Edexcel A (SNAB) A Level Biology:复习笔记2.3.8 The Role of 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 undergoing permanent change
  • They speed up reactions by reducing the activation energy of reactions

• Enzymes are globular proteins with complex tertiary structures
  • Some are formed from a single polypeptide, whilst others are made up of two or more polypeptides and therefore have a quaternary structure
• 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
• All enzymes are proteins that are produced via the process of protein synthesis inside cells
• Some enzymes remain inside cells, whilst others are secreted to work outside of cells
• Enzymes can be intracellular or extracellular referring to whether they are active inside or outside the cell respectively
  • Intracellular enzymes are produced and function inside the cell
  • Extracellular enzymes are secreted by cells and catalyse reactions outside cells (eg. digestive enzymes in the gut)

Intracellular and Extracellular Enzymes Table

Enzymes and the lowering of activation energy

• 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 reduce the stability of bonds in the reactants
  • The destabilisation of bonds in the substrate makes it more reactive
• Rather than lowering the overall energy change of the reaction, enzymes work by providing an alternative energy pathway with a lower activation energy
• Without enzymes, extremely high temperatures or pressures would be needed to reach the activation energy for many biological reactions
  • Enzymes avoid the need for these extreme conditions (that would otherwise kill cells)

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