The Role of Enzymes (OCR A Level Biology): 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

• 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

Site of action of enzymes

• 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 therefore 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