BCHS-4361-09-PEROXIS_39258

BCHS-4361-09-PEROXIS_39258 - PEROXISOMAL DISEASES The...

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PEROXISOMAL DISEASES The peroxisome is a cytoplasmic organelle present in almost all eukaryotic cells (they are termed glyoxysomes in plants). They are round or oval-shaped structures bounded by a single bilayer membrane. Human hepatocytes and renal tubule cells have numerous large (0.5-1.5 mm) peroxisomes. In other tissues, they are smaller and are sometimes called microbodies. Catalase is a marker enzyme for peroxisomes. This enzyme removes accumulated H 2 O 2 : 2H 2 O 2 2H 2 0 + O 2 There are ca. 50 biochemical reactions that take place in or involve peroxisomes. Several inherited involve peroxisome malfunction. We will discuss Refsum’s disease, Zellweger’s syndrome, and X-linked adrenoleukodystrophy.
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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 H 2 O 2
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What are peroxisomes? • Single-membraned organelles in eukaryotic cells • Lots in liver and kidney (for detoxification) • Contain various enzymes (catalase and various oxidases) • Enzymes are transported into peroxisomes from the cytosol Rat liver peroxisome; internal dark areas are urate oxidase crystals
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The Peroxisome • Single membrane • Roughly spherical – 0.2 - 1.7 μ m • Composition varies
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Peroxisomal Variability Extreme ranges Human hepatocytes Shape Spherical, ovoid, tubular, Ovoid square, triangular Internal Amorphous, Amorphous Structure paracrystalline Size 0.1-2.0 microns 0.5 microns No. per cell 1-1000s 100s % volume of cell 0.1-80% 1%
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PROTEINS THAT ARE TARGETED TO PEROXISOMES Peroxisomal oxidases that bypass mitochondrial ATP generation α -hydroxyacid Oxidase Glycolate Oxidase D-amino acid oxidase Xanthine Oxidase Urate Oxidase NADH Oxidase Acyl-CoA Oxidase The peroxisomal oxidases generate reactive oxygen species, e.g. Acyl-CoA + O 2 ----> Enoyl-CoA + H 2 O 2 Xanthine + O 2 ----> Urate + O 2 Catalase scavenges Hydrogen Peroxide 2 H 2 O 2 ----> H 2 O + O 2 Ascorbate Peroxidase is also in membrane of plant peroxisomes
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I. PEROXISOME METABOLISM Among metabolic activities are: Oxidation of very long chain fatty acids (VLCFA) Oxidation of phytanic acid Biosynthesis of ether bond-containing phospholipids (plasmalogens). Most of these processes involve coordinated interactions between the peroxisomes and other organelles. Thus the decomposition of the VCLFAs and phytanic acid is a process shared by the peroxisomes and the mitochondria. The final steps in the synthesis of the plasmologens occur in the endoplasmic reticulum, but the process depends on precursors which are synthesized in the peroxisomes.
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1. Lipid -oxidation (VLCFA only in mammals) ( acyl-CoA-oxidase, bi/tri-functional enzyme, acyl-CoA-thiolase ) 2. Steroid side chain oxidation / bile acid formation 3. Dolichol and cholesterol synthesis 4. Plasmalogen (ether lipid) synthesis
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BCHS-4361-09-PEROXIS_39258 - PEROXISOMAL DISEASES The...

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