An organelle that breaks down the proteins the cell doesn't need. It uses hydrogen peroxide and catalase to achieve this. Unlike mitochondria or chloroplasts it doesn't have any DNA or any means to transcribe proteins, so it has to get all its proteases and so on from the host cell.

It is possible that this is an ancient remnant of a symbiotic bacterium that has gradually lost the ability to look after itself. However, since it has no genetic material, there is no way of looking at its phylogeny to test this. It has been shown, in leaves, that plant glyoxysomes are converted to peroxisomes during greening. Photorespiration is carried out in plant peroxisomes and the necessary enzymes are transported into the organelle. At the same time, the glyoxysomal enzymes are broken down. This suggests that the two distinct organelles are actually different developmental stages of the same microbody; tailored by the cell for separate purposes.

More specifically, the major enzymes in peroxisomes break down purines, which are of of the two major types of nitrogenous bases.

In plants, the peroxisomes are where a specific set of photosynthesis-related reactions take place when cells containing a high quantities of oxygen are exposed to sunlight. These reactions result in the production of hydrogen peroxide, which would be quite corrosive to the cell if it were not for the presence of yet another enzyme in the peroxisomes that immediately breaks the H2O2 down into harmless water and oxygen molecules.

In human beings, peroxisomes play an important role in the body's ability to break down fatty acids, the formation of cholesterol, the production of bile acids, and the production of lipids that help form cell membranes (including myelin, which sheaths the nerves). Peroxisomes are also important in preventing the excess production of oxalate, which can cause kidney stones.

Thus, when a person's peroxisomes fail to function properly, a variety of symptoms can develop. Diseases such as malonic aciduria, pseudo-Zellweger syndrome, Refsum disease, and X-linked adrenoleukodystrophy are all related to peroxisome failure.

Information sources: Biology by Helena Curtis and

What are Peroxisomes?

Peroxisomes are microbodies which range in size from 0.5-1.5 ┬Ám and thus are about the size of lysosomes. Like these organelles, peroxisomes contain enzymes bounded by a single membrane. These enzymes work by dehydrogenating their substrate and combining the hydrogen thus removed with oxygen, to produce hydrogen peroxide, H2O2. This substance is toxic to the cell, so peroxisomes also contain an enzyme which is capable of converting hydrogen peroxide into water and oxygen. In the same way as lysosomes, peroxisomes are excellent examples of compartmentalized structure relating to cellular function. Peroxisomes reproduce by binary fission and are thought to have descended from bacteria.

Peroxisomal Targeting Signals

The enzymes and other proteins which are present in peroxisomes are synthesized in the cytoplasm and each contains a peroxisomal targeting signal (PTS). This PTS binds to a receptor molecule on the peroxisome which transports the protein into the microbody.

So far, researchers have identified two peroxisomal targeting signals:

  • a 9-amino acid sequence at the N-terminal of the protein
  • a tripeptide at the C-terminal of the protein Each of these has its own receptor which selectively transports it into the peroxisome.

  • Functions of peroxisomes in the liver

  • Breakdown of excess fatty acids through oxidation
  • Breakdown of hydrogen peroxide, a toxic product of fatty-acid oxidation, a process which is catalysed by the enzyme catalase
  • The synthesis of cholesterol
  • The synthesis of bile acids
  • The synthesis of the lipids used to make myelin
  • Breakdown of excess purines (nitrogenous bases such as AMP) to uric acid

  • Peroxisome Disorders

    There are several rare, inherited disorders of peroxisome function which occur in humans, most of which involve mutations in the DNA coding for certain enzymes found within the peroxisomes. For example X-linked adrenoleukodystrophy (X-ALD) is a disorder resulting from a failure to produce the enzymes which metabolize fatty acids, the end result of which is a deterioration in the myelin sheaths of neurons. Because the DNA coding for these enzymes is located on the X-chromosome, the disease is found predominently in young boys who have only a single copy of the X-chromosome.

    Some diseases, such as Zellweger syndrome, result from a failure to produce functional peroxisomes. This disorder results from the inheritance of two recessive genes for coding for one of the receptors (PXR1) needed to transport proteins into the peroxisome.

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