Enzymes & Metabolism (AQA GCSE Biology): Revision Note

Enzymes & metabolism

• Enzymes are biological catalysts made from protein that speed up chemical reactions in cells without being used up

• Without enzymes the metabolic reactions that occur at body temperature would take place too slowly

  • Examples of essential metabolic reactions that are controlled by enzymes include:

    • breaking down carbohydrates, lipids and proteins in digestion

    • respiration

    • production of new proteins

Enzymes: How they work

• The lock and key theory is one simplified model that is used to explain enzyme action

• In this model the substrate, or reactant, fits into the enzyme's active site like a key into a lock

• An enzyme-controlled chemical reaction occurs as follows

  1. Enzymes and substrates move about randomly in solution

  2. When an enzyme and its complementary substrate collide, the substrate attaches to the active site of the enzyme

  3. After the substrate has attached to the active site, the chemical reaction occurs

  4. A product forms from the substrate, which is then released from the active site; the enzyme is unchanged and will go on to catalyse further reactions

Use this image

Enzyme specificity

• Each type of enzyme catalyses a specific process in living organisms

• This is because the shape of an enzyme's active site is complementary to its substrate

  • Proteins are formed from chains of amino acids held together by bonds

  • The order of amino acids determines the shape of an enzyme's active site

Use this image

Effect of temperature and pH on enzyme activity

The effect of temperature

• Enzymes work fastest at their optimum temperature – in the human body, the optimum temperature is around 37 °C

• Heating to high temperatures (beyond the optimum) will start to break the bonds that hold the enzyme together – the enzyme's active site will lose its shape, reducing the effectiveness of substrate binding

• Eventually the shape of the active site is lost completely and the enzyme is described as having been denatured

  • Substrates cannot fit into denatured enzymes as the specific shape of their active site has been lost

Use this image

• Increasing temperature from 0 °C to the optimum increases the activity of enzymes; this is because the more energy the molecules have, the faster they move and the more collisions there are between enzymes and substrate molecules

• Low temperatures do not denature enzymes, but at lower temperatures there is less kinetic energy available, so enzymes and their substrates move more slowly and collide at a lower rate

Use this image

The effect of pH

• Enzymes activity is highest when the enzymes are at their optimum pH

  • Many human enzymes have an optimum pH of 7

  • Stomach enzymes have an optimum pH of 2

  • Small intestine enzymes have a higher optimum pH of 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 break

• This changes the shape of the active site, so the substrate can no longer fit into it

  • The enzymes and substrate are no longer complementary

  • The enzyme has denatured

Use this image

• As conditions deviate further from an enzyme's optimum pH, enzyme activity gradually slows

• At extreme pH levels there will be no enzyme activity

Use this image