peroxisome, single
membrane-bound organelle occurring in the cytoplasm of eukaryotic cells.
Peroxisomes play a key role in the oxidation of specific biomolecules, long and
branched fatty acid chains, D-amino acids, and polyamines. They also contribute
to the biosynthesis of membrane lipids known as plasmalogens and
etherphospholipids that are necessary for proper brain and lung function.
Peroxisomes resemble organelles found in other organisms as they are related to
glyoxysomes of plants fungi and also glycosomes of
kinetoplastids. Peroxisomes contain oxidative enzymes, such as catalase,This
reaction is important in liver and kidney cells, where the peroxisomes detoxify
various toxic substances that enter the blood.D-amino acid oxidase, Urate
oxidase is also present along with catalase.
Urate
oxidase is
mainly localised in the liver, where it forms a large electron-dense
paracrystalline core in many peroxisomes.In purine degradation pathway and
catalyzes uric acid into the 5-hydroxyisourat to
allantoin.Excessive concentration of uric acid in the blood stream leads to
gout.Urate oxidase is found in nearly all organisms, from bacteria to mammals.
A major function of the peroxisome is the breakdown
of fatty acid molecules, in a process called beta-oxidation. In this process,
the fatty acids are broken down two carbons at a time, converted to Acetyl-CoA,
which is then transported back to the cytosol for further use. In animal cells,
beta-oxidation can also occur in the mitochondria.
in peroxisomes is very long chain fatty acids, longer than those that
mitochondria can efficiently oxidize. In yeast and plant cells, this
process is exclusive for the peroxisome. oxidation in peroxisomes is not
coupled to ATP synthesis. Instead, the high-potential electrons are transferred
to O2, which yields H2O2. It does generate
heat however. The enzyme catalase, found exclusively in peroxisomes, converts
the hydrogen peroxide into water and oxygen.
D-amino acid oxidase digest D amino acids,D-amino-acid oxidase is mainly localized in the
kidney, liver and brain.The enzyme is present in peroxisomes of cells of the
proximal tubules of the kidney ,peroxisomes of hepatocytes of the liver and
in the microperoxisomes of cerebellum.
D-amino acids oxidases to imino acids ,producing
ammonia and hydrogen peroxide
Those oxidation reactions produce hydrogen peroxide, hydrogen peroxide is potentially
toxic to the cell, because it has the ability to react with many other
molecules. Therefore, peroxisomes also contain enzymes such as catalase
that convert hydrogen peroxide to water and oxygen.
Plasmalogen synthesis:Fatty acids enter in to peroxisomes , peroxisomes matrix enzymes
glycerone phosphate acyl transferase and alkyl-glycerone phosphate synthase
on the luminal side of the peroxisomal membrane,which one of the hydrocarbon
chains is joined to glycerol by an ether bond rather than an ester bond,
Plasmalogen’s are important membrane components in some tissues, mainly in
heart and brain, although they are absent in others. Plasmalogen is the most
abundant phospholipid in myelin. Deficiency of plasmalogens causes profound abnormalities
in the myelination of nerve cells, which is one reason why many peroxisomal
disorders affect the nervous system.
Peroxisomes also play a role in the production of
bile salts.
Bile salts biosynthesis involves at least 13
different enzymes located to various organelles in liver hepatocytes. The
bile salt biosynthesis pathway consists of two main routes and starts either
in the ER classical pathway or in the mitochondria acidic pathway. Both
routes yield CoA-activated C-27 bile acid biosynthesis intermediates and
converge in peroxisomes where the bile salt cholic acid and chenodeoxycholic
acid are formed. The final step of bile salt synthesis is the conjugation of
cholic acid and chenodeoxycholic acid to either taurine or glycine. The Nacytransferase
enzyme responsible for bile acid conjugation in liver
peroxisomes.
Origin of peroxisomes:
Peroxisomes can be derived from the Endoplasmic reticulum and replicate by fission. |
