AQA A Level Chemistry复习笔记7.7.2 Proteins



  • 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



The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)


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


Drug-receptor Interactions

  • Receptors are proteins found on enzymes, cell membranes or DNA
  • Most drugs work by their ability to bind to receptors stopping their normal biological activity and interrupting the development of disease
  • Drugs bind to receptors generally using intermolecular forces or ionic bonds
  • The stronger the interaction the more effective the drug activity
  • Drug-receptor interaction has become very important in drug design
  • Computers are widely used to model drug-receptor interactions



Only the correct orientation of one enantiomer enable the drug to bind to the biomolecule making it stereoselective

  • Many naturally occurring organic molecules consist of enantiomers, in which only one enantiomer is biologically active
  • Similarly, many drugs have enantiomeric forms in which only one form of the drug is active
  • The site where the drug binds to the biomolecule can only accept one orientation; it is said to be stereoselective