PEROXISOMES: CHARACTERISTICS, FUNCTIONS, STRUCTURE, BIOGENESIS - BIOLOGY - 2025

The peroxisomes, also known as microbodies, are small organelles, very similar to the lysosomes, which are suspended in the cytosol of most eukaryotic cells.

Just as the human body has organs that perform different functions to keep it alive, cells have them too and they are what we call "organelles" or "organelles".

Schematic of a microbe cell showing a peroxisome (peroxisome), a mitochondrion (mitochondriom) and the nucleus (nucleus) (Source: CNX OpenStax / CC BY (https://creativecommons.org/licenses/by/4.0) via Wikimedia Commons)

Just as the heart pumps blood to the rest of the body, the nose and lungs are used to breathe, the stomach receives food and begins with its digestion, and the brain is in charge of coordinating everything (to give a few examples). Organelles are essential for many of the functions of cells.

Among some of the cellular organelles are peroxisomes, which were described in 1960 by Christian René de Duve, the same researcher who developed subcellular fractionation techniques to separate the different cellular organelles based on their density.

de Duve shared, in 1974, the Nobel Prize in Physiology and Medicine with Albert Claude and George Palade thanks to their work with these techniques and the discovery of peroxisomes.

The name of these organelles derives from the internal production of hydrogen peroxide (H ₂ O ₂), a by-product of the oxidation-reduction reactions that occur in them and which is potentially toxic to cells (it can react with many other molecules), so it is quickly degraded.

In a cell there can be up to 500 peroxisomes "swimming" in the cytosol, but the number and size of these organelles depends not only on the type of cell in question, but on the physiological state of the cell and the environment that surrounds it.

General characteristics of peroxisomes

There are many characteristics that peroxisomes have that make them similar to other cellular organelles and, at the same time, very different. Here is a short list of some of the most important:

- They are small organelles surrounded by a simple membrane, which separates them from the rest of the molecules and organelles in the cytosol.

- Much of what is inside them, especially proteins and enzymes, are synthesized in the cytosol of the cell to which they belong by means of free ribosomes, which are protein complexes capable of mediating the translation of messenger RNA (mRNA) from the nucleus and derived from the transcription of a given gene.

- They do not have their own genome, that is, inside there is no DNA or the machinery necessary for its processing (replication, transcription and translation, for example).

- They are multiplied by division.

- Inside you can find up to 50 different digestive enzymes and their secondary products (dangerous for cells).

- Their size and number can vary greatly from one cell to another, as they depend on intracellular conditions (they are inducible) and the type of cell.

Features

Peroxisomes fulfill different functions within a cell, many of them related to the enzymes that are inside it.

- Oxidative reactions

Inside peroxisomes, many oxidation-reduction reactions occur, this is the exchange of electrons between one compound and another, generally catalyzed by proteins with enzymatic activity (enzymes).

These oxidation-reduction reactions in peroxisomes commonly produce hydrogen peroxide (H ₂ O ₂), a compound that is harmful to cells.

However, inside peroxisomes there is an enzyme called catalase, which is responsible for breaking down hydrogen peroxide to form water or using it to oxidize other compounds.

The ability to contain these reactions within is closely related to the other functions that these cellular organelles perform, since the metabolic degradation of many molecules implies their oxidation.

Without the oxidative reactions of peroxisomes, the accumulation of compounds like long-chain fatty acids, for example, could cause considerable damage to nerve cells in the brain.

- Energy metabolism

Peroxisomes participate in the production of ATP, which is the main energy "currency" of a cell.

One of the ways they do this is by breaking down fatty acids (what fats and many lipids are made of), digesting ethanol (a type of alcohol) and amino acids (the "building blocks" that make up proteins), and so on.

In animal cells, most of the fatty acids are degraded in the mitochondria and a small portion is processed in the peroxisomes, but in yeast and plants this function is practically exclusive to peroxisomes.

- Biosynthesis

Peroxisomes also function in the production of molecules that are part of cell membranes. These molecules are known as plasmallogens and are a very important type of lipid for the brain and cardiac (heart) cells of humans and other mammals.

Other lipids synthesized in peroxisomes and with the participation of the endoplasmic reticulum (another very important cellular organelle) are cholesterol and dolichol, essential for the functioning of cells.

In many mammalian animals, for example, the peroxisomes of the liver cells also participate in the synthesis of bile acids, which are derived from cholesterol and are very necessary for the digestion of the fats contained in food that are processed in the stomach and then in the small intestine.

Structure

Peroxisomes are membranous organelles, but unlike the membranes seen in other organelles such as mitochondria and chloroplasts, for example, they have a single membrane and not a double membrane system.

Its appearance is not constant, that is, it can change. However, they are usually spherical organelles that have an average diameter between 0.2 and 1 μm, that is, one millionth of a meter.

Basic diagram of the structure of a peroxisome (Source: Thuresson / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/) via Wikimedia Commons)

When these are not spherical in shape, then they can be seen as small tubules of different sizes, which are connected to each other (surely they are peroxisomes in division).

