Peroxisomes structure, functions and assembly
By: Dr. Akalesh K Verma, Asst. Professor, Cotton University, Assam, India(Materials: Educational)
Peroxisome Definition
Peroxisomes are membrane-bound organelles in most eukaryotic cells, primarily involved in lipid metabolism and the conversion of reactive oxygen species such as hydrogen peroxide into safer molecules like water and oxygen.
Structure of Peroxisomes
Peroxisomes are organelles that can vary in shape, size and number depending on the energy needs of the cell. In yeast cells, a carbohydrate-rich growth medium shrinks peroxisomes. On the other hand, the presence of toxins or a lipid-rich diet can increase their number and size. These organelles are made of a phospholipid bilayer with many membrane-bound proteins – especially those that act as protein transporters and translocators. The enzymes involved in detoxification and lipid metabolism are synthesized on free ribosomes in the cytoplasm and selectively imported into peroxisomes, making them more similar to mitochondria and chloroplasts when compared to lysosomes that bud off from the endoplasmic reticulum (ER). However, there is also some evidence linking ER-mediated protein synthesis to the enzymes present in peroxisomes.
Fig: 1. Peroxisome structure
Peroxisomes Proliferations
Upon induction of peroxisome proliferation, new peroxisomes can be made by the division of pre-existing organelles in a process called fission. Peroxisomal fission can be broken down into three steps i) elongation of the peroxisomal membrane ii) constriction of the elongation at a certain site and iii) the actual scission step, that separates the daughter peroxisome from the mother.
Fig: 2. Peroxisomes division (Proliferation).
Peroxisome Assembly
The assembly of peroxisomes is fundamentally similar to that of mitochondria and chloroplasts, rather than to that of the endoplasmic reticulum, Golgi apparatus, and lysosomes. Proteins destined for peroxisomes are translated on free cytosolic ribosomes and then transported into peroxisomes as completed polypeptide chains. Phospholipids are also imported to peroxisomes, via phospholipid transfer proteins, from their major site of synthesis in the ER. The import of proteins and phospholipids results in peroxisome growth, and new peroxisomes are then formed by division of old ones. Most proteins are targeted to peroxisomes by the simple amino acid sequence Ser-Lys-Leu at their carboxy terminus (peroxisome targeting signal 1, or PTS1). Other proteins are targeted by a sequence of nine amino acids (PTS2) at their amino terminus, and some proteins may be targeted by alternative signals that have not yet been well defined. PTS1 and PTS2 are recognized by distinct receptors and then transferred to a translocation complex that mediates their transport across the peroxisome membrane. However, the mechanism of protein import into peroxisomes is less well characterized than the mechanisms of protein translocation across the membranes of other subcellular organelles. In contrast to the translocation of polypeptide chains across the membranes of the endoplasmic reticulum, mitochondria, and chloroplasts, targeting signals are usually not cleaved during the import of proteins into peroxisomes. Cytosolic Hsp70 has been implicated in protein import to peroxisomes, but the possible role of molecular chaperones within peroxisomes is unclear. Moreover, it appears that proteins can be transported into peroxisomes in at least partially folded conformations, rather than as extended polypeptide chains. Some peroxisome membrane proteins are similarly synthesized on cytosolic ribosomes and targeted to the peroxisome membrane by distinct internal signals. However, other experiments suggest that some peroxisomal membrane proteins may be synthesized on membrane-bound polysomes of the endoplasmic reticulum and then transported to peroxisomes, suggesting a role for the endoplasmic reticulum in peroxisome maintenance. The import of proteins into peroxisomes thus appears to have several novel features, making it an active area of investigation. Mutations associated with serious human diseases involving disorders of peroxisomes. In some such diseases, only a single peroxisomal enzyme is deficient. However, in other diseases resulting from defects in peroxisome function, multiple peroxisomal enzymes fail to be imported to peroxisomes, instead being localized in the cytosol. The latter group of diseases results from deficiencies in the PTS1 or PTS2 pathways responsible for peroxisomal protein import. The prototypical example is Zellweger syndrome, which is lethal within the first ten years of life. Zellweger syndrome can result from mutations in at least ten different genes affecting peroxisomal protein import, one of which has been identified as the gene encoding the receptor for the peroxisome targeting signal PTS1.
Fig: 3. Peroxisomes assembly.
Metabolic functions
A major function of the peroxisome is the breakdown of very long chain fatty acids through beta-oxidation. In animal cells, the very long fatty acids are converted to medium chain fatty acids, which are subsequently shuttled to mitochondria where they are eventually broken down to carbon dioxide and water.In yeast and plant cells, this process is exclusive for the peroxisomes.
The first reactions in the formation of plasmalogen in animal cells also occur 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 reason why many peroxisomal disorders affect the nervous system. However the last enzyme is absent in humans, explaining the disease known as gout, caused by the accumulation of uric acid. 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 (H2O2, itself toxic):
peroxidase, another peroxisomal enzyme, uses this H2O2 to oxidize other substrates, including phenols, formic acid, formaldehyde, andalcohol, by means of the peroxidation reaction:
,thus eliminating the poisonous hydrogen peroxide in the process.
This reaction is important in liver and kidney cells, where the peroxisomes detoxify various toxic substances that enter the blood. About 25% of the ethanol humans drink is oxidized to acetaldehyde in this way. In addition, when excess H2O2 accumulates in the cell, catalase converts it to H2O through this reaction:
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 (O2•-) and nitric oxide (•NO) radicals. The peroxisome of plant cells is polarised when fighting fungal penetration. Infection causes a glucosinolate molecule to play an antifungal role to be made and delivered to the outside of the cell through the action of the peroxisomal proteins (PEN2 and PEN3).
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All images: Google image
