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Structure and Function of Glyoxysomes

  • Botany

In this structure and function of glyoxysomes post we have briefly explained about definition, glyoxysomes structure and function (Fatty Acid Metabolism, and Glyoxylate Cycle)

Structure and Function of Glyoxysomes

Glyoxysomes are specialised peroxisomes found in plants (particularly in germinating seed fat storage tissues) and filamentous fungi. The seedling consumes these sugars, which are synthesised from fats, until it is mature enough to produce them through photosynthesis. It was involved in fatty acid breakdown and conversion to acetyl-CoA for the glyoxylate bypass.

Glyoxysomes

In terms of morphology, glyoxysomes structure are a temporary organelle of the plant that is similar to peroxisomes. They appear during a brief period in the life cycle of plants, most notably in the fat storage tissues of germinating seeds in certain beans and nuts, as well as in filamentous fungi.

Glyoxysomes structure appear in the endosperm of the plant cell in the first few days after seed germination and are associated with the lipid bodies. The fatty acid from the stored fat is broken down and converted into carbohydrates in the endosperm cell. Organelles that were present prior to fat conversion are no longer present. It is coincidental with the conversion of fats into carbohydrates during seed germination that these organelles appear.

Glyoxysomes Structure

Glyoxysomes structure are specialised peroxisomes found in plants (particularly in germinating seed fat storage tissues) and filamentous fungi. These sugars synthesised from fats are used by the seedling until it is mature enough to produce them through photosynthesis.

Glyoxysomes structure are prominent organelles in the cells of lipid-storing endosperm and cotyledons, according to an ultra-structural examination of the germinating seed of several higher plants.

Most of the glyoxysomes structure features of the organelles are similar to those of the microbodies of other plant organs, including their association with the endoplasmic reticulum. They were also associated with the numerous storage lipid droplets that appeared in many cases to partially envelop the latter glyoxysomes structure.

Glyoxysomes

Glyoxysomes Structure

Many of these glyoxysomes structure contain crystalline inclusions, and dense Nucleo amorphous nucleoid may also be present. During certain stages following germination, glyoxysomes pusses invagination of cytoplasm containing ribosomes in some fat-storing cotyledons.

As of now, there is no convincing evidence that glyoxysomes contain nucleic acid. There have been reports of RNA in glyoxysomes isolated from caster endosperm and pine megagametophytes.

However, due to the possibility of contamination with other types of particles characterised by a ribosome studded membrane and referred to as “dilated cisternae” or “ribosomes,” According to researchers, there are no cellular DNA species that are unique to glyoxysomes.

Functions

The glyoxysomes structure is a plant peroxisome found primarily in germinating seeds that is involved in the breakdown and conversion of fatty acids to acetyl-CoA for the glyoxylate bypass. Because it contains catalase, the glyoxysome may be related to or derived from microbodies or peroxisomes. Glyoxysomes are responsible for the following biochemical activities in plant cells:

Fatty Acid Metabolism

During the germination of oily seeds, the stored lipid molecules of spherosomes are hydrolyzed to glycerol and fatty acids by the enzyme lipase (glycerol ester hydrolase). The enzyme phospholipase hydrolyzes the phospholipid molecules.

The hydrolysis-released long chain fatty acids are then broken down by the sequential removal of two carbon or C2 fragments in the β-oxidation process.

Trans-2-enoyl-CoA is hydrated by an enzyme enoyl hydratase or crotonase to produce the L-3- hydroxyacyl-CoA, which is oxidized by a NAD linked L-3-hydroxyacyl- CoA dehydrogenase to produce 3-Keto acyl-CoA.

The 3-keto acyl-CoA loses a two-carbon fragment under the action of the enzyme thiolase or β-keto thiolase to generate an acetyl-CoA and a new fatty acyl-CoA with two less carbon atoms than the original.

This new fatty acyl-CoA is then recycled through the same series of reactions until the final two molecules of acetyl-CoA are produced. The complete β-oxidation chain can be represented as follows:

Glyoxysomes

In plant seeds β-oxidation occurs in glyoxysomes. But in other plant cells, β-oxidation occurs in glyoxysomes and mitochondria. The glyoxysomal β-oxidation requires oxygen for oxidation of reduced flavoprotein produced as a result of the fatty-acyl-CoA dehydrogenase activity.

In animal cells β-oxidation occurs in mitochondria. In-plant cells, the acetyl-CoA, the product of the β-oxidation chain is not oxidized by the Krebs cycle, because it remains spatially separated from the enzymes of the Krebs cycle, instead of it, acetyl-CoA undergoes the glyoxylate cycle to be converted into succinate.

Glyoxylate Cycle

The glyoxylate pathway occurs in Glyoxysomes and it involves some of the reactions of the Krebs cycle in which citrate is formed from oxaloacetate and acetyl-CoA under the action of citrate synthetase enzyme.

The citrate is subsequently converted into isocitrate by aconitase enzyme. The cycle then involves the enzymatic conversion of isocitrate to glyoxylate and succinate by isocitratase enzyme:

The glyoxylate and another mole of acetyl-CoA form a mole of malate by malate synthetase:

This malate is converted to oxaloacetate by malate dehydrogenase for the cycle to be completed. Thus, overall, the glyoxylate pathway involves:

2 Acetyl-CoA + NAD+ → Succinate + NADH + H+

Succinate is the end product of the glyoxysomal metabolism of fatty acid and is not further metabolized within this organelle. The synthesis of hexose or gluconeogenesis involves the conversion of succinate to oxaloacetate, which presumably takes place in the mitochondria since the glyoxysomes do not contain the enzymes fumarase and succinic dehydrogenase.

Two molecules of oxaloacetate are formed from four molecules of acetyl-CoA without carbon loss. This oxaloacetate is converted to phosphoenolpyruvate in the phosphoenolpyruvate carboxykinase reaction with the loss of two molecules of CO2:

2 Oxaloacetate + 2ATP ⇌ 2 Phosphoenol pyruvate + 2CO+ 2ADP

Glyoxysomes

The phosphoenol pyruvate is converted into monosaccharides (e.g., glucose, fructose), disaccharide (sucrose) and polysaccharide (starch) by following reaction:

We hope glyoxysomes structure and function article helps you understand definition, glyoxysomes structure and function (Fatty Acid Metabolism, and Glyoxylate Cycle).

Further Readings

Reference

Structure and Function of Glyoxysomes