Enzyme Action (Cambridge (CIE) AS Biology): Revision Note

Mode of enzyme action

• 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 enzyme's active site and this must happen at the correct orientation and speed in order for a reaction to occur

• 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 and therefore the enzyme's shape and active site will change

• 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

• Enzyme reactions can either be catabolic or anabolic

• Catabolic reactions involve the breakdown of complex molecules into simpler products

  • Thishappens when a single substrate is drawn into the active site and broken apart into two or more distinct molecules

• Examples of catabolic reactions include cellular respiration and hydrolysis reactions

• Anabolic reactions involve the building of more complex molecules from simpler ones by:

  • Drawing two or more substrates into the active site

  • Forming bonds between them

  • Releasing a single product

• Examples of anabolic reactions include protein synthesis and photosynthesis

Enzymes work by lowering the 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 just 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 and in doing so they provide an alternative energy pathway