1.4.2 Enzyme Specificity

• 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)

• 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

• 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

The temporary formation of an enzyme-substrate complex

• Enzyme reactions can either be catabolic or anabolic
• Catabolic reactions involve the breakdown of complex molecules into simpler products, which happens 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

A catabolic reaction

• 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 and releasing a single product
• Examples of anabolic reactions include protein synthesis and photosynthesis

An anabolic reaction

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

The activation energy of a chemical reaction is lowered by the presence of a catalyst (ie. an enzyme)