Peroxisome

Size and Shape

Peroxisomes differ in shape, size, and number depending on the cell’s energy requirements. For example, a smaller number of peroxisomes are found when yeast cells are grown on a sugar-rich medium. In contrast, when exposed to an alcohol-containing medium, peroxisome number increases significantly, occupying almost half the total cell volume.

However, they are generally spherical-shaped organelles averaging 0.2 to 1.5 μm in size. Peroxisomes can either exist as individual microperoxisomes or as a network of interconnected tubules called peroxisome reticulum.

Composition

It is a single, membrane-bound vesicle containing a fine, granular matrix. The membrane consists of lipids and proteins. At times, a condensed crystalline core is observed in the center of the organelle. The core contains all the enzymes necessary for its function.

How are they Formed

The assembly or biogenesis of peroxisomes is fundamentally similar to that of mitochondria and chloroplasts. Since peroxisomes lack their own DNA, 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, through phospholipid transfer proteins, from their major site of synthesis in the endoplasmic reticulum. The intake of proteins and phospholipids results in the growth of the peroxisome, and the division of existing ones then forms new peroxisomes.

Any anomaly related to peroxisome biogenesis may cause peroxisome biogenesis disorders (PBD).

Peroxisomes contain at least 50 oxidative enzymes involved in various biochemical pathways in different types of cells. The oxidative enzymes produce H₂O₂. Since H₂O₂ is harmful to the cell, peroxisomes also contain catalase. The enzyme decomposes hydrogen peroxide by converting it to water or by using it to oxidize another organic compound. Some other substances like uric acid, amino acids, and fatty acids are also broken down by those oxidative enzymes.

Among all oxidations performed by peroxisomes, β–oxidation of fatty acid is most important as it provides a significant source of metabolic energy in the cell.

β–Oxidation of Fatty Acids

This pathway, mediated by peroxisomes, provides a significant source of metabolic energy in eukaryotes. In animals, fatty acids are oxidized in peroxisomes and mitochondria, but in yeasts and plants, fatty acid oxidation is restricted to peroxisomes.

In β–oxidation, long chain fatty acids are progressively broken down to generate acetyl coenzyme A (acetyl CoA), releasing energy as ATP. The acetyl CoA then combines oxaloacetate to form citrate, which enters the citric acid cycle to produce more energy for the cell. The H₂O₂ formed is detoxified by the enzyme catalase. 

Some other oxidations performed by peroxisomes are the degradation of phytanic acid by alpha-oxidation and the synthesis of bile acids and plasmalogen.

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

Other Roles of Peroxisome in Animal Cells

• It helps in lipid biosynthesis involving synthesizing long-chain fatty acids, such as cholesterol and dolichol. Bile acid also gets synthesized from the liver’s cholesterol.
• Synthesizing plasmalogens (family of phospholipids) are essential membrane components of heart and brain tissues.
• Producing bioluminescence in fireflies containing the enzyme Luciferase. It helps flies find a suitable mate for their reproduction and search for food.

Other Roles of Peroxisome in Plant Cells

• Inducing germination of seeds by converting the stored fatty acids to carbohydrates that act as raw materials for germination
• It causes photorespiration in many higher plants where they take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis.
• Causing degradation (catabolism) of purines, polyamines, and amino acids by the enzyme uric acid oxidase