5.2 Enzyme

• Nucleus


  Contains DNA which carries the information for the synthesis of proteins
• Protein synthesis and transport

  1. Proteins synthesised at ribosomes

  2. Transported through space within rough endoplasmic reticulum

  3. Proteins leave rough endoplasmic reticulum bound in transport vesicles

  4. Transport vesicles move towards Golgi apparatus

  5. Transport vesicles fuse with Golgi apparatus membranes and empty contents into membranous space
• Protein modification in Golgi apparatus


  Proteins further modified into enzymes
• Enzyme secretion

  1. Secretory vesicles form at tip of Golgi apparatus

  2. Secretory vesicles fuse with plasma membrane

  3. Extracellular enzymes secreted outside the cell
• Enzyme - A metabolic pathway begins with a specific molecule of a substance, which is then altered in a series of steps or reactions, resulting in a certain product.
• 1. Each step of the pathway or metabolic reaction is catalysed by an enzyme
  2. An enzyme is an organic catalyst that acts as a biochemical agent which speeds up a reaction without being consumed
  3. Enzymes are mostly proteins and synthesised by the cells of living organisms
• Enzyme-substrate


  To form an enzyme-substrate
• Enzyme nomenclature


  Naming system for enzymes
• The International Union of Biochemistry and Molecular Biology (IUBMB) introduced the enzyme nomenclature in the 1960s
• Enzyme naming

  The name of an enzyme is derived from the name of the substrate it catalyses
• Enzymes


  • Highly specific in their action
  • Names of most enzymes are derived by adding the suffix -ase to the name of the substrates they hydrolyse
• Enzyme names

  • Lactase
  • Sucrase
  • Lipase
• Enzyme-catalysed reaction

  1. Enzyme name written above reaction arrow

  2. Lactase hydrolyses the breakdown of lactose to glucose and galactose
• There are a few enzymes that were named before a systematic way of naming enzymes was formulated, e.g. pepsin, trypsin and rennin
• Enzymes

  • Act rapidly to speed up biochemical reactions
  • Needed in small quantities and are reusable
  • Highly specific - active site binds to specific substrate(s)
  • Not affected by the reactions they catalyse
  • Most reactions catalysed are reversible
  • Activity can be slowed down or stopped by inhibitors
  • Require non-protein helpers called cofactors for catalytic activity
• Inorganic cofactors


  • Iron
  • Copper
• Organic cofactors/coenzymes


  • Water-soluble vitamins such as vitamin B complex
• Without regulation by enzymes, the metabolic pathways would be very congested because many biological reactions would take such a long time to complete
• Intracellular enzymes

  Synthesised in a cell and act within the cell, e.g. hexokinase which catalyses glycolysis
• Extracellular enzymes


  Produced in the cell, then packed and secreted from the cell to catalyse reactions outside the cell, e.g. trypsin produced by pancreatic cells and secreted into the duodenum to break down polypeptides