2. Microbody
organelles
found in the
cytoplasm
PEROXISOMES
in higher
eukaryotic cells.
GLYCOSOMES in
kinetoplastids
GLYOXYSOMES
in plant cells
• Related microbody organelles that perform differing
metabolic functions tailored to their cellular
environments.
• The close evolutionary relationship of these
organelles is most clearly evidenced by the
conservation of proteins involved in matrix protein
import and biogenesis.
3. Oxidation reactions leading to the production of hydrogen peroxide. Because hydrogen peroxide is
harmful to the cell, peroxisomes also contain the enzyme catalase, which decomposes hydrogen
peroxide either by converting it to water or by using it to oxidize another organic compound.
Peroxisomes are involved in lipid biosynthesis. In animal cells, cholesterol and Dolichol are
synthesized in peroxisomes as well as in the endoplasmic reticulum
In plants, peroxisomes in seeds are responsible for the conversion of stored fatty acids to
carbohydrates, which is critical to providing energy and raw materials for growth of the
germinating plant.
Peroxisomes in leaves are involved in photorespiration, which serves to metabolize a side product
formed during photosynthesis.
MAIN FUNCTIONS OF PEROXISOMES
4. OXIDATION REACTIONS OF PEROXISOMES
A variety of substrates are broken down by such oxidative reactions in peroxisomes, including uric acid, amino acids, and fatty acids. The
oxidation of fatty acids is a particularly important example, since it provides a major source of metabolic energy. In animal cells, fatty
acids are oxidized in both peroxisomes and mitochondria, but in yeasts, fatty acid oxidation is restricted to peroxisomes.
Beta oxidation of fatty acids occurs in the peroxisomes (a eukaryotic organelle) in addition to the mitochondria, though in a modified
form.
A primary function of peroxisomal beta oxidation is to shorten very long chain fatty acids so they can be degraded in the mitochondrion.
Peroxisomal oxidation has advantages over mitochondrial oxidation - fatty acids can diffuse across the peroxisomal membrane. After
very long chain fatty acids are partly degraded, they are attached to carnitine for transport to mitochondria.
5.
6. Peroxisomes contain oxidative enzymes, such as catalase, D-amino acid oxidase, and uric acid oxidase.
Certain enzymes within the peroxisome, by using molecular oxygen, remove hydrogen atoms from specific organic
substrates in an oxidative reaction, producing hydrogen peroxide (H2O2, itself toxic)
Catalase, another peroxisomal enzyme, uses this H2O2 to oxidize other substrates, including phenols, formic acid,
formaldehyde, and alcohol, by means of the peroxidation reaction, thus eliminating the poisonous hydrogen peroxide.
This reaction is important in liver and kidney cells, where the peroxisomes detoxify various toxic substances that enter the
blood. About 25% of the ethanol humans drink is oxidized to acetaldehyde in this way. In addition, when excess H2O2
accumulates in the cell, catalase converts it to H2O through this reaction
DETOXIFICATION OF REACTIVE OXYGEN SPECIES
7. LIPID SYNTHESIS IN PEROXISOMES
• Plasmalogens are relatively rare in some tissues but form a significant fraction of the membranes in nervous tissue. Some
of the steps in plasmalogen biosynthesis are localized in peroxisomes, and some peroxisomal disorders impair plasmalogen
biosynthesis.
• Cholesterol: There is now considerable evidence that peroxisomes not only have a role in cholesterol oxidation but also in
cholesterol biosynthesis.
Acetyl CoA Acetoacetyl CoA
THIOLASE
Mevalonate Mevalonate-5-Phosphate
MEVALONATE
KINASE
3-Hydroxy-3-
methylglutaryl CoA
HMG
REDUCTASE
Initiation steps in cholesterol synthesis
• Bile Acids: Peroxisomes play an important role in the biosynthesis of bile acids because a peroxisomal β-oxidation step is
required for the formation of the mature C24-bile acids from C27-bile acid intermediates. In addition, de novo synthesized
bile acids are conjugated within the peroxisome.
9. GLYOXYLATE CYCLE IN PEROXISOMES
(occurs in plants)
• Plants are capable of synthesizing carbohydrates from fatty acids via the glyoxylate cycle,
which is a variant of the citric acid cycle.
• The Glyoxylate cycle provides the means to convert C2-units to C4-precursors for
biosynthesis, allowing growth on fatty acids and C2-compounds.
• The operation of the Glyoxylate cycle requires transport of several intermediates across
the peroxisomal membrane.
Glyoxysome
reactions
11. GLYCOSOMES OF KINETOPLASTIDS
The glycosome is a membrane-enclosed organelle that contains the glycolytic enzymes.
Found in a few species of protozoa including the Kinetoplastida which included the suborders Trypanosomatina and
Bodonina, and Leishmania.
The organelle is bounded by a single membrane and contains a dense proteinaceous matrix. It is believed to have
evolved from the peroxisome.
There has been a variety of evidence found biochemically to give evidence that glycosomes are present in cells. In
the organelle that is assumed to be a glycosome, numerous proteins are found. These include glycogen synthase,
phosphorylase, and branching and de-branching enzymes for glycogen. All of these are regulatory enzymes that are
needed in glycogen synthesis.
12. SOME OTHER FUNCTIONS OF PEROXISOMES
• 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 fibers in the brain.
• In normal human hepatocytes, the intermediary-metabolic enzyme alanine-glyoxylate aminotransferase (AGT) is located
within the peroxisomes. This 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.
• 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.
• A 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.
13. ISOLATION OF PEROXISOMES
Tissue culture is homogenised. Kidney cells and hepatocytes contain
large peroxisomes as compared to those found in fibroblasts or muscle
cells. Isolated peroxisomes can be used to study fatty acid oxidation,
amino acid metabolism lipid synthesis etc.
Serial centrifugations at 1000g, 2000g and 25000g remove cell debris,
nuclei, mitochondria and lipids to obtain a crude peroxisomal fraction
(lysosomes, ER, peroxisomes, light mitochondria)
Density gradient ultra-centrifuge to obtain peroxisomal fraction at the
bottom of the tube.
Degree of purification of peroxisomes can be evaluated by comparing
concentrations of catalase (peroxisome marker) and cytochrome C
oxidase (mitochondrial marker)