They often have a crystalline center or nucleus, which scientists describe in this way by how they look at it under the microscope, probably as a result of the huge amount of protein inside them.

Biogenesis (origin)

Although peroxisomes do not contain DNA inside, that is, they do not have their own genome, they can be divided by budding or fission.

This process depends on the amount of proteins and materials to build new membranes that they have at their disposal, which are "imported" from the cytosol.

Those who participate?

The endoplasmic reticulum is responsible for both the synthesis of the phospholipids that form the peroxisome membrane, and the synthesis of some of its proteins, this through its associated ribosomes.

Ribosomes (actually present in the cytosol as "free polyribosomes") are the ones that translate most of the proteins. These proteins can only enter the interior of peroxisomes if they have a special label or “mark”.

Without these marks, proteins cannot be recognized by other proteins on the peroxisome membrane and therefore cannot pass through.

So, if ribosomes attached to the rough endoplasmic reticulum (RER) and those that are free in the cytosol “send” enough material to the peroxisomes, they can divide in two.

Peroxisomes in animal cells

Animal cells have many peroxisomes and lysosomes, similar organelles that are responsible for "recycling" other organelles and different types of molecules of different sizes.

The cells of some animals (but not those of humans), for example, have peroxisomes capable of breaking down uric acid, which is generally a nitrogen-rich metabolic waste, the accumulation of which in the blood can have deleterious effects.

"Weird" functions

In addition to all the functions mentioned above, peroxisomes perform very particular functions in some animals. Fireflies and other insects, for example, use an enzyme in the peroxisomes of their cells to find mates and, in some cases, to locate their food.

This enzyme is known as luciferase. Luciferase helps males produce a bright "flash" of light, which can be green or yellow, and which serves to attract females of the same species.

The duration of each flash and the interval in which they appear is specific to each species, so that females can distinguish males in the dark of night. In certain species, the female also produces a flash, and in others, she emits a light that attracts the male to eat it.

Modified peroxisomes

Just as plants possess glyoxysomes, which are a type of peroxisome specialized in a specific metabolic pathway, some animal cells possess modified peroxisomes.

Kinetoplastids, a group of parasites that cause different diseases in humans and other animals, have a type of "modified peroxisome" known as a glycosome.

Glycosomes are given this name because they contain the enzymes necessary for the processing of glucose (glycolytic enzymes), as well as other enzymes that participate in other metabolic pathways to obtain energy.

Peroxisomes in plant cells

Plant cells also contain peroxisomes and these have very important functions for the functioning of plants, in addition to the functions that are shared with those of peroxisomes of other types of cells.

- Glyoxylate cycle

In seeds, for example, the peroxisomes of their cells are responsible for converting stored fats into carbohydrates, which are the raw material necessary for the development of the seedling that will germinate.

The process by which plant peroxisomes perform this function is known as the glyoxylate cycle, which is considered a variant of the Krebs cycle, which is why some texts refer to these peroxisomes as glyoxysomes.

- Photorespiration

In plants, these organelles are also involved in a process known as photorespiration, which consists of a metabolic path "contrary" to photosynthesis, since oxygen is not produced, but rather is consumed, and carbon dioxide is released without obtaining ATP..

Despite the above, this process is also known as "carbon recovery", since peroxisomes receive from chloroplasts (another organelle of plant cells) a chemical compound called glycolate, which they convert into another compound called glycine (a amino acid).

The glycine produced in plant peroxisomes is transported to the mitochondria (the organelle where respiration and the synthesis of large amounts of ATP occur). In the mitochondria, this glycine is converted to serine, another amino acid, which is returned to the peroxisome.

Serine, once in the peroxisome, is converted to glycerate and from there it is sent to the chloroplast again. All this process does not lead to the production of energy, but it does lead to the use of the carbon atoms that are attached to the glycolate.

Peroxisome diseases

There are different types of "disorders" related to peroxisomes. Generally, these disorders have to do with mutations in the genes that are involved in the biogenesis of these organelles or, even, in those genes that encode the enzymes or transport proteins.

Since they have a genetic component, these disorders are usually congenital (they are inherited from parents to children) that can have moderate or severe consequences, depending on the case.

Zellweger syndrome

This syndrome, although rare, includes some of the most severe conditions. It is characterized by the complete absence or by a considerable reduction in the number of chromosomes in the cells of the body.

The genetic mutations that cause this syndrome also cause the accumulation of compounds rich in elements such as iron and copper, and of very long chain fatty acids in the blood and other tissues such as the liver, brain and kidneys.

What are the consequences?

Young children affected by this syndrome are usually born with facial (facial) deformities and some intellectual disabilities. They can suffer from vision and hearing problems, as well as gastrointestinal and liver problems, so they usually do not live more than a year.

Other related syndromes

There are other diseases related to defects in peroxisomes. These include Neonatal Adrenoleukodystrophy (NALD, Neonatal Adrenoleukodystrophy) and Childhood Refsum Disease.

Both diseases are characterized by the late onset of symptoms, which are usually seen during childhood, so that patients can survive into early adulthood.

References

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