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Peroxisome

What are Peroxisomes

Peroxisomes, formerly known as microbodies, are small, vesicular, single membrane-bound cell organelles present in all eukaryotic cells, including both plant and animal cells. They contain various enzymes, which primarily function together to eliminate the cellular toxic substances, particularly hydrogen peroxide, a common by-product of cellular metabolism.

In 1967, Belgian cytologist Christian de Duve identified peroxisomes. Duve and his co-workers discovered that these organelles contain several oxidases involved in the production of hydrogen peroxide (H2O2). They also observed that peroxisomes contain another enzyme, catalase that decomposes H2O2 into oxygen and water. Due to their role in peroxide metabolism, Duve named them ‘peroxisomes’, replacing the formerly used term ‘microbodies’.

Where are They Found in a Cell

Peroxisomes are found free-floating in the cytoplasm of all plant and animals cells.

Structure

Peroxisomes are small, membrane-enclosed organelles, usually appearing circular in cross-section. As they lack their own DNA, proteins are transported from the cytosol after translation. They can either exist as individual microperoxisomes or as a network of interconnected tubules called peroxisome reticulum.

Size and Shape

Peroxisomes are small, spherical, membrane-bound bodies of 0.2 to 1.5 μm diameter.

Composition

As stated, peroxisome is a membrane-bound vesicle containing a fine, granular matrix. It remains covered by a single membrane of lipid and protein, which encloses the granular matrix.

At times, a condensed crystalline core can be observed in the center of the organelle. The core contains various enzymes for various cell functions, mainly metabolism. Sometimes, the enzymes, catalase or urate oxidase, can become so concentrated that they aggregate to form this crystalline core in the center of the organelle.

The number, size, and protein composition of peroxisomes vary, depending on cell type and environmental conditions. For instance, in yeast (Saccharomyces cerevisiae), only a few small peroxisomes are present if the cells have an abundant glucose supply. In contrast, when the yeasts were supplied with long-chain fatty acids as the sole nutrient source, cells generated more large peroxisomes to meet the metabolic demand.

How are they Formed

Peroxisomes are formed by two distinct pathways: (i) de novo biogenesis; and (ii) growth and division of existing peroxisomes.

The biogenesis of peroxisome follows three main stages: 

(a) Formation of the membrane: In de novo biogenesis, two pre-peroxisomal vesicles fuse to give rise to early peroxisome. Out of these two vesicles, one is derived from the endoplasmic reticulum (ER) and the other one from mitochondria. These two vesicles contain two peroxisome-biogenesis-initiating proteins or peroxins, PEX16 and PEX3. Both proteins import peroxisomal membrane proteins (PMPs) into the lipid bilayer.

(b) Import of proteins into the matrix: These early peroxisomes then import more PMPs followed by matrix (lumen) proteins, eventually forming mature peroxisome. Mature peroxisome continues to import both PMPs and matrix proteins.

(c) Proliferation of the organelles: After maturing, the peroxisomes rapidly increase in numbers by a growth and division mechanism, which involves the elongation of peroxisomes followed by fission. Two to five peroxisomes can be formed from a single peroxisome.

Functions: What Does the Peroxisome Do

Peroxisomes contain oxidative enzymes that play an essential role in beta-oxidation of long-chain fatty acids, degradation of phytanic acid by alpha-oxidation, and the synthesis of bile acids and plasmalogen.

The oxidation of fatty acids is vital since it provides a major source of metabolic energy. These oxidation reactions also produce hydrogen peroxide as a by-product. Being a reactive oxygen species (ROS), hydrogen peroxide is considered toxic to the cell, as it reacts with many other molecules. Peroxisome neutralizes the toxicity by converting hydrogen peroxide into water and oxygen under the influence of enzyme catalase present in it. In this way, peroxisomes provide a safe site for the oxidative metabolism of the molecules mentioned above.

These functions of peroxisomes, i.e., metabolism of hydrogen peroxide and oxidation of fatty acids, are common in both plant and animal cells. However, in animal cells, the oxidation of fatty acids occurs in both peroxisomes and mitochondria, but in plants, it is only limited to peroxisomes.

Some more functions of peroxisomes, specific to plant and animal cells are as follows:

In Plant Cells

Germination of Seeds: In seeds, peroxisomes convert the stored fatty acids to carbohydrates, which provide energy and raw materials for their growth.

Photorespiration: In green leaves, peroxisomes carry out the photorespiration process along with chloroplasts.

Degradation of purines: Peroxisomes conduct the catabolism of purines, polyamines, and amino acids under the influence of uric acid oxidase.

In Animal Cells

Lipid biosynthesis: Synthesis of long-chain fatty acids, such as cholesterol and dolichol, occurs in peroxisomes. Bile acid also gets synthesized from the liver cholesterol.

Peroxisomes also contain enzymes to synthesize plasmalogens, a family of phospholipids that are essential membrane components of tissues of the heart and brain.

Bioluminescence: Peroxisomes of fireflies contain an enzyme Luciferase that aid in bioluminescence, thus helping the flies to find a mate or meal.

FAQs

Q.1. Why peroxisomes are not considered part of the endomembrane system?

Ans. Peroxisomes are not considered a part of the endomembrane system because their functions are not coordinated with the endoplasmic reticulum, Golgi complex, lysosomes, and vacuoles.

Article was last reviewed on Thursday, October 28, 2021

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