Peroxisome

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.

What are Peroxisomes?

Peroxisomes are microbodies which range in size from 0.5-1.5 µm 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: