IGCSE Biology 0610

5.1 Enzymes

• Enzymes are proteins that are involved in all metabolic reactions, where they function as biological catalysts
• A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction
• Enzymes are important in all living organisms as they maintain reaction rate necessary to sustain life

Enzyme activity can be measured by measuring how much product is formed or how much substrate is used over a period of time

Effect of Temperature on Enzymes

• The temperature when the maximum rate of reaction occurs is called optimum temperature
  Example: Human enzymes optimum temperature – 37°C
• Increasing the temperature increases the rate of reaction to the optimum temperature. This is because the enzyme and substrate molecules have greater kinetic energy. The frequency of effective collisions between them increases.
• If temperature continues to increase:
  Enzymes are folded into a specific shape. This shape is maintained by bonds. At higher temperatures, the bonds holding the enzyme molecule together break down. The shape of the active site changes and the substrate will no longer fit. The enzyme has been denatured and will no longer catalyse the reaction.

Effect of pH on Enzymes

• The pH where the maximum rate of reaction occurs is called optimum pH
• Most enzymes work best with neutral pH (7). However, some enzymes like stomach enzymes have acidic optimum pH (<7).
• If pH is too high or low (i.e. too different from the optimum pH), the bonds that hold the enzyme structure break. The shape of active site changes and the substrate will no longer fit. The enzyme has been denatured and will no longer catalyse the reaction.