What is the Structure and Importance of Peroxisomes in both plants and Animals?
Peroxisomes are usually spherical and about 0.8µ in diameter. It was first discovered by Tolbert (1968) from Spinach leaf and was obtained by density gradient centrifugation. Both plants and animal cells contain a different kind of membrane-bounded structures called Peroxisome which participates in metabolic oxidations involving hydrogen peroxide. Peroxisome contains very dense crystalline material in a dark granular matrix, enzymes, and other materials. The enzymes chiefly included are catalase which decomposed H2O2, urate oxidase, D-amino acid oxidase and α-hydroxy acid oxidase.
The main function of peroxisome is that it is involved in purine metabolism and the conversion of fat to carbohydrate. Peroxisome contains typical enzymes which are involved in glyoxilate metabolism and photorespiration during photosynthesis.
They contain a fine, granular substance that many condense in the center, forming an opaque and homogeneous core. is J.Rhodin described intracellular components called “Microbodies” in 1954.
Historical Aspects of Peroxisomes
The Peroxisomes were isolated by De Duve and co-workers in 1965, were enriched with some oxidative enzymes, such as peroxide, catalase, D-amino oxidase and Urate oxidase, acetyl~coA oxidase, polyamine oxidase, NADH-glyoxylate reductase, NADP-isocitrate dehydrogenase and Catalase.
- The Peroxidase was applied because this organelle is specifically involved in the formation and decomposition of hydrogen peroxide (H2O2).
- The second function of peroxisome is identified in 1976. It involves in β-oxidation of fatty acid/ the peroxisomes have also been found in protozoa, yeast and many cell types of higher plants.
- In plants, some organelles are similar to peroxisomes of animal cells. The plant peroxisomes are called “Glyoxisomes,” the enzymatic makeup includes the enzymes of the glyoxylate cycle, hence the name “Glyoxysomes.”
- Glyoxysomes also contain enzymes for the oxidation of alcohols and alkenes, which may be included in the substrate.
Morphology of Peroxisomes (or) Peroxisome Structure
The Peroxisomes structure
- Peroxisomes are ovoid granules
- In rat liver cells the average diameter of peroxisomes was shown to be 0.6 to 0.7 µm.
- The number of peroxisomes per cell varied between 70 to 100, whereas 15 to 20 lysosomes were found per liver cell.
- Tissues peroxisomes show a crystal-like body made of tubular subunits.
- The core containing peroxisomes found in liver, kidney. They are smaller and lack a core. These are “Microperoxisomes”.
- Both peroxisomes and microperoxisomes are demonstrated under Electron Microscope (EM) using the histochemical reaction. This demonstration carried out, 3’-3’ diaminobenzidine (DAB) oxidation in the presence of H2O2.
Biogenesis of Peroxisomes
The Mechanism of biogenesis of Peroxisomes is complex and not completely known. It is possible that the peroxisomal membrane proteins are synthesized in the Endoplasmic Reticulum (ER), but both organelles have different protein composition.
Peroxisomal, as well as glyoxysomal, enzymes are synthesized in the cytosol on free ribosomes and are then incorporates post-translationally into these organelles.
- The main Peroxisomal enzyme “Catalase” is made as a monomeric precursor (apoenzyme) which after being in the cytosol with a half-life of 14 minutes, is translocated into the peroxisome.
- The final enzyme (holoenzyme) with its tetrameric form becomes active.
- The half-life of Catalase has been calculated as 36 hours.
- Peroxisome destroyed after the lifespan of 5 to 6 days.
A Mixed model of Peroxisome Biogenesis
- The membrane proteins are synthesized on membrane-bound ribosomes.
- The peroxisomal enzymes are made in the cytosol on free ribosomes and are translocated into the organelle.
Enzymes in Peroxisomes:
The organelles contain four enzymes related to the metabolism of H2O2.
- Urate Oxidase
- D-Amino oxidase
- Α-anhydroxylic acid oxidase
The above three enzymes produce peroxide (H2O2) and catalyze will be destroyed.
- Catalase – in the matrix and liver peroxides
Plant Peroxisomes are involved in Photorespiration
In green leaves, there are peroxisomes that carry out a process called “Photorespiration”. In this process Glycolic acid, a two-carbon product of photosynthesis that is released from Chloroplasts is oxidized by glycolic acid oxidase, an enzyme present in Peroxisomes.
Photorespiration is so called because light induces the synthesis of glycolic acid in Chloroplasts. The entire process involves the intervention of two basic organelles: Chloroplasts and Peroxisomes. Glyoxysomes, which are special plant cell organelle involved in the metabolism of stored lipids.
Aditional points on Peroxisomes
Peroxisomes are small, membranebound cytoplasmic organelles found in both plant and animal cells. Beaufaytt and Berther (1963) called these micro-bodies as peroxisome.
- These organelles mainly occur in photosynthesizing cells of higher plants, algae, liverworts, mosses, ferns and also in fungi.
- Their number varies from 70-100 per cell. Peroxisomes are rounded bodies whose diameter varies from 0.2-1.5 u.
- Peroxisome contains a granular stroma bounded by a unit membrane. A centrally placed opaque area is present in the matrix which is made up of parallel tubules or twisted strands. Animal and plant peroxisomes vary considerably in their enzymatic context. The matrix of peroxisome contains peroxide-destroying enzymes (catalases) and peroxide producing enzymes. They prevent the peroxides from acting on the cellular contents.
Although the exact function of peroxisomes is not known, present evidence suggests that these organelles are associated with glycolate metabolism linked with photorespiration in plants and lipid metabolism in animal cells.
Microbodies are also single membrane-bound organelles, associated with oxidation reaction other than that respiration. These organelles often posses a crystalloid core and granular matrix. There are two types of Microbodies namely Peroxisomes and glyoxysomes.
The peroxysomes have enzymes for peroxide biosynthesis. These occur in most of the animals and plants but are quite common in photosynthetic cells. Their number can be 70 to 100 per Mesophyll cell where they interact with mitochondria and chloroplasts to take part in photo-respiration.
The glyoxysomes usually occur in fat-rich plant cells and are associated with triglyceride metabolism through Glyoxylate cycle.
Peroxisomes are organelles rich in peroxidase, catalase, D-amino acid oxidase, and urate oxidase. They are abundant in the liver, kidney, and in many cell types of animals and plants. They have 0.6 to 0.7µm granules with a single membrane and a dense matrix. Frequently, a crystalline-like condensation is observed.
Many cells contain micro-peroxisomes that can be demonstrated by the DAB reaction of peroxidase. The use of the specific immunochemical method for catalyzes also serves to detect peroxisomes and micro peroxisomes. The enzymes of this organelle are synthesized in the cytosol on free ribosomes and are then transferred by a post-translational mechanism. A mixed model for the biogenesis of peroxisomes has been postulated.
Three of the enzymes – urate oxidase, D-amino oxidase, and α-hydroxylic acid oxidase-produce H2O2 and catalase decompose it. These organelles are also involved in β-oxidation of fatty acids and play a role in thermogenesis. The metabolic pathways of peroxisomes are interrelated with steps taking place in the cytosol and mitochondria. Catalase acts a “safety valve” to deal with peroxides that are dangerous to the cell.
In plants, peroxisomes carry out the process of photorespiration, which involves the cooperation of chloroplast and peroxisomes. Glyoxysomes are special plant organelles involved in the metabolism of stored lipids.