PEROXISOMES

• Peroxisomes


  Simple membrane bound vesicles with a diameter of 0.1 to 1.0 micrometers, which contains a dense, crystalline core of oxidative enzymes

  
• Microbody


  Another name for peroxisomes

  
• Cytosome


  Another name for peroxisomes

  
• Peroxisomes are multifunctional organelles containing more than 50 enzymes involved in diverse activities
• Peroxisomes


  The site of synthesis and degradation of hydrogen peroxide, a highly reactive and toxic oxidizing agent
• Peroxisomes are not derived from the endoplasmic reticulum and therefore are not part of the endomembrane system
• Locations of peroxisomes


  • Eukaryotic cells
  • Mammalian liver and kidney cells
  • Algae
  • Photosynthetic cells of plants
  • Germinating seedlings of plant species that store fats in their seeds
• The role of peroxisomes are not as well understood in animals as they are in plant cells
• Important enzymes present in peroxisomes


  • Catalase
  • D-amino acid oxidase
  • Urate oxidase
  • Glycolate oxidase
  • Luciferase
• Regardless of where they occur, peroxisomes are characterized by the presence of catalase, an enzyme that plays a vital role in the breakdown of hydrogen peroxide
• Hydrogen peroxide metabolism in peroxisomes


  1. Oxidases transfer electrons from substrates to oxygen, forming hydrogen peroxide

  2. Catalase breaks down hydrogen peroxide in two ways: catalytic mode and peroxidative mode
• Catalytic mode of catalase


  One molecule of H2O2 is oxidized to O2 and the second one is reduced to water
• Peroxidative mode of catalase


  Hydrogen peroxide is reduced to water using electrons derived from an organic donor
• Catalase is the most abundant protein in peroxisomes, representing up to 15% of the protein content
• Detoxification of toxic compounds by peroxisomes


  • Catalase can use a variety of substances including methanol, ethanol, formic acid, formaldehyde, nitrites and phenols as electron donors, detoxifying these harmful compounds
• Oxidation of fatty acids in peroxisomes


  • 25-50% of fatty acid oxidation in animal tissues occurs in peroxisomes, the rest in mitochondria
  • Peroxisomal beta-oxidation is especially important for long chain fatty acids
• Metabolism of nitrogen-containing compounds in peroxisomes


  • Peroxisomes contain urate oxidase (uricase) which oxidizes urate to allantoin, releasing hydrogen peroxide which is then broken down by catalase
• Breakdown of unusual substances in peroxisomes


  • Peroxisomes can metabolize D-amino acids and alkanes which are not recognized by other degradative pathways
• Leaf peroxisomes in plants


  • Closely associated with chloroplasts and mitochondria, involved in photorespiration
  • Key enzymes of photorespiration are localized to leaf peroxisomes
• Glyoxysomes


  A functionally distinct type of plant peroxisomes that occur transiently in seedlings, involved in the metabolism of stored fats
• Peroxisome biogenesis


  1. Occurs by division of pre-existing peroxisomes

  2. Proteins destined for peroxisomes are synthesized on cytosolic ribosomes and imported post-translationally into pre-existing peroxisomes