AQA A Level Chemistry复习笔记7.7.3 Enzymes

• Enzymes are biological catalysts
  • ‘Biological’ because they function in living systems
  • ‘Catalysts’ because they speed up the rate of chemical reactions without being used up or changed

   

• Enzymes are also globular proteins
• Critical to the enzyme’s function is the active site where the substrate binds
• Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions
  • Virtually every metabolic reaction within living organisms is catalysed by an enzyme – enzymes are therefore essential for life to exist

   

• Enzymes have an active site where specific substrates bind forming an enzyme-substrate complex
• 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
• Substrates collide with the enzymes active site and this must happen at the correct orientation and speed in order for a reaction to occur

Enzyme Specificity

• 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 (and therefore the specificity of the enzyme) 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

   

An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not