Peroxisome

Lore & Background

Peroxisomes are small, membrane-bound organelles, typically 0.1 to 1 micrometer in diameter, found in the cytoplasm of nearly all eukaryotic cells. They possess a single surrounding biomembrane and contain 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, only a few small peroxisomes are present when glucose is abundant, but when the yeast must rely on long-chain fatty acids as a sole carbon source, the cell may produce 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. Peroxisomes are critical for the catabolism of very long chain fatty acids and branched chain fatty acids, breaking them down into simpler substrates that mitochondria can use for energy production. They also play a key role in the biosynthesis of plasmalogens, ether phospholipids essential for normal brain and lung function in mammals. In animal cells, the initial steps of plasmalogen formation occur within the peroxisome. Additionally, peroxisomes are involved in the production of bile acids, which are necessary for the absorption of fats and fat-soluble vitamins. They contain oxidative enzymes such as D-amino acid oxidase and uric acid oxidase, though the latter is absent in humans, a fact linked to the disease gout. The enzyme catalase, also present in peroxisomes, decomposes hydrogen peroxide into water and oxygen, and can use the peroxide to oxidize other substrates, including alcohol—about 25% of consumed ethanol is oxidized to acetaldehyde in this manner in liver and kidney cells. In higher plants, peroxisomes contain additional antioxidative enzymes like superoxide dismutase and components of the ascorbate-glutathione cycle.

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.