Enzyme Specificity (AQA AS Biology): Revision Note

Mode of enzyme action

• Enzymes have an active site where specific substrates bind, forming an enzyme-substrate complex

  • Substrates collide with the enzyme's active site, and this must happen at the correct orientation and speed for a reaction to occur

Specificity

• The active site of an enzyme has a specific shape to fit a specific substrate

  • Extremes of heat or pH can change the shape of the active site, preventing substrate binding – this is called denaturation

• The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)

• The shape of the active site is determined by the complex tertiary structure of the protein that makes up the enzyme:

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

  • The order of amino acids determines the shape of an enzyme

  • If the order is altered, the resulting three-dimensional shape changes

Enzyme-substrate complex

• An enzyme-substrate complex forms when an enzyme and its substrate join together

• The enzyme-substrate complex is only formed temporarily, before the enzyme catalyses the reaction and the product(s) are released

  • This way, enzymes are free to be recycled for future reactions

Activation energy of a reaction

• All chemical reactions are associated with energy changes

• For a reaction to proceed, there must be enough activation energy

• Activation energy is the amount of energy needed by the substrate to become unstable enough for a reaction to occur and for products to be formed

  • Enzymes speed up chemical reactions because they influence the stability of bonds in the reactants

  • The destabilisation of bonds in the substrate makes it more reactive

• Enzymes work by lowering the activation energy of a reaction by providing an alternative energy pathway