Peroxisomes were discovered as biochemical entities by De Duve’s group. They were identified as small sedimentable particles containing marker enzymes that distinguished them from other known organelles. The presence of hydrogen-peroxide-producing oxidases and catalase inspired the name peroxisomes and focused attention on their role in oxidative metabolic transactions.
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.
Isolated peroxisomes are permeable to small molecules such as sucrose. During isolation, they often lose proteins that are normally confined to the peroxisomal matrix. This loss of peroxisomal content was initially taken as the evidence for the permeability of the peroxisomal membrane in vivo but is now known to be an isolation artifact. In living cells, peroxisomes are the sealed vesicles surrounded by a single membrane.
Functions of Peroxisomes
The principal function of peroxisomes is to house different metabolic pathways that are involved in various aspects of lipid metabolism. These include the following:
- Enzymes involved in the degradative oxidation (e.g., b-oxidation of very long chain fatty acids, 2-methyl-branched fatty acids, dicarboxylic acids, leukotrienes, bile acid intermediates and cholesterol side chains and both a- and b-oxidation of 3-methyl branched-chain fatty acids)
- The synthesis of ether glycerolipids or plasmalogens.
- The formation of bile acids, dolichol, and cholesterol.
- The catabolism of purines, polyamines, and amino acids, and the detoxification of reactive oxygen species such as hydrogen peroxide, superoxide anions, and epoxides. In methylotrophic yeasts, peroxisomes are also involved in the metabolism of methanol and methylamines.
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.
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