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Episode 9 - Peroxisomes
Peroxisomes are membrane-bound sacs that store enzymes that oxidize other compounds (like catalase and uric acid oxidase) into hydrogen peroxide. In short, peroxisomes are a type of vesicle that functions in intracellular digestion. They are composed of a one-layer thick phospholipid membrane, and a crystalline core.
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Peroxisome enzymes break down fatty acids, amino acids, and other inorganic substances in the cell into hydrogen peroxide. The H2O2, which is still dangerous, is then converted into water and oxygen by another peroxisome enzyme. One of the main functions of peroxisomes: breakdown of fatty acids via Beta-Oxidation into two-carbon compounds, then converting them into Acetyl Co-A for use in the Kreb’s cycle.
Liver cells are rich in peroxisomes to break down toxins like alcohol. Peroxisomes contain alcohol dehydrogenases that oxidize ethanol into the less toxic ethanal. Special peroxisomes, called glyoxisomes, are found in the fat-storing tissues of plant seeds. Upon release of gibberellins during germination, glyoxisomes digest fatty acids into sugars that feed the seed until it can photosynthesize for itself.
Enzymes of peroxisomes that are synthesized in the cytosol travel into the peroxisome via a signal amino acid sequence of -Ser-Lys-Leu near the –COOH terminal of the protein and a receptor for this sequence in the peroxisome. Like the lysosome, peroxisomes, too, travels through the cytoplasm and fuses with transport vesicles to break down substances brought into the cell by endocytosis. But unlike lysosomes, peroxisomes are not produced by the secretory pathway (via the Golgi), but instead grow larger by using proteins made in the cytosol, lipids made in the smooth ER and in the peroxisomes themselves. They can divide by themselves, like mitochondria and chloroplasts, but do not contain their own DNA - so we tend to class them as "possibly semi-autonomous". They divide before the mitosis phase, during G2.
Transmission electron microscopy (TEM) micrograph of a peroxisome from the marine snail Gibulla umbilicalis. (Source: Lobo-da-Cunha A, 2001)
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