The Role of Enzymes (Edexcel A Level Biology (A) SNAB): Revision Note

Enzymes as Catalysts

• 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)