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
• Lock and key model of enzyme activity

  • Enzyme has a specific 3-D shape
  • Active site of the enzyme is a complementary shape to the substrate
• Enzyme action


  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

  4. The enzyme is unchanged and will go on to catalyse further reactions
• 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
• Effect of temperature on enzyme activity

  • Enzymes work fastest at their 'optimum temperature'
  • 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
  • Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop
• Effect of pH on enzyme activity

  • 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