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Peroxisomes

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  1. Peroxisomes Aimee Terauchi and Valerie Villareal

  2. History of Peroxisomes • First observed by electron microscopy in animal cells (1950s), then in plant cells (1960s) • Christian deDuve (1965) • Isolated from liver cells by centrifugation • Called them peroxisomes because they generate and destroy H2O2

  3. The Peroxisome • Single membrane • Roughly spherical • 0.2 - 1.7m • Composition varies

  4. Number and Size of Peroxisomes Vary Depending on Environment Glucose limited More glucose limited Methanol limited Hansenula polymorpha cells

  5. Protein Import • C-terminal signal sequence: SKL • N-terminal signal sequence: RLX5HL • Proteins involved in import: peroxins • Import driven by ATP hydrolysis • Don’t have to be unfolded for import

  6. Two Models for Peroxisome Biogenesis

  7. Metabolic Functions of Peroxisomes Yeasts Biosynthesis: lysine Degradation: amino acids, methanol, -oxidation of fatty acids, decomposition of hydrogen peroxide, glyoxylate cycle Fungi Biosynthesis: penicillin Degradation:  -oxidation of fatty acids, decomposition of hydrogen peroxide, glyoxylate cycle Plants Degradation: purines, some reactions of photorespiration (the conversion of glycolate to glycine and of serine to glycerate), -oxidation of fatty acids, decomposition of hydrogen peroxide, glyoxylate cycle Mammals Biosynthesis: ether phospholipids (plasmalogens), cholesterol and bile acids, polyunsaturated fatty acids Degradation: amino acids, purines, prostaglandin, polyamines, -oxidation of fatty acids, -oxidation of fatty acids, decomposition of hydrogen peroxide Humans Biosynthesis: ether phospholipids (plasmalogens), cholesterol and bile acids, polyunsaturated fatty acids Degradation: amino acids, purines, -oxidation of fatty acids, -oxidation of fatty acids, decomposition of hydrogen peroxide

  8. Major Metabolic Functions of the Peroxisome in Plants • -oxidation of fatty acids • Glyoxylate cycle • Photorespiration (Glycolate pathway) • Degradation of purines • Decomposition of hydrogen peroxide

  9. Two Types of Peroxisomes in Plants • Leaves • Catalyzes oxidation of side product of CO2 fixation in photorespiration • Germinating seeds • Converts fatty acid in seed lipids into sugars needed for growth in the young plant

  10. Glyoxysomes

  11. -oxidation occurs in mitochondria and peroxisomes in mammals, but exclusively in the peroxisome in plants and yeast.

  12. Glyoxysomes and Leaf Peroxisomes are Interconverted During Development • Immunogold particles of 2 sizes bound to: • Enzymes of glyoxylate cycle • Peroxisomal enzymes • The same population of peroxisomes assumes different metabolic roles depending on developmental stage of cotelydon Greening cotelydons

  13. Photorespiration and Glycolate • Oxygenase activity of rubisco • Consumption of O2 • Glycolate cycle • Production of CO2 • Involves 3 organelles (chloroplasts, peroxisomes, & mitochondria)

  14. The Glycolate Cycle Glycolic acid oxidase H2O2 production

  15. The Glycolate Cycle

  16. Purine Degradation • Nucleic acid purine moieties (adenine and guanine) are degraded to uric acid xanthine uric acid allantoin O2 H2O2 O2 H2O2 Xanthine oxidase Urate oxidase Del Rio et al., J. Exper. Botany 2002

  17. Urate Oxidase • High urate oxidase concentrations contribute to formation of crystalline inclusions • All purine degradation leads to uric acid

  18. Oxidases • The oxidases use molecular oxygen to remove hydrogen atoms from specific organic substrates • A variety of compounds, including L-amino acids, D-amino acids, polyamines, methanol, urate, xanthine, and very-long-chain fatty acids, serve as substrates for the different oxidases

  19. Peroxide Detoxification Oxidases use O2 to oxidize organic substances and produce hydrogen peroxide (H2O2) -- e.g., H2O2 generated by glycolate oxidase reaction, -oxidation of fatty acids Peroxisomes also contain catalase, the enzyme that degrades H2O2.

  20. Importance of H2O2 degradation • 2H2O2 2H2O + O2 • Peroxisomes contain a high concentration of catalase, a heme protein • Other reactive oxygen species (ROS) are formed in peroxisomes catalase H - - O - - O - -H HO- -OH (?)

  21. Reactive Oxygen Species • Cause damage to lipids, proteins, DNA • Amount ROS is reduced by catalase, and superoxide dismutase (SOD) 2O2- O2 + H2O2 • • • • Superoxide anion (radical) Hydrogen peroxide Hydroxyl radical

  22. Radical Chemistry Initiation: RH + O2 -->R· + ·OH Propagation:  R· + O2 --> · + ROO· ROO· + RH --> R· + ROOH ROOH--> RO· + HO· Termination:  R· + R· --> RR R· + ROO·--> ROOR ROO· + ROO· --> ROOR + O2

  23. Other Peroxisomal Enzymes

  24. Conclusions • Compartmentalize! To protect the cell from these destructive byproducts, such reactions are segregated.   

  25. Peroxisomal Diseases Adrenoleukodystrophy: Deficiency in -oxidation of very long- chain fatty acids Zellweger syndrome: Defect in protein import, giving rise to “ghost peroxisomes”