The Action of Enzymes (Edexcel GCSE Biology: Combined Science)

Enzymes

• Enzymes are proteins that act as biological catalysts to speed up the rate of a chemical reaction without being changed or used up in the reaction
• They are biological because they are made in living cells
• Enzymes are necessary to all living organisms as they allow all metabolic reactions to occur at a rate that can sustain life
  • For example, 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

The mechanism of enzyme action

• 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, the enzyme-substrate complex is formed
• After the reaction has occurred, the products leave the enzyme’s active site, which is then free to take up another substrate
• The steps of an enzyme catalysed reaction are shown in the diagram below and can be summarised as follows:
  • Step One: Enzymes and substrates randomly move about in solution
  • Step Two: When an enzyme and its complementary substrate randomly collide, an enzyme-substrate complex forms and the reaction occurs
  • Step Three: A product (or products) forms (from the substrate) and is then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions

How enzymes work

Denaturation of enzymes

• Enzymes are proteins and have a specific shape, held in place by bonds
• This is extremely important around the active site, as the specific shape of this area of the enzyme is what ensures the substrate will fit into the active site and enable the reaction to proceed
• If the bonds that hold the enzyme together are disrupted or broken the active site it will lose its shape - this is known as denaturation
  • The enzyme is said to be denatured
  • 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 the reaction they are catalysing will stop
  • Denaturation can occur due to high temperatures or extremes of pH