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
• Lock and key model of enzyme activity

  • The product is made from the substrate(s) and is released
• 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. Time taken for the reaction to be completed is recorded

  6. Experiment is repeated at different temperatures
• The quicker the reaction is completed


  The faster the enzyme is working
• Investigating the effect of temperature on enzyme activity
• 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 action and 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
  • This is because the enzyme is a protein and has a specific 3-D shape
  • This is known as the lock and key hypothesis
  • When the substrate moves into the enzyme's active site 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
• How enzymes work


  1. Enzymes and substrates randomly move about in solution

  2. When an enzyme and its complementary substrate randomly collide - with the substrate fitting into the active site of the enzyme - an enzyme-substrate complex forms, and the reaction occurs.

  3. A product (or products) forms from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions.
• Enzymes and temperature


  • Enzymes are proteins and have a specific shape, held in place by bonds
  • This is extremely important around the active site area as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed
  • Enzymes work fastest at their 'optimum temperature' – in the human body, the optimum temperature is 37°C
  • Heating to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and it will lose its shape -this is known as denaturation
  • Substrates cannot fit into denatured enzymes as the shape of their active site has been lost
  • Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop
• Increasing the temperature from 0°C to the optimum

  Increases the activity of enzymes as the more energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction
• Low temperatures do not denature enzymes, they just make them work more slowly
• Enzymes and pH


  • The optimum pH for most enzymes is 7 but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
  • If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be destroyed
  • This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity
  • Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop
• Effect of pH on enzyme activity
• Graph showing the effect of pH on rate of activity for an enzyme from the duodenum