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:
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.