Everything about Peroxisome totally explained
Peroxisomes are
ubiquitous organelles in
eukaryotes that participate in the metabolism of
fatty acids and other metabolites. Peroxisomes have enzymes that rid the
cell of toxic
peroxides. They have a single
lipid bilayer membrane that separates their contents from the
cytosol (the internal fluid of the cell) and contain membrane proteins critical for various functions, such as importing proteins into the organelles and aiding in proliferation. Like
lysosomes, peroxisomes are part of the
secretory pathway of a cell, but they're much more dynamic and can replicate by enlarging and then dividing. Peroxisomes were identified as cellular organelles by the Belgian cytologist
Christian de Duve in
1967 after they'd been first described in a Swedish PhD thesis a decade earlier.
Occurrence and evolution
Peroxisomes are found in virtually all eukaryotic cells. Peroxisomes contain enzymes for certain oxidative reactions, like the
beta-oxidation of very-long-chain fatty acids.
Prokaryotes lack peroxisomes. The enzymatic content of peroxisomes varies across species, but the presence of certain proteins common to many species has been used to suggest an
endosymbiotic origin; that is, peroxisomes evolved from bacteria that invaded larger cells as parasites, and very gradually evolved a symbiotic relationship. However, this view has been challenged by recent discoveries. For example, peroxisome-less mutants can restore peroxisomes upon introduction of the wild-type gene, and peroxisomes have been observed to be formed from the
endoplasmic reticulum.
An evolutionary analysis of the peroxisomal
proteome found homologies between the peroxisomal import machinery and the
ERAD pathway in the
endoplasmic reticulum, along with a number of metabolic enzymes that were likely recruited from the
mitochondria. These results indicate that the peroxisome doesn't have an endosymbiotic origin; instead, it likely originates from the ER, and its proteins were recruited from pools existing within the primitive eukaryote, as quoted in the science textbook Biozone.
Function
Peroxisomes contain oxidative
enzymes, such as
catalase,
D-amino acid oxidase, and
uric acid oxidase. Certain enzymes within the peroxisome, by using molecular oxygen, remove hydrogen atoms from specific organic substrates (labeled as R), in an oxidative reaction, producing
hydrogen peroxide (H
2O
2, itself toxic):
»
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 yeast and plant cells, this process is exclusive for the peroxisome.
The first reactions in the formation of
plasmalogen in animal cells also occurs in peroxisomes. Plasmalogen is the most abundant phospholipid in
myelin. Deficiency of plasmalogens causes profound abnormalities in the myelination of
nerve cells, which is one of the reasons that many peroxisomal disorders lead to neurological disease.
Peroxisomes also play a role in the production of
bile acids and proteins.
In higher plants, peroxisomes contain also a complex battery of antioxidative
enzymes such as superoxide dismutase, the components of the ascorbate-glutathione cycle, and the NADP-dehydrogenases of the pentose-phosphate pathway. It has
been demonstrated the generation of superoxide (O
2•-) and
nitric oxide (
•NO) radicals.,.
Protein import
Proteins are selectively imported into peroxisomes. Since the organelles contain no DNA or ribosomes and thus have no means of producing proteins, all of their proteins must be imported across the membrane. It is believed that necessary proteins enter through the
endoplasmic reticulum during biogenesis as well as through membrane proteins.
A specific protein signal (PTS or
peroxisomal targeting signal) of three amino acids at the
C-terminus of many peroxisomal proteins signals the membrane of the peroxisome to import them into the organelle. Other peroxisomal proteins contain a signal at the
N-terminus. There are at least 32 known peroxisomal proteins, called
peroxins, which participate in the process of importing proteins by means of
ATP hydrolysis. Proteins don't have to unfold to be imported into the peroxisome. The protein receptors, the peroxins
PEX5 and
PEX7, accompany their cargoes (containing a PTS1 or a PTS2, respectively) all the way into the peroxisome where they release the cargo and then return to the
cytosol - a step named
recycling. Overall, the import cycle is referred to as the
extended shuttle mechanism.
Evidence now indicates that ATP hydrolysis is required for the recycling of receptors to the
cytosol. Also,
ubiquitination appears to be crucial for the export of PEX5 from the peroxisome, to the cytosol. Little is known about the import of PEX7, although it has helper proteins that have been shown to be ubiquitinated.
Deficiencies
Peroxisomal disorders are a class of condtions that lead to disorders of
lipid metabolism. One well-known example is
Zellweger syndrome.
Peroxisomes matrix proteins are synthesized on free ribosomes in the cytosol and that these proteins are imported posttranslationally in pre-existing peroxisomes.
Genes
Gene which encode peroxisomal proteins include:
References
Further Information
Get more info on 'Peroxisome'.
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