Peroxisomes are tiny vesicles, single membrane-bound organelles located around the eukaryotic cells. They hold digestive enzymes for tearing down toxic materials in the cell and oxidative enzymes for metabolic action. They are a heterogeneous group of organelles and the existence of the marker enzymes differentiated them from other cell organelles. Peroxisomes play a necessary role in lipid production and are also engaged in the transformation of reactive oxygen species such as hydrogen peroxide into safer molecules like water and oxygen.
The peroxisome evolves its name from the phenomenon that many metabolic enzymes that develop hydrogen peroxide as a by-product are segregated here because peroxides are toxic to cells. Within peroxisomes, hydrogen peroxide is discredited by the enzyme catalase to water and oxygen. Peroxisomes are circled by a single membrane and they range in the diameter from 0.1 to 1 mm. They lie either in the form of a network of interconnected tubules (peroxisome reticulum) as in liver cells or as individual micro peroxisomes in alternative cells such as tissue culture fibroblasts.
Peroxisomes are small, membrane-enclosed cellular organelles consisting of oxidative enzymes that are engaged in a variety of metabolic reactions, including several conditions of energy metabolism. They are recognized as an important type of microbody found in both plants and animal cells.
They were diagnosed as organelles by Belgian cytologist Christian de Duve in 1967 after previously been represented. First peroxisomes to be discovered were disengaged from leaf homogenate of spinach. They are most richly found in detoxifying organs such as the liver and kidney cells. However, they can be activated to proliferate in response to metabolic needs.
Structure of Peroxisomes
- Peroxisomes fluctuate in shape, size, and number depending upon the energy demands of the cell. These are formed of a phospholipid bilayer with many membrane-bound proteins.
- The enzymes engaged in lipid metabolism are synthesized on independent ribosomes and selectively imported to peroxisomes. These enzymes include one of the two signaling sequences– Peroxisome Target Sequence 1 being the most common one.
- The phospholipids of peroxisomes are generally synthesized in smooth Endoplasmic reticulum. Due to the ingress of proteins and lipids, the peroxisome grows in size and breaks into two organelles.
- They are membrane-bound spherical figures of 0.2 to 1.5 μm in diameter located in all eukaryotic organisms including both plants and animal cells.
- They are gotten floating freely in the cytoplasm in close association of ER, mitochondria, or chloroplast within the cell.
- They are among the smoothest of eukaryotic organelles.
- They lie either in the form of a network of interconnected tubules called peroxisome reticulum or as individual microperoxisomes.
- They are irregular in size and shape according to the cell and usually circular in cross-section.
- They range from 0.2 -1.5 μm in diameter.
- It consists of a single inhibiting membrane of lipid and protein molecules encircling the granular matrix.
- The matrix consists of fibrils or a crystalloid structure containing enzymes.
Almost 60 known enzymes are present in the matrix of peroxisomes. They are accountable to carry out oxidation reactions leading to the production of hydrogen peroxide. The main groups of enzymes include:
- Urate oxidase
- D-amino acid oxidase
The main function of peroxisome is the lipid metabolism and the processing of reactive oxygen species. Other peroxisome 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.
The main reason for the high energy density of fats is the low ratio of oxygen atoms in every fatty acid molecule. For occurrence, palmitic acid, a fatty acid containing 16 carbon atoms and containing a molecular mass of more than 250 gms/mole, has only two oxygen atoms.
While this formes lipid good storage molecules, they cannot be directly burnet as fuel or quickly catabolized in the cytoplasm through glycolysis. They need to be processed before they can be shunted into the mitochondria for complete oxidation through the citric acid cycle and oxidative phosphorylation.
When these molecules need to be oxidized to release ATP, they need to be first broken down into smaller molecules before they can be processed in the mitochondria. Within peroxisomes, long-chain fatty acids are progressively broken down to generate acetyl coenzyme A (acetyl CoA) in a process called β–oxidation.
Acetyl CoA then combines with oxaloacetate to form citrate. While most carbohydrates enter the citric acid cycle as a three-carbon molecule called pyruvate which is then decarboxylated to form acetyl CoA, peroxisomal β–oxidation allows fatty acids to access the citric acid cycle directly.
One of the main byproducts of β–oxidation is hydrogen peroxide which can be harmful to the cell. This molecule is also carefully detoxified by the enzyme catalase within peroxisomes.
In plants, peroxisomes play important roles in seed germination and photosynthesis. During seed germination, fat stores are mobilized for anabolic reactions that lead to the formation of carbohydrates. This is called the glyoxalate cycle and begins with β–oxidation and the generation of acetyl CoA as well.
In leaves, peroxisomes prevent the loss of energy during photosynthetic carbon fixation through recycling the products of photorespiration. A crucial enzyme called Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is necessary for photosynthesis, catalyzing the carboxylation of ribulose-1,5-bisphosphate (RuBP).
This is the central reaction for the fixation of carbon dioxide to form organic molecules. However, RuBisCO, as the name suggests, can also oxygenate RuBP, using molecular oxygen, releasing carbon dioxide – in effect, reversing the net result of photosynthesis. This is particularly true when the plant is exposed to hot, dry environments and the stomata close in order to prevent transpiration.
When RuBisCO oxidizes RuBP, it generates a 2-carbon molecule called phosphoglycolate. This is captured by peroxisomes where it is oxidized to glycine. Thereafter, it is shuttled between the mitochondria and peroxisomes, undergoing a series of transformations before it is converted into a molecule of glycerate that can be imported into chloroplasts to participate in the Calvin cycle for photosynthesis.