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Peroxisomes. Aimee Terauchi and Valerie Villareal. 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

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Peroxisomes

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Peroxisomes l.jpg

Peroxisomes

Aimee Terauchi and Valerie Villareal


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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


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The Peroxisome

  • Single membrane

  • Roughly spherical

    • 0.2 - 1.7m

  • Composition varies


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Number and Size of Peroxisomes Vary Depending on Environment

Glucose limited

More glucose limited

Methanol limited

Hansenula polymorpha cells


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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


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Two Models for Peroxisome Biogenesis


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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


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Major Metabolic Functions of the Peroxisome in Plants

  • -oxidation of fatty acids

  • Glyoxylate cycle

  • Photorespiration (Glycolate pathway)

  • Degradation of purines

  • Decomposition of hydrogen peroxide


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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


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Glyoxysomes


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


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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


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Photorespiration and Glycolate

  • Oxygenase activity of rubisco

    • Consumption of O2

  • Glycolate cycle

    • Production of CO2

    • Involves 3 organelles (chloroplasts, peroxisomes, & mitochondria)


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The Glycolate Cycle

Glycolic acid

oxidase

H2O2 production


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The Glycolate Cycle


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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


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Urate Oxidase

  • High urate oxidase concentrations contribute to formation of crystalline inclusions

  • All purine degradation leads to uric acid


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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


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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.


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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 (?)


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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


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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


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Other Peroxisomal Enzymes


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Conclusions

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


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Peroxisomal Diseases

Adrenoleukodystrophy: Deficiency in -oxidation of very long-

chain fatty acids

Zellweger syndrome: Defect in protein import, giving rise to “ghost peroxisomes”


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