3. The Discovery of Glyoxysomes: the Work of
Harry Beevers(in 1961)
They analyzed the linear
sucrose gradients of endosperm
homogenates and showed that
the glyoxylate cycle.
Enzymes were found in an
organelle fraction that was not
mitochondria .
Beevers and Breidenbach
called these new organelles
glyoxysomes.
Beevers and their postdoctoral
fellow was Bill Breidenbach.
4. Glyoxysomes are specialized peroxisomes found in
plants (particularly in the fat storage tissues of
germinating seeds) and also in filamentous fungi.
glyoxysomes possess the key enzymes of glyoxylate
cycle (isocitrate lyase and malate synthase).
INTRODUCTION OF GLYOXYSOMES
Glyoxysomes are found in contact with lipid bodies in
cotyledons or endosperm where fatty acids are being
converted to carbohydrate (sugars) during germination.
5. 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.
Glyoxysomes also participate in photorespiration and
nitrogen metabolism in root nodules.
6. Succinate produced in glyoxysomes is ultimately
converted to sucrose in the cytosol.
It is presumed that presence of glyoxysomes in
senescent organs is in response to the mobilization of
membrane lipids.
7. Glyoxysomes have the following characteristics:
(1) They have a single membrane.
(2) They have high equilibrium density in sucrose
gradient centrifugation.
(3)Their matrix (internal content) is finely granular.
CHARACTERISTICS OF GLYOXYSOMES
9. Glyoxylate cycle
The glycosylate cycle a variation of the tri-
carboxylic acid cycle, is an anaerobic pathway
occurring in plant, bacteria, fungi and protists.
The glyoxylate cycle centres on the conversion of
acetyl-coA to succinate for the synthesis of
carbohydrate.
Glyoxylate cycle allows cell to naturalise simple
carbon compound as a carbon source when
complex source such as glucose is not available.
10.
11. It is an anabolic pathway occurring in plants, and
several microorganisms but not in animals.
The pathway occurs only in glyoxysome.
The enzymes common to the TCA cycle and the
glyoxysomes are isoenzymes, one is specific to
mitochondria and the other is to glyoxysomes.
The glyoxylate cycle allows plants to use acetyl-CoA
derived from β-oxidation of fatty acids for carbohydrate
synthesis (use fats for the synthesis of carbohydrates).
Acetyl-CoA is totally oxidized to CO2
12. -oxidation occurs in mitochondria
and peroxisomes in mammals, but
exclusively in the peroxisome in plants
and yeast.
13. Function
This cycle allows seeds to use lipid as a source of energy
to form shoot and root during germination.
The lipid stores of germinating seeds are used for the
formation of the carbohydrates that fuel the growth and
development of the organisms.
Glyoxysomes also functions in photorespiration and
nitrogen fixation.
14. Role in Gluconeogenesis
Fatty acid from lipid are commonly used as an energy
source by vertebrates as fatty acid degraded through beta
oxidation into acetate molecule.
This acetate, bound to the active thiol group of co-A,
enters the citric acid cycle where it is fully oxidised to
carbon dioxide.
This pathway thus allows cell to obtain energy from fat.
To utilize acetate from fat for biosynthesis of
carbohydrates, the glyoxylate cycle whose initial
reaction are identical to the TCA cycle is used.
15. Summary
Pyruvate is converted to acetyl-CoA by the action of
pyruvate dehydrogenase complex, a huge enzyme
complex.
Acetyl-CoA is converted to 2CO2 via the eight-step
citric acid cycle, generating three NADH, one FADH2,
and one ATP (by substrate-level phophorylation).
Intermediates of citric acid cycle are also used as
biosynthetic precursors for many other biomolecules,
including fatty acids, steroids, amino acids, heme,
pyrimidines, and glucose.
16. Oxaloacetate can get replenished from pyruvate, via
a carboxylation reaction catalyzed by the biotin-
containing pyruvate carboxylase.
Net conversion of fatty acids to glucose can occur in
germinating seeds, few invertebrates and some
bacteria via the glycoxylate cycle.
Cycle shares three steps with the citric acid cycle
but bypasses the two decarboxylation steps,
converting two molecules of acetyl-CoA to one
succinate.
