Gyloxysomes are
single membrane bounded organelles found
only in plant cells,containing enzymes that participate in photorespiration in
leaves, nitrogen metabolism in root nodules and fat conversions in seeds.
Glyoxysomes are found in Fungi and advanced plants.
Glyoxysomes are found in Fungi and advanced plants.
In many plant species in which lipid is the main storage material
in the cotyledons or endosperm of the seed. During the first few days of
seed germination there is a dramatic decrease in the lipid content of
the seed and sugars, principally sucrose are formed. These sugars are
subsequently translocated to the growing embryo or embryonic axis.
This lipid to carbohydrate conversion has been correlated with an
increase in the activity of the glyoxysomal enzymes, malate synthetase
and isocitrate lyase, and as germination proceeds and the lipid reserves
are depleted, the number of lipid storage bodies or spherosomes
decrease and there is a drop in the activities of malate synthetase and
isocitrate lyase. The elucidation of the metabolic processes involved in
this process of gluconeogenesis, ultimately led to the isolation of particles in which were localized the crucial enzymes of this pathway.
In fat-storing cells of germinating fatty seeds contain
enzymes of the glyoxylate cycle in addition to peroxisomal enzymes. Glyoxysomes
contains enzymes like isocratic lyase, malate synthetase, glycolate oxidases,
etc. for glyoxylate cycle ,not only enzymes specific to the glyoxylate cycle
but they also contain several of the essential enzymes of the Krebs cycle,
which therefore function simultaneously in both mitochondria and glyoxysomes.
Fatty acid oxidation in plants occurs in the peroxisomes of leaf tissues and glyoxyomes of germinating seeds. Plants peroxisomes and glyoxysomes are similar in structure and function.Glyoxysomes occure during seed germination and may be consider specialized peroxisomes.
This peroxisomes involve in photorespiration .Intracellular concentration of oxygen is high RUBP carboxylase act as oxygenase and splits ribulose disphosphate into phosphoglycerate and phospho glycoliate. Then transport into peroxisomes where phospho glycoliate is dephosphorylated and converted to glyoxalate.Then glyoxylate converted to glycein in turn converted to serine in mitochondria by decarboxylation process.As oxygen is utilization this process is called photorespiration.
The B-oxiddation pathway is not an important for
energy source in plants, in fact plant mitochondria does not contain
B-oxidation enzymes. During germination triacyle glycerol stored in seed are
converted into other essential metabolites like glucose etc.
B-oxidation reaction sequence in mitochondria is
essentially the same as that in peroxisomes and glyoxysomes ,in mitochondrial
enzymes differ significantly from their isoenzyme in those compartments. these
difference apparently reflects in evolutionary divergence that occurred very
early ,with the separation of Gram + and Gram- Bacteria. In mitochondrial each
of the four enzymes of B-oxidation is a separate, soluble protein ,but in case
of peroxisomes and glyoxysomes are part of a compex of proteins.
Triacyleglycerols converts in to fatty acids in lipid bodies .This fatty acids enter in to glyoxysome ,in glyoxysomes fatty acides break down into Acetyl coA.
Acetyl coA coverted into citricate with oxaloacetate, oxaloacetate from mitochondria.
Citrate is converted into Isocitrate . Isocitrate is converted into Succinate and Glyoxylate.
Succinate again transport to mitochondria and participate TCA cycle.
Glyoxylate is convert into Malate .
Malate enter into cytosol where it involve in Gluconeogenesis produce Glucose.
Isocitrate is crucial intermediate ,standing at the branch point between the glyoxylate and citric acid cycle.
