Peroxisome

PEROXISOME - the organelle behind the film 'Lorenzo's Oil'
Peroxisomes are small rounded organelles found free floating in the cell cytoplasm. These structures contain at least 50 enzymes and are separated from the cytoplasm by a lipid bilayer single membrane barrier. They are called peroxisomes because they all produce hydrogen peroxide.

There is a good and very readable article entitled ‘Parsing Peroxisomes’, about Lorenzo’s oil and peroxisomes in the Bulletin of the Howard Hughes Medical Institute (HHMI) at the following website: http://www.hhmi.org/bulletin/feb2008/features/parsing.html
If this URL does not connect properly go to basic URL and then look in HHMI Bulletin, February 2008 issue for article entitled ‘Parsing Peroxisomes’.

Breaking down
The enzymes in peroxisomes break down long chain fatty acids by the process of oxidation. If allowed to accumulate these fatty acids can damage structures such as the myelin 'insulation' sheath surrounding nerve fibres in the brain. (This is the 'Lorenzo's Oil' connection - see later)

Enzymes in peroxisomes also break down uric acid and amino acids. The decomposition of fatty acids by peroxisomes to the chemical acetyl CoA produces a great deal of metabolic energy and supplements that produced by mitochondria.

The main chemical produced by oxidation in peroxisomes is the very cytotoxic (cell toxic) hydrogen peroxide. Fortunately peroxisomes produce copious amounts of the enzyme catalase and this helps break down hydrogen peroxide to water and oxygen. Peroxisomes are major users of oxygen and the oxygen produced from hydrogen peroxide is used within the organelle.

One peroxisomal protein is involved in preventing excess quantities of the chemical 'oxalate' from building up. A type of kidney stone is produced when oxalate joins with calcium to produce calcium oxalate. The proper functioning of this enzyme is therefore important.

Peroxisomes can vary in size and abundance
Peroxisomes can vary in size and adapt to changing situations. When yeast cells are grown on a sugar base small peroxisomes are produced. An alcohol containing base causes large peroxisomes to be produced and the number of peroxisomes can grow to occupy half the volume of the cell.

Peroxisomes are particularly abundant in organs such as liver where lipids are stored, broken down or synthesised

Building up
Peroxisomes produce chemicals as well as breaking them down. They make cholesterol in animal cells and peroxisomes in liver cells produce bile acids. They also contain the enzymes for making phospholipids, and a group of chemicals called plasmalogens, found in heart and brain tissue.

Peroxisomes in plants
Peroxisomes present in germinating seeds convert fatty acids and lipids to sugars for metabolism. This metabolic cycle is called the glyoxylate cycle and the specialised peroxisomes in which it takes place are called a glyoxysomes.

Peroxisomes are also involved in the process of photorespiration connected with photosynthesis. This is complicated but it is basically a side reaction to photosynthesis in which 'carbon recovery' takes place. Peroxisomes receive a chemical called glycolate from chloroplasts. They turn this into another chemical called glycine. This is then sent to mitochodria, which acts as a sub-contractor. In mitochondria it is turned into serine and passed back to the peroxisome where it is turned into glycerate and then sent to the chloroplast. There appears to be no obvious energy gain but it is thought that this pathway is a carbon recovery system.

The lighter side of peroxisomes - a peroxisomal enzyme helps fireflies find a mate or a meal!
One of the enzymes found in peroxisomes from fireflies is called luciferase. This helps some males produce a bright flash of green/yellow light to attract females. The duration of the flash and the interval between the flashes is species specific and easily seen by females flying at night. In some species the female will flash in reply.
In another species the female emits light to attract a male but on landing the male finds he becomes a meal rather than a mate!

The enzyme luciferase is now used extensively in cell biology.

The darker side of peroxisomes
Have you heard of the film 'Lorenzo's Oil'? This movie is based on the story of a boy who suffered from an inherited single enzyme deficiency disorder called X-linked aldrenoleukodystrophy (ALD).

Peroxisomes in the cells of boys who have this disorder are unable to oxidise long chain fatty acids. These fatty acids then accumulate in the brain where they can destroy the myelin sheath 'insulation' around nerve cells. Fortunately the inherited condition ALD is fairly rare, but it is an example of one of several inherited conditions that cause the absence of a single enzyme in the peroxisome and prevent that organelle functioning properly. ALD manifests itself in boys in mid-childhood and usually leads to death within a few years. So what happened to the boy in the film? - You will have to view the video.

Another rare but fatal disorder of peroxisome molecular biology is Zellweger Syndrome. It is an inherited condition in which peroxisomal enzymes produced in the cytoplasm are unable to cross the membrane barrier and enter the matrix of the peroxisome. In this condition peroxisomes are present but only as 'ghost' or empty organelles.

Peroxisomes divide
Peroxisomes are capable of dividing but they do not possess any genetic material. Whether they divide or not depends on the amount of protein and phospholipid material they receive from the ribosomes free floating in the cytoplasm and from those on the endoplasmic reticulum.
If the peroxisomes are well supplied they will increase in size and then divide into two. The proteins required are supplied by ribosomes as complete polypeptide chains and with a destination or security label attached. Without the appropriate peroxisomal target signal or label, access to the inside of the peroxisome (the matrix) will be denied.
With the correct label the protein is recognised by a receptor and guided through a translocation complex to the inside of the organelle. The detailed way in which this works has not yet been elucidated.

SUMMARY

Look at almost any traditional diagram of a cell and you will probably find peroxisomes omitted. If they are included they will probably be shown as small (which they are) and fairly insignificant features (which they are not).

• It is now known that the presence of working peroxisomes is essential for the normal functioning of the cell.
• Peroxisomes carry out the very important oxidation of excess quantities of long chain fatty acids. The accumulation of these acids presents a danger.
• Peroxisomes also break down uric acids and amino acids.
• Peroxisomes contain more than 50 enzymes including copious amounts of catalase. It is catalase that breaks down the very toxic hydrogen peroxide to water and oxygen.
• Peroxisomes produce and export to the cytoplasm cholesterol and an important a group of phospholipids called plasmalogens that are found in brain and heart tissue. Energy released from the oxidation of fatty acids is also exported to the rest of the cell.
• In germinating seeds specialist peroxisomes called glyoxysomes convert fatty acids and lipids to sugars.
• Several fairly rare inherited diseases cause peroxisome malfunction and can lead to death.
• There is a good and very readable article entitled ‘Parsing Peroxisomes’, about Lorenzo’s oil and peroxisomes in the Bulletin of the Howard Hughes Medical Institute (HHMI) at the following website: http://www.hhmi.org/bulletin/feb2008/features/parsing.html
  If this URL does not connect properly go to basic URL and then look in HHMI Bulletin, February 2008 issue for article entitled ‘Parsing Peroxisomes’.