Question

Which enzymes of the citric acid cycle are missing from the glyoxylate cycle?

Short answer

Isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and succinyl-CoA synthetase.

Step-by-step solution

- Understand the Citric Acid Cycle

The citric acid cycle (also known as the Krebs cycle or TCA cycle) consists of a series of enzymatic reactions that produce energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The main enzymes involved are citrate synthase, aconitase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase, and malate dehydrogenase.

- Understand the Glyoxylate Cycle

The glyoxylate cycle is a variation of the citric acid cycle that occurs in plants, bacteria, protists, and fungi. It bypasses the two decarboxylation steps of the citric acid cycle. The main enzymes in the glyoxylate cycle include isocitrate lyase and malate synthase. These enzymes help the cycle convert isocitrate to glyoxylate and succinate, and glyoxylate to malate, respectively.

- Identify Missing Enzymes

In comparing both cycles, the glyoxylate cycle does not include the enzymes that catalyze the decarboxylation steps of the citric acid cycle. Specifically, the enzymes that are missing are isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and succinyl-CoA synthetase.

Key concepts

citric acid cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a critical metabolic pathway. It takes place in the mitochondria of eukaryotic cells. Its main role is to generate energy by oxidizing acetyl-CoA derived from nutrients like carbohydrates, fats, and proteins.
The cycle consists of several enzyme-catalyzed reactions.
You'll often hear about the following enzymes:

• Citrate synthase
• Aconitase
• Isocitrate dehydrogenase
• α-Ketoglutarate dehydrogenase
• Succinyl-CoA synthetase
• Succinate dehydrogenase
• Fumarase
• Malate dehydrogenase

These enzymes work together to transform acetyl-CoA into CO₂, while also producing ATP, NADH, and FADH₂.
These molecules are essential for producing energy in cells.

enzymes

Enzymes are biological catalysts that speed up chemical reactions in cells.
They are essential for life as they regulate metabolic pathways.
In the citric acid cycle and glyoxylate cycle, specific enzymes play unique roles.
For instance:

• Citrate synthase helps combine acetyl-CoA and oxaloacetate to form citrate.
• Aconitase converts citrate to isocitrate.
• Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase catalyze important decarboxylation steps.

Each enzyme has a specific function, ensuring the proper flow of the cycle.
Without these enzymes, the reactions would not occur as efficiently, if at all.

decarboxylation

Decarboxylation is a chemical reaction that removes a carbon group from a molecule, releasing it as CO₂. In the citric acid cycle:

• Isocitrate dehydrogenase removes a carbon from isocitrate to form α-ketoglutarate and CO₂.
• α-Ketoglutarate dehydrogenase then removes another carbon to form succinyl-CoA and CO₂.

These steps are crucial for energy production.
Interestingly, the glyoxylate cycle bypasses these decarboxylation steps.
This allows organisms like plants and bacteria to convert fats into carbohydrates more efficiently.
Hence, they do not rely on the decarboxylation enzymes, making the cycles distinct from each other.