Peroxisomes are small, membrane-bound structures inside most human cells. They help cells break down certain fats, process potentially harmful molecules, and carry out several chemical reactions that are essential for normal metabolism.
Although peroxisomes are much smaller and less widely discussed than mitochondria, they perform specialized jobs that other cell compartments cannot handle as efficiently. Their importance becomes especially clear in disorders caused by defects in peroxisome formation or function.
What is a peroxisome?
A peroxisome is a tiny organelle surrounded by a single membrane. An organelle is a specialized structure within a cell that performs particular functions.
Peroxisomes are found in nearly all eukaryotic cells, including human cells. They contain enzymes—proteins that speed up chemical reactions—that allow them to carry out specific metabolic processes.
The number and enzyme content of peroxisomes vary between cell types. Cells that perform especially active lipid metabolism, such as liver and kidney cells, contain substantial numbers of peroxisomes.
Peroxisomes do not contain their own DNA. Instead, the proteins they need are encoded by genes in the cell’s nucleus and then transported into the organelle.
What do peroxisomes do?
Peroxisomes have several related functions, but their most important roles involve lipid metabolism and chemical detoxification.
They break down certain fatty acids
One of the best-known functions of peroxisomes is the breakdown of fatty acids through a process called beta-oxidation.
Fatty acids are molecules that cells use for energy and as building materials. Some fatty acids, however, are too long or chemically specialized to be handled efficiently by mitochondria alone.
Peroxisomes help shorten certain very-long-chain fatty acids. During this process, enzymes progressively remove two-carbon units from the fatty acid.
Unlike mitochondrial fatty-acid oxidation, which is closely tied to ATP production, peroxisomal beta-oxidation primarily prepares fatty acids for further metabolism. The shortened products can then be transferred to other cellular pathways, including mitochondrial metabolism.
Peroxisomes also participate in the metabolism of certain branched-chain fatty acids and other unusual lipid molecules that require specialized processing.
They help control hydrogen peroxide
The name peroxisome reflects another important feature of the organelle: its involvement in reactions that produce and break down hydrogen peroxide.
Hydrogen peroxide is a reactive molecule. Cells produce it naturally during several metabolic reactions, but excessive amounts can damage proteins, lipids, and DNA.
Peroxisomes contain catalase, an enzyme that converts hydrogen peroxide into water and oxygen. This provides an important way to control hydrogen peroxide generated within the organelle.
Peroxisomes also contain other enzymes involved in oxidation reactions. These reactions can be chemically useful but may generate reactive byproducts, so keeping them compartmentalized and tightly regulated helps protect the rest of the cell.
They help make important lipids
Peroxisomes are involved not only in breaking down fats but also in making certain lipids.
One particularly important group is the plasmalogens, specialized phospholipids found in cell membranes. Plasmalogens are especially abundant in tissues such as the brain and heart and are important components of cellular membranes.
The early steps of plasmalogen production occur in peroxisomes before the pathway continues elsewhere in the cell. If peroxisomes cannot perform these steps properly, plasmalogen production can be severely impaired.
Peroxisomes also participate in pathways involved in the metabolism of bile-acid-related molecules and other specialized lipids.
How are peroxisomes different from mitochondria?
Peroxisomes and mitochondria both participate in metabolism, and both can oxidize fatty acids, but they are not interchangeable.
| Feature | Peroxisomes | Mitochondria |
|---|---|---|
| Membrane | One surrounding membrane | Two surrounding membranes |
| Own DNA | No | Yes |
| Major roles | Specialized lipid metabolism, oxidation, detoxification | ATP production, cellular respiration, many metabolic pathways |
| Fatty-acid oxidation | Especially important for very-long-chain and certain unusual fatty acids | Major pathway for many fatty acids |
| Hydrogen peroxide | Generated in some reactions and broken down by catalase | Reactive oxygen species are also generated, but mitochondria do not contain catalase as their principal hydrogen-peroxide defense |
A useful distinction is that mitochondria are central to energy production, whereas peroxisomes specialize in particular oxidative and lipid-processing reactions. The two organelles cooperate rather than simply performing the same job.
How do peroxisomes handle potentially harmful molecules?
Peroxisomes are sometimes described as cellular detoxification centers, although that description is incomplete.
Their enzymes carry out oxidation reactions that can alter or break down various molecules. Catalase and other antioxidant systems then help manage reactive products generated by these reactions.
The liver contains many peroxisomes because liver cells perform extensive metabolic processing. Peroxisomal enzymes contribute to the metabolism of fatty acids and other compounds, including some molecules that can otherwise be harmful when they accumulate.
Peroxisomes therefore contribute to cellular protection, but they are only one part of the body’s overall system for handling reactive and potentially toxic substances.
How are peroxisomes made?
Peroxisomes are unusual among organelles because they do not arise from a simple process of copying an existing organelle in the same way that mitochondria and chloroplasts replicate their genomes and structures.
Peroxisomal membranes and proteins are assembled through a coordinated cellular process. Specialized proteins called peroxins, encoded by PEX genes, help create peroxisomes, import their proteins, and maintain their functions.
Most peroxisomal proteins are made on free ribosomes in the cell and then delivered to the peroxisome. Many contain specific molecular signals that allow the cellular transport machinery to recognize them as peroxisomal proteins.
This protein-import system is essential because peroxisomes need a constantly replenished supply of enzymes to perform their metabolic reactions.
What happens when peroxisomes do not work properly?
Defects in peroxisomal function can cause substances that normally would be metabolized to accumulate while essential products become deficient.
Some inherited disorders affect the formation or maintenance of peroxisomes as a whole. These are known as peroxisome biogenesis disorders. Others result from defects in individual peroxisomal enzymes or transport proteins.
Because peroxisomes perform several different jobs, the effects can involve multiple organs. The nervous system, liver, kidneys, eyes, and other tissues may be affected, depending on the particular disorder.
One example is Zellweger spectrum disorder, a group of inherited conditions caused by defects in peroxisome biogenesis. Severe forms can cause profound problems involving the brain, liver, and other organs.
Another example is X-linked adrenoleukodystrophy, in which impaired peroxisomal processing leads to accumulation of very-long-chain fatty acids. Depending on the form of the disease, the nervous system and adrenal glands can be affected.
These disorders demonstrate why peroxisomes are more than cellular housekeeping compartments: their metabolic activities are essential to normal human physiology.
Why are peroxisomes important?
Peroxisomes occupy a specialized position in cellular metabolism. They help process fatty acids that require particular oxidative pathways, participate in the production of important membrane lipids, and manage hydrogen peroxide generated by oxidation reactions.
Their work is closely coordinated with other parts of the cell. A fatty acid may begin being processed in a peroxisome and then undergo further metabolism elsewhere. Likewise, lipid products made through peroxisomal pathways can become components of cellular membranes or precursors for other molecules.
The result is a division of labor among organelles. Peroxisomes handle a set of chemically demanding reactions in a controlled compartment, helping the cell obtain useful metabolic products while limiting exposure to reactive intermediates.
