Peroxisome

Lore & Background

Peroxisomes are small, membrane-bound organelles, typically 0.1 to 1 micrometer in diameter, found in the cytoplasm of virtually all eukaryotic cells. They possess a single biomembrane surrounding a fine, granular matrix. Their number, size, and protein composition are not fixed; they vary depending on the cell type and environmental conditions. For instance, in baker’s yeast, few and small peroxisomes are present when glucose is abundant, but when long-chain fatty acids are the sole carbon source, the cell may produce up to 20 to 25 large peroxisomes. These organelles are oxidative in nature, often using molecular oxygen as a co-substrate to generate hydrogen peroxide, from which they derive their name. They contain enzymes that both produce and break down hydrogen peroxide, notably catalase, which decomposes the toxic compound into water and oxygen. Peroxisomes are critical for lipid metabolism, including the beta-oxidation of very long chain fatty acids—converting them to medium chain fatty acids in animal cells for further breakdown in mitochondria—and the alpha-oxidation of phytanic acid. They also initiate the biosynthesis of plasmalogens, ether phospholipids essential for normal brain and lung function, and contribute to bile acid production. In mammalian cells, peroxisomes house the first steps of plasmalogen formation and the conjugation of cholic acid. They contain oxidative enzymes like D-amino acid oxidase and uric acid oxidase (the latter absent in humans, leading to gout from uric acid buildup). Additionally, peroxisomes hold about 10% of the total activity of two pentose phosphate pathway enzymes, supporting energy metabolism. In plants, they perform the glyoxylate cycle in germinating seeds and photorespiration in leaves, while in some yeasts they handle methanol oxidation.

Reader's Guide

Peroxisomes are small (0.1–1 μm diameter) organelles with a fine, granular matrix, surrounded by a single biomembrane. Their number, size, and protein composition vary depending on cell type and environmental conditions. A major function is the breakdown of very long chain fatty acids through beta oxidation; in animal cells, these are converted to medium chain fatty acids and shuttled to mitochondria, while in yeast and plant cells the process occurs exclusively in peroxisomes. Peroxisomes also initiate plasmalogen formation, produce bile acids, and contain oxidative enzymes such as D-amino acid oxidase and uric acid oxidase (the latter absent in humans, explaining gout). They detoxify toxic substances, including about 25% of consumed ethanol, and in plants contain antioxidative enzymes. Peroxisomes generate reactive oxygen species that are important signaling molecules in plants and animals, contributing to healthy aging and age-related disorders. They also contribute to anti-viral defense and combat pathogens. Peroxisomes are derived from the smooth endoplasmic reticulum under certain conditions and replicate by membrane growth and division. Matrix proteins are imported via specific targeting signals (PTS1 or PTS2) without needing to be unfolded, using peroxins (36 known, 13 in mammals).

Did You Know?

• Peroxisomes contain approximately 10% of the total activity of two enzymes in the pentose phosphate pathway.
• In baker's yeast, only a few small peroxisomes are present with good glucose supply, but up to 20–25 large ones form when supplied with long-chain fatty acids.
• About 25% of ethanol consumed by humans is oxidized to acetaldehyde in peroxisomes.
• Uric acid oxidase is absent in humans, explaining the disease known as gout.