GLYOXYLATE CYCLE: CHARACTERISTICS, REACTIONS, REGULATION, FUNCTIONS - BIOLOGY - 2024

The glyoxylate cycle is a metabolic pathway present in plants, in some microorganisms and in invertebrate animals (absent in all vertebrates), through which these organisms can convert fats into carbohydrates (sugars).

This route was discovered in 1957, while Kornberg, Krebs, and Beevers were trying to elucidate how bacteria such as Escherichia coli could grow in the presence of acetate as the sole carbon source, and how germinating seedlings of spurge (Ricinus communis) could convert fats into carbohydrates.

Schematic of the glyoxylate cycle (Source: Agrotman via Wikimedia Commons)

Studies by these three researchers led to the discovery of two enzymes known as isocitrate lyase and malate synthase, which, together with the enzymes of the Krebs cycle, allow the synthesis of succinate from two acetyl-coA molecules.

The succinate thus produced is converted to malate through the tricarboxylic acid cycle, and can later be used for the production of glucose through gluconeogenesis.

This route occurs, in plants, in special organelles called glyoxysomes and is essential for the survival of seedlings during the early stages of germination.

characteristics

The glyoxylate pathway can be seen as a "modification" of the Krebs cycle, with the difference that oxidative decarboxylation does not occur in the former, but four carbon dicarboxylic acids can be formed from acetate units of two carbons.

This characteristic of the glyoxylate cycle has been described as a way that some organisms have to avoid ("bypass") the loss of carbon atoms in the form of carbon dioxide that identifies the Krebs cycle.

In plants, the glyoxylate cycle occurs within cytosolic organelles surrounded by a simple membrane known as glyoxysomes. In other organisms such as yeasts and algae, on the other hand, this route occurs in the cytosol.

Glyoxysomes are structurally similar to peroxisomes (some authors consider them "specialized peroxisomes"), other organelles responsible for part of the β-oxidation of fatty acids and the elimination of reactive oxygen species in eukaryotic organisms.

Inside, the fatty acids are oxidized to produce acetyl-CoA, which is subsequently condensed into compounds with four carbon atoms. These compounds are selectively transported to the mitochondria, where they are converted to malate or transported to the cytosol to enter the gluconeogenic pathway (glucose synthesis).

The enzymes shared between the glyoxylate pathway and the tricarboxylic acid cycle exist in the mitochondria and the glyoxysome as isoenzymes, meaning that both pathways work more or less independently of each other.

Occurrence of glyoxysomes

Glyoxysomes are not permanently present in plant tissues. They are especially abundant during the germination of oilseeds, which have little photosynthetic capacity to produce the carbohydrates they need to grow.

In fully developed plants, their participation in the metabolism of fats is not so essential, since sugars are obtained mainly by photosynthesis.

Reactions

Acetate from the breakdown of fatty acids functions as an energy-rich fuel and as a source of phosphoenolpyruvate for the synthesis of glucose through gluconeogenesis. The process is as follows:

Steps of the glyoxylate cycle

1- The glyoxylate pathway, similar to that of the Krebs cycle, begins with the condensation of an acetyl-CoA molecule with another of oxaloacetate to yield citrate, a reaction catalyzed by the enzyme citrate synthase.

2- The Aconitase enzyme converts this citrate into isocitrate.

3- Isocitrate is used as a substrate for the enzyme isocitrate lyase to form the compounds succinate and glyoxylate.

Molecular structure of the enzyme Isocitrate Liasa (Source: Vrabiochemhw via Wikimedia Commons)

4- The glyoxylate is taken up by the enzyme malate synthase to produce malate through its condensation with a second molecule of acetyl-CoA.

5- Malate is converted into oxaloacetate by malate dehydrogenase and this compound can serve as a precursor for the gluconeogenic pathway or be condensed with another acetyl-CoA to restart the cycle once more.

6- The succinate produced can also be converted to fumarate and this to malate, providing a greater quantity of oxaloacetate molecules for the formation of glucose. Otherwise, this molecule can also be exported into the mitochondria to function in the Krebs cycle.

Oxaloacetate enters the gluconeogenic pathway for glucose production thanks to its conversion to phosphoenolpyruvate, which is catalyzed by the enzyme phosphoenolpyruvate carboxykinase.

Regulation

Since the glyoxylate and tricarboxylic acid cycles share numerous intermediates with each other, there is a coordinated regulation between the two.

In addition, it is necessary that control mechanisms exist, since the synthesis of glucose and other hexoses from acetyl-CoA (from the degradation of fats) implies the participation of at least four routes:

- The β-oxidation of fatty acids that produces the acetyl-CoA molecules necessary for both the glyoxylate cycle and the Krebs cycle and which, in plants, takes place in glyoxysomes.

- The glyoxylate cycle, which also occurs in glyoxysomes and which, as mentioned, produces intermediates such as succinate, malate and oxaloacetate.

- The Krebs cycle, which takes place in the mitochondria and in which the intermediates succinate, malate and oxaloacetate are also produced.

- Gluconeogenesis, which occurs in the cytosol and involves the use of oxaloacetate converted to phosphoenolpyruvate to synthesize glucose.

The main control point is in the enzyme isocitrate dehydrogenase, whose regulation involves a covalent modification by adding or removing a phosphate group.

When the enzyme is phosphorylated it is inactivated, so the isocitrate is directed towards the glyoxylate pathway for the production of glucose.

Features

For plants, the glyoxylate cycle is essential, especially during the germination process, since the degradation of the fats stored in the seeds is used for the synthesis of glucose in photosynthetically underdeveloped tissues.

Glucose is used as a source for obtaining energy in the form of ATP or for the formation of more complex carbohydrates with structural functions, but some of the intermediates generated during the glyoxylate pathway can also serve the synthesis of other cellular components.

In microorganisms

The main function of the glyoxylate cycle in microorganisms is to provide an "alternative" metabolic pathway, so that microorganisms are able to take advantage of other sources of carbon and energy for their growth.

Such is the case of the bacterium Escherichia coli, in which, when the levels of some intermediates of glycolysis and the citric acid cycle decrease (isocitrate, 3-phosphoglycerate, pyruvate, phosphoenolpyruvate and oxaloacetate), the enzyme isocitrate dehydrogenase (which participates in the Krebs cycle) is inhibited and isocitrate is directed towards the glyoxylate pathway.

If this pathway is active when the bacteria grow in a medium rich in acetate, for example, this metabolite can be used to synthesize carboxylic acids with four carbon atoms that, later, can lead to the formation of energetic carbohydrates..

For other organisms such as fungi, for example, pathogenicity has been shown to be highly dependent on the presence of an active glyoxylate cycle, apparently for metabolic reasons.

References

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2. Ensign, SA (2006). Revisiting the glyoxylate cycle: alternate pathways for microbial acetate assimilation. Molecular Microbiology, 61 (2), 274–276.
3. Garrett, R., & Grisham, C. (2010). Biochemistry (4th ed.). Boston, USA: Brooks / Cole. CENGAGE Learning.
4. Lorenz, MC, & Fink, GR (2001). The glyoxylate cycle is required for fungal virulence. Nature, 412, 83-86.
5. Mathews, C., van Holde, K., & Ahern, K. (2000). Biochemistry (3rd ed.). San Francisco, California: Pearson.
6. Rawn, JD (1998). Biochemistry. Burlington, Massachusetts: Neil Patterson Publishers.
7. Vallarino, JG, & Osorio, S. (2019). Organic Acids. In Postharvest Physiology and Biochemistry of Fruits and Vegetables (pp. 207–224). Elsevier Inc.