Enzymes

• Enzymes


  • Catalysts that speed up the rate of a chemical reaction without being changed or used up in the reaction
  • Proteins
  • Biological catalysts (biological because they are made in living cells, catalysts because they speed up the rate of chemical reactions without being changed)
  • Necessary to all living organisms as they maintain reaction speeds of all metabolic reactions (all the reactions that keep an organism alive) at a rate that can sustain life
• If we did not produce digestive enzymes, it would take around 2 - 3 weeks to digest one meal; with enzymes, it takes around 4 hours
• Enzyme substrate specificity


  • Enzymes are specific to one particular substrate (molecule/s that get broken down or joined together in the reaction) as the enzyme is a complementary shape to the substrate
  • The product is made from the substrate(s) and is released
• Enzyme specificity: lock and key model of enzyme activity


  • The substrate moves into the enzyme's active site and they become known as the enzyme-substrate complex
  • After the reaction has occurred, the products leave the enzyme's active site as they no longer fit it and it is free to take up another substrate
• Investigating the effect of temperature on amylase

  1. Starch solution is heated to a set temperature

  2. Iodine is added to wells of a spotting tile

  3. Amylase is added to the starch solution and mixed well

  4. Every minute, droplets of solution are added to a new well of iodine solution

  5. This is continued until the iodine stops turning blue-black (this means there is no more starch left in the solution as the amylase has broken it all down)

  6. Time taken for the reaction to be completed is recorded

  7. Experiment is repeated at different temperatures
• The quicker the reaction is completed, the faster the enzyme is working
• Investigating the effect of pH on amylase

  1. Place single drops of iodine solution in rows on the tile

  2. Label a test tube with the pH to be tested

  3. Use the syringe to place 2cm³ of amylase in the test tube

  4. Add 1cm³ of buffer solution to the test tube using a syringe

  5. Use another test tube to add 2cm³ of starch solution to the amylase and buffer solution, start the stopwatch whilst mixing using a pipette

  6. After 10 seconds, use a pipette to place one drop of mixture on the first drop of iodine, which should turn blue-black

  7. Wait another 10 seconds and place another drop of mixture on the second drop of iodine

  8. Repeat every 10 seconds until iodine solution remains orange-brown

  9. Repeat experiment at different pH values - the less time the iodine solution takes to remain orange-brown, the quicker all the starch has been digested and so the better the enzyme works at that pH
• Describing and explaining experimental results for enzyme experiments is a common type of exam question so make sure you understand what is happening and, for a 7, 8 or 9, can relate this to changes in the active site of the enzyme when it has denatured, or if it is a low temperature, relate it to the amount of kinetic energy the molecules have.
• Enzyme specificity


  Enzymes are specific to one particular substrate(s) as the active site of the enzyme, where the substrate attaches, is a complementary shape to the substrate
• Enzymes


  • They are proteins and have a specific 3-D shape
• Lock and key hypothesis

  The enzyme's active site is a complementary shape to the substrate
• Enzyme-substrate complex formation


  1. Substrate moves into the enzyme's active site

  2. Reaction occurs
• After the reaction, the products leave the enzyme's active site as they no longer fit it and it is free to take up another substrate
• Enzymes are unchanged at the end of the reaction
• How enzymes work


  1. Enzymes and substrates randomly move about in solution

  2. Enzyme-substrate complex forms when enzyme and complementary substrate collide

  3. Product(s) form and are released, enzyme is unchanged
• Optimum temperature


  Temperature at which enzymes work fastest, 37°C for human body
• Denaturation


  Breaking of bonds that hold enzyme together, causing loss of shape
• Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop
• Increasing temperature from 0°C to optimum

  Increases enzyme activity as molecules have more energy and collisions with substrate increase
• Optimum pH


  Most enzymes have optimum pH of 7, some have lower (pH 2) or higher (pH 8-9) optimum depending on where they work in the body
• Effect of pH on enzymes

  pH too high or low can destroy bonds holding enzyme structure, changing active site shape so substrate no longer fits
• Moving too far from optimum pH will cause enzyme to denature and activity to stop