of the enzyme, no enzyme -substrate complexes form
    • questions
      • enzyme reactions are slow at low temps because there is less kinetic energy therefore fewer collisions between enzymes and their substrates
        • Optimum temp: the temp at which enzymes function most efficiently
          • salivary amylase stops working when food is mixed and enters the stomach because it is an enzyme and therefore a protein, and is denatured at a low pH
            • protease can breakdown protein but not starch as enzymes are specific to one substrate
    • Catalase in living tissue
      • breaks down hydrogen peroxide into water and oxygen
        • hydrogen peroxide is a harmful by-product of metabolism and has to be broken down within the cell before it causes damage
          • most living cells produce catalase with those cells which are more metabolically active and have a role in removing toxic by-products such as the liver, produce greater concs of the enzyme

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