GLIOXISOMES: CHARACTERISTICS, STRUCTURE AND FUNCTIONS - BIOLOGY - 2024

The glyoxisomes are a specialized class of microbodies usually found in germinated seeds of plants rich in oils (oil).

They contain enzymes that help convert the oils contained as reserve substances in the seeds into carbohydrates. This conversion occurs during the germination process.

A glyoxysome and other organelles within a plant cell. Taken and edited from: Gevictor, from Wikimedia Commons.

Carbohydrates are easier to mobilize towards the young plant to be used during growth. Similar organelles have been observed in some protists and fungi.

These organelles have been termed "glyoxysome-like." Glyoxysomes are so named because they contain the enzymes that participate in the glyoxylate cycle.

The glyoxylate cycle is a metabolic pathway that occurs in the glyoxysomes of plant cells, some fungi, and protists. This is a modification of the citric acid cycle.

It uses fatty acids as a substrate for the synthesis of carbohydrates. This metabolic pathway is very important for the seeds during the germination process.

Microbodies

Microbodies are vesicle-shaped organelles present in the cell cytoplasm. They are spherical in shape and are surrounded by a single membrane.

They act as vessels that contain metabolic activities. Besides glyoxysomes, there are other microbodies such as: peroxisomes, glycosomes or glucosomes, and Woronin bodies.

Peroxisomes

Peroxisomes are microbodies unique to eukaryotes, containing oxidase and catalase enzymes. They were first described by Christian de Duve and his collaborators in 1965.

Peroxisomes are essential in the metabolism of fats, as they contain ß-oxidation enzymes capable of acting on them. These enzymes break down lipids and produce Acetyl-CoA.

They act mainly on high molecular weight lipids, breaking them down for oxidation in the mitochondria. They are also involved in the degradation of cholesterol for the synthesis of bile acids.

They also contain enzymes for many important metabolic pathways, such as the metabolism of harmful compounds in the liver (eg, alcohol). They participate in the synthesis of phospholipids, triglycerides and isoprenoids.

Their name comes from the fact that they oxidize substrates using molecular oxygen to form hydrogen peroxide.

Woronin bodies

Woronin bodies are specific microbodies of Ascomycota fungi. Its functions are not entirely clear. One of these is believed to be closing the pores in the septa of the hyphae. This occurs when hyphal damage occurs, to minimize the possible loss of cytoplasm.

The glucosomes

Glycosomes are peroxisomes that contain enzymes for glycolysis and the reuse of purines. They are found in kinetoplastid protozoa (Kinetoplastea). These organisms depend exclusively on glycolysis for the production of ATP.

Simple diagram of a peroxisome. Taken and edited from: Agateller.

The discovery of glyoxysomes

Glyoxysomes were discovered by the English botanist Harry Beevers and a postdoctoral student named Bill Breidenbach. The discovery of these organelles was made during a study of the linear sucrose gradients of endosperm homogenates.

These two researchers demonstrated in that study that the enzymes of the glyoxylate cycle were found in a fraction of the organelle that was not a mitochondrion. This organelle was called glyoxysome due to the participation of its enzymes in the glyoxylate cycle.

Beever's discovery of glyoxysomes paved the way for other researchers to find peroxisomes. The latter are organelles similar to glyoxysomes, which are found in the leaves of plants.

This discovery also greatly improved the understanding of peroxisome metabolism in animals.

General characteristics of glyoxysomes

One of the characteristics that allows glyoxysomes to be recognized is their catalase content, as well as their proximity to lipid bodies.

They are found in the seeds of plants, they can also be found in filamentous fungi.

Structure

They are spherical, with a diameter ranging from 0.5 to 1.5 μm, and have a granular interior. Sometimes they have crystalline protein inclusions.

They originate from the endoplasmic reticulum, forming part of the endomembrane system. They lack a genome and are linked by a single membrane.

Features

Participation in gluconeogenesis

Glyoxysomes participate in gluconeogenesis. Plants are the only organisms capable of converting lipids into sugars. These reactions occur in reserve tissues of the seeds that store fat.

In vegetables, ß-oxidation occurs in the microbodies present in the leaves (peroxisomes) and in the seeds (glyoxysomes) of those of oilseeds that are in the germination process.

This reaction does not occur in the mitochondria. The function of ß-oxidation is to provide sugar precursor molecules from fats.

The process of ß-oxidation of fatty acids that occurs in both types of microbodies is similar. The acetyl-CoA that is obtained by this oxidation enters the glyoxylate cycle, to produce sugar precursors before the developing plants can carry out the photosynthetic process.

The glyoxylate cycle

Basically, the glyoxylate cycle of glyoxysomes is a modified metabolic pathway of the mitochondrial Krebs cycle. The glyoxylate cycle avoids decarboxylation steps.

This jump allows the production of carbohydrate precursors (oxaloacetate). On this route there is no loss of CO2. Acetyl-CoA, from the oxidation of fatty acids, participates in the reactions of the glyoxylate cycle.

Hydrogen peroxide detoxification

In seeds, the β-oxidation of fatty acids produces hydrogen peroxide. The catalase of the glyoxysomes plays a vital role during the detoxification process of this compound.

These reactions, in which the mitochondria are also involved, include the glyoxalate cycle, which occurs in the cotyledons of seeds of some oilseed species.

Later in development, the cotyledons emerge from the ground and begin to receive light. At that time there is a sharp drop in the activity of glyoxysomal enzymes in the glyoxysomes.

At the same time, there is an increase in the production of enzymes typical of peroxisomes. This fact shows that a gradual transformation from glyoxysomes to peroxisomes that participate in photorespiration is occurring. This progressive transformation from one type of microbody to another has been proven experimentally.

References

1. Glyoxylate cycle. On Wikipedia. Recovered from
2. Glyoxysome. On Wikipedia. Recovered from
3. IA Graham (2008). Seed Storage Oil Mobilization. Annual Review of Plant Biology.
4. N. Kresge, RD Simoni & RL Hill (2010). The discovery of glyoxysomes: the Work of Harry Beevers. Journal of Biological Chemestry.
5. K. Mendgen (1973). Microbodies (glyoxysomes) in infection structures of Uromyces phaseoli. Protoplasm
6. M. Parsons, T. Furuya, S. Pal, P. Kessler (2001). Biogenesis and function of peroxisomes and glycosomes. Molecular & Biochemical Parasitology.