PEROXISOME and GLYOXYSOMES
A peroxisome is a membrane-bound organelle (formerly known as a microbody), found in
the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles.
Frequently, molecular oxygen serves as a co-substrate, from which hydrogen peroxide (H2O2)
is then formed.
Peroxisomes owe their name to hydrogen peroxide generating and scavenging activities.
They perform key roles in lipid metabolism and the conversion of reactive oxygen species.
They also contain approximately 10% of the total activity of two enzymes (Glucose-6-
phosphate dehydrogenase and 6-Phosphogluconate dehydrogenase) in the pentose phosphate
pathway , which is important for energy metabolism.
Peroxisomal functions include the glyoxylate cycle in germinating seeds
("glyoxysomes"), photorespiration in leaves, glycolysis in trypanosomes ("glycosomes"),
and methanol and/or amine oxidation and assimilation in some yeasts.
History
Peroxisomes (microbodies) were first described by a Swedish doctoral student, J. Rhodin in
1954. They were identified as organelles by the Belgian cytologist Christian de Duve in
1967, De Duve and co-workers discovered that peroxisomes contain several oxidases
involved in the production of hydrogen peroxide (H2O2) as well as catalase involved in the
decomposition of H2O2 to oxygen and water.
Peroxisome Structure
Most organelles are created by budding off of the endomembrane system, but that is not the
case with peroxisomes. Peroxisomes are created by taking in proteins and lipids from
the cytoplasm of the cell. You remember the cytoplasm. It's the gel-like fluid in the cell in
which the organelles are suspended. The influx of proteins and lipids makes the peroxisome
grow in size. Once the peroxisome is large enough, it divides through fission to create two
peroxisomes. Peroxisomes are created in this manner because they don't have their own DNA
to give instructions on making the proteins that they need to function. So peroxisomes must
be created already containing all the proteins that they need.
Since peroxisomes are not created from the endomembrane system, they are bound by a
single membrane instead of a double membrane like most organelles. The end result is a
single membrane-bound organelle with lipids and proteins that act as enzymes.
Functions
A major function of the peroxisome is the breakdown of very long chain fatty
acids through beta oxidation.
In animal cells, the long fatty acids are converted to medium chain fatty acids, which
are subsequently shuttled to mitochondria where they eventually are broken down to
carbon dioxide and water.
In yeast and plant cells, this process is carried out exclusively in peroxisomes.
In higher plants, peroxisomes contain also a complex battery of antioxidative enzymes
such as superoxide dismutase, the components of the ascorbate-glutathione cycle, and
the NADP-dehydrogenases of the pentose-phosphate pathway. It has been
demonstrated that peroxisomes generate superoxide (O2•−) and nitric oxide (•NO)
radicals.
There is evidence now that those reactive oxygen species including peroxisomal
H2O2 are also important signalling molecules in plants and animals and contribute to
healthy ageing and age-related disorders in humans.
GLYOXYSOMES
Glyoxysomes are specialized peroxisomes found in plants (particularly in the fat storage
tissues of germinating seeds) and also in filamentous fungi. Seeds that contain fats and oils
include corn, soybean, sunflower, peanut and pumpkin.
As in all peroxisomes, in glyoxysomes the fatty acids are oxidized to acetyl-CoA by
peroxisomal β-oxidation enzymes. When the fatty acids are oxidized hydrogen peroxide
(H2O2) is produced as oxygen (O2) is consumed. Thus the seeds need oxygen to germinate.
Besides peroxisomal functions, glyoxysomes possess additionally the key enzymes
of glyoxylate cycle (isocitrate lyase and malate synthase) which accomplish the glyoxylate
cycle bypass.
Thus, glyoxysomes (as all peroxisomes) contain enzymes that initiate the breakdown of fatty
acids and additionally possess the enzymes to produce intermediate products for the synthesis
of sugars by gluconeogenesis. The seedling uses these sugars synthesized from fats until it is
mature enough to produce them by photosynthesis. Plant peroxisomes also participate
in photorespiration and nitrogen metabolism in root nodules.
The key difference between glyoxysomes and peroxisomes is that glyoxysomes are present
only in plant cells and filamentous fungi while peroxisomes are present in almost all
eukaryotic cells. Glyoxysomes are abundant in plant cells of germinating seeds
while peroxisomes are abundant in liver and kidney cells.
