5. 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 active site of the enzyme, where the substrate attaches, is a complementary shape to the substrate
• Investigating the effect of temperature on amylase

  1. Heat starch solution to a set temperature

  2. Add iodine to wells of a spotting tile

  3. Add amylase to the starch solution and mix well

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

  5. Record the time taken for the reaction to be completed

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


  The faster the enzyme is working
• Investigating the effect of pH on amylase

  1. Place drops of iodine solution in rows on a tile

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

  3. Add amylase and buffer solution to the test tube

  4. Add starch solution and start the stopwatch

  5. Every 10 seconds, place a drop of the mixture on the iodine

  6. Repeat 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
• 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
  • 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 work fastest at their 'optimum temperature' – in the human body, the optimum temperature is 37⁰C
  • 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 0⁰C 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