Question

What are the unique reactions of the glyoxylate cycle?

Short answer

The unique reactions are catalyzed by isocitrate lyase and malate synthase.

Step-by-step solution

Identify the pathway

The glyoxylate cycle is a variation of the citric acid cycle. It occurs in plants, bacteria, fungi, and protists and allows them to convert fatty acids into carbohydrates.

List all steps of the cycle

The glyoxylate cycle includes the following steps: Condensation of acetyl-CoA with oxaloacetate to form citrate, isomerization to isocitrate, and then the key unique reactions.

Highlight unique reactions

The unique reactions differentiating the glyoxylate cycle from the citric acid cycle are: 1. Conversion of isocitrate to glyoxylate and succinate by the enzyme isocitrate lyase. 2. Condensation of acetyl-CoA with glyoxylate to form malate by the enzyme malate synthase.

Summarize the unique reactions

The two unique reactions are catalyzed by isocitrate lyase and malate synthase. These steps bypass the decarboxylation steps of the citric acid cycle, avoiding CO2 release.

Key concepts

isocitrate lyase

Isocitrate lyase is a crucial enzyme in the glyoxylate cycle, primarily responsible for one of the unique reactions that differentiate this cycle from the citric acid cycle. This enzyme catalyzes the cleavage of isocitrate into glyoxylate and succinate. In more detail, isocitrate is split into two smaller molecules:

• Succinate (which can enter the citric acid cycle)
• Glyoxylate (which continues in the glyoxylate cycle)

This reaction is significant because it helps bypass the steps of the citric acid cycle where carbon dioxide is usually released. Thus, isocitrate lyase plays a vital role in conserving carbon atoms that can later be used to produce glucose in organisms capable of the glyoxylate cycle.

malate synthase

Malate synthase is another key enzyme in the glyoxylate cycle, working right after isocitrate lyase. This enzyme catalyzes the reaction that combines glyoxylate with another molecule of acetyl-CoA to produce malate. Let's break down the process: Glyoxylate, produced by the action of isocitrate lyase, reacts with acetyl-CoA, a product of fatty acid degradation. The enzyme malate synthase facilitates this reaction, resulting in the production of malate. Malate can then enter the citric acid cycle or be used in gluconeogenesis to form glucose. By doing so, malate synthase plays a critical role in converting fatty acids into carbohydrates, highlighting its importance in the metabolism of plants, bacteria, fungi, and protists.

citric acid cycle bypass

The glyoxylate cycle serves as a modified version of the citric acid cycle, found in certain organisms like plants, bacteria, fungi, and protists. One of its main features is the citric acid cycle bypass, which is achieved through the unique reactions catalyzed by isocitrate lyase and malate synthase. By bypassing the carbon dioxide-releasing steps of the citric acid cycle:

• It offers a way to retain carbon that would otherwise be lost as CO2.
• This bypass is crucial for organisms that need to convert fatty acids into carbohydrates.
• The two-step process involves converting isocitrate to glyoxylate and succinate, followed by converting glyoxylate to malate.

Thus, the citric acid cycle bypass enables these organisms to efficiently produce carbohydrates, pivotal for their growth and energy needs, especially under conditions where carbohydrates are scarce but fatty acids are available.