Peroxisomes are ubiquitous organelles found in most known eukaryotic cells. They perform both vital catabolic and anabolic functions within cells to ensure the host’s health and survival are promoted.
Peroxisomes are a membrane-bound organelle with an electron-dense matrix, lacking an obvious internal structure. They have several functions:
- Oxidation and detoxification: Peroxisomes have the odd quality of both generating and degrading hydrogen peroxide (H²O²) in exactly the same compartment. The organelle contains many oxidative enzymes which use oxygen to remove hydrogen ions from organic substrates.
R + O² > R’ + H²O²
H²0² is removed by peroxidase, by peroxidation:
2H²O² > 2H²O + O²
Enzymes which detoxify are also present. They work in the same manner: oxidising toxic substances (such as formaldehyde, or alcohol) by peroxidation reaction:
H²O² + R’ H² > R’ + 2H²O
This detoxification is particularly prevalent in kidney cells. Accumulation of hydrogen peroxide is incredibly dangerous and can cause massive amounts of cell damage. As you get older and your peroxisomes and mitochondria become less effective, one of the damaging substances which accumulates is H²O² and is one of the main factors of aging.
- Fatty acid breakdown: Also known as beta-oxidation. Although the mitochondria is more famously associated with this particular task, the peroxisome is capable of quickly breaking down fatty acids of more than 12 carbons long: something the mitochondria has issues with. The long chains are oxidised, two carbons at a time, until they become small enough for the mitochondria to handle.
- Biosynthesis of plasmalogens: Plasmalogens are a very important class of lipid which constitutes ~80-90% of myelin (a material which wraps around the axons of neurons to increase conduction of electrical signalling). Unsurprisingly, people with peroxisome disorders tend to have defects within their nervous system.
There are many other, more minor, functions of the peroxisome, and if you are interested I do suggest reading up on them.
Unlike mitochondria, peroxisomes do not have their own genome. Therefore, all proteins are shuttled into the peroxisome after being coded for by the cell nucleus and translated and modified by the ER/Golgi. Most proteins destined for the peroxisome have a three amino acid signal sequence at their C-terminal end, recognised by soluble peroxisome receptors called PEX. PEX receptors bind the proteins in the cytosol and embed themselves in the membrane of the peroxisome, releasing the cargo inside, before returning to the cytosol. It is this mechanism of transportation which, when faulty, forms the basis of peroxisomal disorders.
The PEX3 receptor is responsible for the docking of proteins destined for the membrane, intended to become receptors themselves. In Zellweger syndrome, this PEX3 is faulty. This means other receptors can not dock within the membrane, and if there are no receptors, no cargo can be delivered. This creates ‘ghost peroxisomes’ - essentially membrane-bound bags of absolutely nothing. It is an autosomal recessive disorder characterised by leukodystrophy. Myelin is severely impaired, white matter in the brain is heavily underdeveloped, and long-chain fatty acids float freely in the blood. The condition is lethal, and patients do not usually survive past one year of age. There are no treatments available.
Photo: Peroxisomes individually fluorescently tagged in green, showing how ubiquitous they are within your cells. Other structures present: nucleus (blue), mitochondria (red).