Presented by Ajit Kumar Dansena

M.Sc. Life Science(1st sem) Central University of Guajrat

Glyoxysomes

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.

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.

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.

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.

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

The Glyoxylate Cycle

Another Process Involving Glycolytic

Enzymes and Metabolites

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.

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

-oxidation occurs in mitochondria and peroxisomes in mammals, but exclusively in the peroxisome in plants and yeast.

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.

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.

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.

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.