Glyoxylate cycle

Diagram of the glyoxylate cycle

The glyoxylate cycle is a series of biochemical reactions that enables certain organisms, such as plants, bacteria, and fungi, to convert fatty acids into carbohydrates. This cycle is a variation of the citric acid cycle and is crucial for organisms that rely on stored lipids as a carbon source, especially during seed germination in plants.

Overview

The glyoxylate cycle bypasses the decarboxylation steps of the citric acid cycle, allowing the conversion of acetyl-CoA into four-carbon dicarboxylic acids, which can then be used for gluconeogenesis. This cycle takes place in specialized peroxisomes known as glyoxysomes in plants and in the cytoplasm of some bacteria.

Key Enzymes

The glyoxylate cycle involves several key enzymes:

• Isocitrate lyase: This enzyme catalyzes the cleavage of isocitrate into succinate and glyoxylate.
• Malate synthase: This enzyme catalyzes the condensation of glyoxylate and acetyl-CoA to form malate.

These enzymes allow the cycle to bypass the steps in the citric acid cycle that release carbon dioxide, thus conserving carbon atoms for the synthesis of carbohydrates.

Biological Significance

The glyoxylate cycle is particularly important in:

• Seed Germination: In plants, the glyoxylate cycle is active in the early stages of seed germination, when the seedling relies on stored lipids for energy and carbon skeletons until it can perform photosynthesis.
• Microbial Growth: Some bacteria and fungi use the glyoxylate cycle to grow on acetate or other simple carbon compounds as their sole carbon source.

Differences from the Citric Acid Cycle

The main differences between the glyoxylate cycle and the citric acid cycle are:

• The glyoxylate cycle bypasses the two decarboxylation steps of the citric acid cycle, preventing the loss of carbon as carbon dioxide.
• It includes the unique enzymes isocitrate lyase and malate synthase.

Related Pages

• Citric acid cycle
• Gluconeogenesis
• Peroxisome
• Metabolism