Peroxisomes: Defintion, Structure & Function

Peroxisomes are small, roughly spherical membrane-bound entities found throughout the cytoplasm of almost all eukaryotic (plant, animal, protist and fungal) cells. Unlike most bodies within cells that are normally classified as organelles, peroxisomes have only a single plasma membrane rather than a double membrane layer.

They represent the most common type of microbody inside eukaryotic cells with lysosomes perhaps being a better-known kind of microbody. Although self-replicating, they do not contain their own DNA as mitochondria do.

Therefore, when they make copies of themselves, they must make use of proteins they import to the scene for this purpose. This is believed to occur via a peroxisomal targeting signal consisting of a specific string of amino acids (the monomeric units of proteins).

  • Peroxisomes vs. Lysosomes: Whereas peroxisomes are self-replicating, lysosomes are usually made in the Golgi complex.

Structure of the Peroxisome

Peroxisomes' location is in the cytoplasm. These organelles have a diameter of about one-tenth of a micrometer to 1 micrometer, or 0.1 to 1 μm.

This tells you not only that peroxisomes are tiny, but also that their size varies considerably, which is what you might expect of what is essentially a biological shipping container. Most boxes used by parcel-delivery companies, after all, look more or less the same except for their dimensions.

The cell membrane and that of most of the cell's organelles (e.g., mitochondria, the nucleus, the endoplasmic reticulum) consist of a double bilayer, with each of these bilayers including a hydrophilic (water-seeking) side and a hydrophobic (water-repelling) side.

This is because a single bilayer consists of mainly of roughly oblong phospholipid molecules, which have a fatty end that does not dissolve easily in water and a phosphate (charged) end that does.

In a double membrane, the two "water-repelling" lipid sides chemically seek each other and hence face each other, forming the center; meanwhile, one of the two "water-seeking" phosphate sides faces the exterior of the cell, and the other faces the cytoplasm.

This results in the construction of, schematically, a pair of identical sheets stuck together in a "mirror-image" manner. In a peroxisome, the fatty portions of the peroxisomal membrane also lie on the interior of the single membrane, facing away from the cytoplasm.

Peroxisomes contain at least 50 different enzymes. Have you ever had a neighbor who seems to have at least one can of every kind of destructive but potentially useful chemical (insecticide, herbicide, pain thinner) in his garage? In the world of organelles, peroxisomes are sort of like that neighbor.

The enzymes they contain help degrade the materials that the peroxisome scoops up from the surrounding cytoplasm, including the waste products of the countless metabolic reactions a cell is undergoing at any moment to propagate the process of life itself. One of these common by-products is hydrogen peroxide, or H2O2; this gives the peroxisome its name.

Peroxisome biogenesis is atypical for a component of eukaryotic cells. Lacking DNA and reproductive machinery of their own, peroxisomes can self-replicate by simple fission in the manner of mitochondria and chloroplasts.

This ultimately occurs once a peroxisome, which is something of a tiny biochemical hoarder, reaches a critical size after importing enough protein products it encounters in the cytoplasm into its lumen (inside space) and membrane. At the time this bloated peroxisome splits, each of the two resulting cells begins its existence with a complement of non-peroxisomal proteins that started as trash somewhere else.

What’s Inside the Peroxisome?

Within the peroxisome is a urate oxidase crystalline core, which looks like dark circular region on microscopy. Urate oxidase is an enzyme that helps break down uric acid. The core is home to a variety of other enzymes as well, although they cannot be as easily visualized.

Peroxisomes are especially rich in the enzyme catalase, which breaks down hydrogen peroxide and either converts it to water or uses it in the oxidation of an organic (carbon-containing) compound. H2O2 itself is present in significant numbers only because it is generated by the breakdown of a number of different compounds that peroxisomes ingest.

Peroxisomes, like mitochondria, take part enthusiastically in fatty-acid oxidation, and they probably started out as free-living primitive aerobic, or oxygen-using, bacteria. (Most free-living bacteria today can rely on anaerobic glycolysis alone.)

Role of the Peroxisome in Metabolism

Although peroxisomes also take part in biosynthesis and manufacture a number of different lipid molecules, including components of bile and cholesterol, their chief role in cell biology is catabolic. Some peroxisomes in the liver detoxify the ethyl alcohol in beverages by removing electrons from the alcohol and placing them elsewhere, which is the definition of oxidation.

Some enzymes in peroxisomes break down the long-chain fatty acids that result from the metabolism of triglycerides in the diet and from other sources. This is a vital function because an accumulation of these fatty acids can be toxic to neural tissue. The enzymes required for these reactions have to be taken up from the cytoplasm after being synthesized as polypeptide chains by ribosomes on the endoplasmic reticulum.

The Peroxisome as an Antioxidant

Reactive oxidative species, or ROS, are chemicals that are inevitably formed in the use of energy for necessary cellular processes, much like car exhaust is an inescapable product of gas-burning automobiles.

As their name implies, they are oxidizing agents, as as such they can contribute to various types of cell damage if not maintained at relatively low concentrations. Yet these oxidative reactions are vital to life itself; ROS can be harmful, but ignoring the molecules serving as their precursors is not an option.

Thus, one area of research interest is examining how peroxisomes achieve a balance between the production of needed ROS, and the clearance of these substances and the enzymes that produce them, before they rise to levels that can do more harm than good to the peroxisome and to the cell as a whole.

Peroxisomes and Nerve Function

All animal cells include peroxisomes, but they play an especially important role in nerve cells, including those in the brain. This is because peroxisomes serve as a site of the synthesis of plasmalogens. These are a special type of phospholipid molecule that are incorporated into the plasma membranes of cells in certain tissues, including the heart and the neurons of the central nervous system.

Plasmalogens are a key component of the substance myelin, which is essential for the normal conduction of nerve impulses. Damage to myelin can lead to diseases such as multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS). Scientists aim to learn the exact connection between disorders involving peroxisome function and the progression of certain nerve disorders.

Peroxisomes and Your Liver and Kidneys

The liver and kidney are major detoxification centers; as such, these organs feature a high density of chemical reactions and a concomitantly high accumulation of potentially deleterious waste products. In the liver, peroxisomes make bile acids, with bile itself being critical to the proper absorption of fat and substances that are easily dissolved in fats, like vitamin B-12.

In the kidney, a particular protein commonly found in peroxisomes helps prevent the formation of kidney stones, or renal calculi. This is an extremely painful condition linked to calcium deposits.

Peroxisome Function in Plants

In plant cells, peroxisomes are involved in the process of photorespiration. This series of reactions serves to rid the plant of phosphoglycerate, an incidental product of photosynthesis that is not required by the plant and becomes an annoyance at significant levels.

The phosphoglycerate is converted to glycerate within peroxisomes and then returned to chloroplasts, where it can take part in the useful reactions of the Calvin cycle.

Peroxisomes also play a role in seed germination in plants. They do this by converting lipids and fatty acids in the vicinity of the nascent organism to sugars, which are a much more useful source of adenosine triphosphate, or ATP (a molecule that provides energy), for the rapidly growing and maturing seed products.

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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