What are Peroxisomes?
Peroxisomes are small vesicles, single membrane-bound organelles found around the eukaryotic cells. They contain digestive enzymes for breaking down toxic materials in the cell and oxidative enzymes for metabolic activity. They are a heterogeneous group of organelles and the presence of the marker enzymes distinguished them from other cell organelles.
Peroxisomes play an important role in lipid production and are also involved in the conversion of reactive oxygen species such as hydrogen peroxide into safer molecules like water and oxygen.
The peroxisome derives its name from the fact that many metabolic enzymes that generate hydrogen peroxide as a by-product are sequestered here because peroxides are toxic to cells. Within peroxisomes, hydrogen peroxide is degraded by the enzyme catalase to water and oxygen.
Peroxisomes are surrounded by a single membrane and they range in the diameter from 0.1 to 1 mm. They exist either in the form of a network of interconnected tubules (peroxisome reticulum) as in liver cells or as individual micro peroxisomes in other cells such as tissue culture fibroblasts.
Also read: Lysosomes
Peroxisomes vary in shape, size and number depending upon the energy requirements of the cell. These are made of a phospholipid bilayer with many membrane-bound proteins.
The enzymes involved in lipid metabolism are synthesised on free ribosomes and selectively imported to peroxisomes. These enzymes include one of the two signalling sequences- Peroxisome Target Sequence 1 being the most common one.
The phospholipids of peroxisomes are usually synthesised in smooth Endoplasmic reticulum. Due to the ingress of proteins and lipids, the peroxisome grows in size and divides into two organelles.
The main function of peroxisome is the lipid metabolism and the processing of reactive oxygen species. Other functions include:
- The synthesis of ether glycerolipids of plasmalogens.
- The formation of bile acids, dolichol, and cholesterol.
- The catabolism of purines, polyamines, and amino acids, and the detoxification of reactive oxygen species
- In methylotrophic yeasts, peroxisomes are also involved in the metabolism of methanol and methylamines.
In plants, peroxisomes facilitate photosynthesis and seed germination. They prevent loss of energy during photosynthesis carbon fixation.
Metabolism of Peroxisomes
Isolated peroxisomes are permeable to small molecules such as sucrose. During isolation process, they often lose proteins that are normally confined to the peroxisomal matrix. In all living cells, peroxisomes are the sealed vesicles surrounded by a single membrane.
Biogenesis of Peroxisomes
As peroxisomes have no DNA, all their proteins must be imported from genes encoded in the nucleus. Most of the proteins that reside in the peroxisome matrix and membrane are synthesized in the cytosol and then imported posttranslationally to the organelle.
About 25 PEX genes, encoding proteins called peroxins are necessary for the biogenesis of the organelle. Most of these genes are found in multiple organisms and 13 are conserved in humans.
Also read: Nucleus
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