Peroxisomes

Peroxisomes exist in all eukaryotic cells, and function to rid the body of toxic substances such as hydrogen peroxide, or other metabolites. They are a major site of oxygen utilization and are found in large quantities in organs where toxic products are bound to accumulate, like the liver. They have a crystalline structure inside a sac which also contains amorphous gray material.

Peroxisomes are only bound by a single membrane, and they do not contain DNA or ribosomes, unlike mitochondria and chloroplasts. However, they are thought to acquire their proteins from the cytosol by using selective import. However, they must import all of their proteins because they have no genome. Found in all eukaryotes, they contain oxidative enzymes, such as catalase and urate oxidase in very high concentrations.

Overall, peroxisomes are widely distributed in both plant and animal kingdoms and contribute to a variety of catabolic and anabolic processes.

In animal cells, peroxisome functions include respiration based on hydrogen peroxide forming oxidases and catalase, fatty acid oxidation, plasmalogen biosynthesis, alcohol oxidation, transaminations, and metabolism of purines, polyamines, bile acids and other substrates.

In seeds of plants rich in lipids, peroxisomes (glyoxysomes) are the site of breakdown of fatty acids to succinate via the glyoxylate cycle, and participate in gluconeogenesis in this way. In leaf tissues, peroxisomes serve as sites of photorespiration aswell.

A major peroxisome-related disorder is "Zellweger Syndrome". This is an autosomal recessive lethal disorder caused by mutations in different genes involved in peroxisome biogenesis. Symptoms include polymicrogyria (enlarged liver), high concentrations of iron and copper in the blood. The liver, kidneys and brain are the most affected organs.

The peroxisomal matrix protein import system shares mechanistic similarities with the Endoplasmic Reticulum-Proteasome Degradation process (ERAD), which indicates a common evolutionary history.