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
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
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
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 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.
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
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’.