12 julio, 2024

Peroxisomes: characteristics, functions, structure, biogenesis

Peroxisomes, also known as microbodies, are small cellular organelles, very similar to lysosomes, that are suspended in the cytosol of most eukaryotic cells.

Just as the human body has organs that fulfill different functions to keep it alive, cells also have them and they are what we call «organelles» or «organelles.»

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 its digestion, and the brain is in charge of coordinating everything (to give a few examples), so the organelles are essential for many of the functions of cells.

Some of the cell organelles include peroxisomes, which were described in 1960 by Christian René de Duve, the same researcher who developed subcellular fractionation techniques to separate the different cell 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 (H2O2), a by-product of the oxidation-reduction reactions that occur in them and that is potentially toxic to cells (it can react with many other molecules), for which is rapidly 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 also on its physiological state and the environment that surrounds it.

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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 brief list summarizing 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, 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 specific gene.

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

– Multiply 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.

functions

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

– 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 (H2O2), a compound that is harmful to cells.

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

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

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

– Energy metabolism

Peroxisomes are involved 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), etc.

In animal cells most of the fatty acids are broken down in mitochondria and a small portion is processed in 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 plasmalogens 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 cell organelle) are cholesterol and dolichol, essential for cell function.

In many mammalian animals, for example, the peroxisomes of the liver cells also participate in the synthesis of bile acids, which are derivatives of cholesterol and very necessary for the digestion of the fats contained in the food that is 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 rather than 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.

When these do not have a spherical shape, then they can be seen as small tubules of different sizes, which are connected to each other (they are probably dividing peroxisomes).

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

Biogenesis (origin)

Although peroxisomes do not contain DNA inside, that is, they do not have their own genome, they can divide 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 in charge of 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 what translate most proteins. These proteins can only enter the interior of peroxisomes if they have a special tag or “mark”.

Without these marks, the proteins cannot be recognized by other proteins in the peroxisome membrane and therefore cannot cross it.

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

peroxisomes in animal cells

Animal cells have many peroxisomes and lysosomes, similar organelles that are responsible for the «recycling» of 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 whose accumulation in the blood can have detrimental effects.

“Rare” features

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 a mate and, in some cases, to locate food.

This enzyme is known as luciferase. Luciferase helps males produce a bright «flash» of light that can be green or yellow, which helps them attract females of the same species.

The duration of each flash and the interval in which they appear is specific to each species, so the females can distinguish the 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 him.

modified peroxisomes

Just as plants have glyoxysomes, which are a type of peroxisome specialized in a specific metabolic pathway, some animal cells have 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 receive this name because the enzymes necessary for glucose processing (glycolytic enzymes) are enclosed inside them, 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 the 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 necessary raw material 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 route «opposite» to photosynthesis, since oxygen is not produced, but rather 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.

The serine, once in the peroxisome, is converted to glycerate and from there it is…

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