Glyoxysomes are specialized peroxisomes found in plants (particularly in the fat storage tissues of germinating seeds) and also in filamentous fungi. The seedling uses these sugars, synthesized from fats until they are mature enough to make them through photosynthesis. It involved the breakdown and conversion of fatty acids to acetyl-CoA for the glyoxylate bypass.
What is Glyoxysomes?
The glyoxysomes are a temporary organelle of the plant that are comparable to peroxisomes in morphological details. They occur during the transient period in the life cycle of plants, particularly in the fat storage tissues of germinating seeds in certain beans and nuts, and also in filamentous fungi.
Glyoxysomes are appeared in the first few days after seed germination in the endosperm of the plant cell and are associated with the lipid bodies. In the endosperm cell, the fatty acid of the stored fat is breakdown and converted into carbohydrates.
After the fat conversion organelles are disappeared. Due to the appearance of these organelles, it’s coincidental with the conversion of fats into carbohydrates during seed germination.
Structure of glyoxysomes
Glyoxysomes are specialized peroxisomes found in plants (particularly in the fat storage tissues of germinating seeds) and also in filamentous fungi. The seedling uses these sugars synthesized from fats until it is mature enough to produce them by photosynthesis.
Ultrastructural examination of the germinating seed of several higher plants shows the glyoxysomes are prominent organelles in the cells of lipid storing endosperm and cotyledons.
The organelles closely resemble the microbodies of the other plant organs in most of their structural features, including association with the endoplasmic reticulum. They are also associated with the numerous storage lipid droplets appearing in many cases to envelop partially the latter structure.
Crystalline inclusions are common in many of these glyoxysomes and dense Nucleo amorphous nucleoids may occur as well. In some fat-storing cotyledons, glyoxysomes characteristically pusses invagination of cytoplasm containing ribosome during certain stages following germination.
As so far there is no substantial evidence that glyoxysome contains nucleic acid. There have been reports that RNA is present in glyoxysomes isolated from caster endosperm and megagametophytes of pine.
However, because of possible contamination with other classes of particles characterized by a ribosome-studded membrane and termed “dilated cisternae” or “ribosomes”. Studies by Douglass, Criddle, Breidenbach, and also Gerhardt, and Beevers conclude that there are no cellular DNA species that are unique to glyoxysomes.
Functions of Glyoxysomes
The glyoxysome is a plant peroxisome, especially found in germinating seeds, involved in the breakdown and conversion of fatty acids to acetyl-CoA for the glyoxylate bypass. Since it is also rich in catalase, the glyoxysome may be related to the microbodies or peroxisomes or derived from them.
Glyoxysomes perform the following biochemical activities of plants cells:
1. Fatty acid metabolism in Glyoxysomes
During the germination of oily seeds, the stored lipid molecules of spherosomes are hydrolyzed by the enzyme lipase (glycerol ester hydrolase) to glycerol and fatty acids. The phospholipid molecules are hydrolyzed by the enzyme phospholipase.
The long-chain fatty acids which are released by the hydrolysis are then broken down by the successive removal of two carbon or C2 fragments in the process of β-oxidation.
β-Oxidation in Glyoxysomes
During β-oxidation process, the fatty acid is first activated by enzyme fatty acid thiokinase to a fatty acyl-CoA which is oxidized by a FAD-linked enzyme fatty acyl-CoA dehydrogenase into trans-2-enoyl-CoA.
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:
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.
2. Glyoxylate cycle of Glyoxysomes
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 the 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 Phosphoenolpyruvate + 2CO2 + 2ADP
The phosphoenolpyruvate is converted into monosaccharides (e.g., glucose, fructose), disaccharide (sucrose), and polysaccharide (starch) by the following reaction: