Enzymes

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  • Enzymes
    • Enzymes are globular proteins with a tertiary structure folded into a spherical shape
      • Enzymes are held together by ionic bonds, hydrogen bonds and disulphide bridge
      • The active site is a small area with a 3D shape
        • The enzyme acts on the substrate by making temporary bonds on the active site
      • Enzymes have a hydrophilic R group which makes enzymes soluble
        • Each enzyme has a unique sequence of amino acids
        • The elements in the R group determine the bonds between the amino acids
    • Site of enzyme action
      • Some enzymes are extracellular- they are secreted from cells by exocytosis and catlyse  extracellular reaction
      • Intracellular enzymes in solution, these are enzymes within the cell in a solution
        • Intracellular enzymes in solution breakdown glucose in the cytoplasm
      • Membrane bound extracellular enzymes, these are enzymes attached to membranes e.g on the cristae of the mitochondria
    • How they work
      • Lock and key model
        • The unique shape of the active site means enzymes catalyse only one type of reaction, it is enzyme specific
      • Induced fit model
        • The enzyme shape is altered by the binding of the substrate
          • This suggests the enzyme is flexible and not rigid
    • Activation energy
      • The minimum amount of energy needed for molecules to react
      • Increasing kinetic energy by increasing the temperature increases the number of successful collisions
        • But if the heat is too high (usually above 40 degrees Celsius) the enzyme denatures
          • As the kinetic energy increases the bonds vibrate, the bonds vibrate so much that they eventually break. The enzyme changes shape and is denatured
    • The rate of reaction of an enzyme controlled reaction can be measured by the amount of product formed or substrate used up in a given time
      • Initially there are many substrate molecules but the concentration decreases as the reaction progresses and the form products
      • Factors affecting rate of reaction
        • Temperature
          • Increasing kinetic energy by increasing the temperature increases the number of successful collisions
            • But if the heat is too high (usually above 40 degrees Celsius) the enzyme denatures
              • As the kinetic energy increases the bonds vibrate, the bonds vibrate so much that they eventually break. The enzyme changes shape and is denatured
        • pH
          • Enzymes will have an optimum pH- the pH which enzymes work best at
          • Small changes in pH cause reversible changes to the enzyme and therefore reduce its activity
          • Extreme changes in pH cause the enzyme to become denatured
            • There are charges on the amino acid chain which are affected by the H+ and OH- ions
              • At high pH the excess OH- ions neutralise the H+ ions and therefore disrupt the hydrogen bonds within the enzyme and cause it to become denatured
        • Substrate concentration
          • When the enzyme concentration is constant the rate of reaction increases as substrate concentration increases
          • When there is a low concentration of substrates enzymes only have a few substrates to collide with, they are not working to their full potential. Substrate concentration is the limiting factor
          • When all active sites are filled the enzyme is saturated and cannot go any faster
        • Enzyme concentration
          • when the product is released from the enzyme it can be reused
          • Only a low concentration of enzymes are needed to catalyse a large number of reactants
          • Turn over is the number of substrate and enzyme can turn into productsin a given time
          • As enzyme concentration increases
    • Enzyme inhibition
      • Decreases the rate of reaction of an enzyme contolled reaction
      • competitive inhibition
        • The competitive inhibitor will have a complementary shape to the active site
          • The substrate and inhibitor compete for the active site
        • Increasing substrate concentration decreasesthe affect of competitive inhibitors
      • Non-competitive inhibiton
        • They bind to a site other than the active site
          • They affect bonds within the enzyme and alter its overall shape so the substrate can no longer bind
        • They don't compete with the substrate
    • Immobilised enzymes
      • Advantages of immobilised enzymes
        • Increased stability as they are able to function over a wider range of temperatures and pH
        • Products are not contaminated wtih enzymes
        • Easily recovered for reuse
        • Enzymes are easily added or removed
        • Can have a sequence of columns at different pH and temperatures
      • Enzymes which are fixed or bound to an inert matrix
      • Uses
        • lactose free milk
          • milk is poured down column containing lactase
            • milk binds to the enzymes active site and is hydolysesd into glucose and galactose
    • Biosensorors
      • Turns a chemical signal into an electrical signal
      • Rapidly and accurately detects concentrations of molecules e.g blood glucose
        • Glucose oxidase, immobilised on a selectively permeable membrane is placed in a blood sample and binds to the glucose and produces a small electrical crrent
      • Enzymes are specific and select one type of molecule

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