Question

When fatty acid \(\beta\) -oxidation predominates in the liver, mitochondrial pyruvate is most likely to be: (A) carboxylated to phosphoenolpyruvate for entry into gluconeogenesis. (B) oxidatively decarboxylated to acetyl-CoA for oxidation in the citric acid cycle. (C) carboxylated to oxaloacetate for entry into gluconeogenesis. (D) reduced to lactate in the process of fermentation.

Short answer

C

Step-by-step solution

Understand the Context

In the liver, fatty acid \( \beta \)-oxidation predominates under conditions where the body requires energy, such as fasting or intense exercise. Fatty acid \( \beta \)-oxidation generates acetyl-CoA, which enters the citric acid cycle.

Evaluate Each Option

Consider what happens to mitochondrial pyruvate under conditions of fatty acid \( \beta \)-oxidation.

Analyze Option A

Phosphoenolpyruvate is an intermediate in gluconeogenesis, but pyruvate first needs to be carboxylated to oxaloacetate. This option skips a necessary step.

Analyze Option B

Pyruvate is converted to acetyl-CoA by the enzyme pyruvate dehydrogenase. However, under fatty acid \( \beta \)-oxidation, the body predominantly uses acetyl-CoA coming from \( \beta \)-oxidation rather than from pyruvate.

Analyze Option C

Pyruvate carboxylation to oxaloacetate is a necessary step in gluconeogenesis, which is a pathway used by the liver to generate glucose during fasting.

Analyze Option D

Lactate production from pyruvate occurs during anaerobic metabolism, not during fatty acid \( \beta \)-oxidation.

Make a Conclusion

Since pyruvate is carboxylated to oxaloacetate to enter gluconeogenesis under conditions of fatty acid \( \beta \)-oxidation, Option C is correct.

Key concepts

Pyruvate Carboxylation

Pyruvate carboxylation is a crucial metabolic process that prepares pyruvate to enter different pathways. When pyruvate is carboxylated, it converts into oxaloacetate. This conversion is catalyzed by the enzyme pyruvate carboxylase. This step is particularly important in gluconeogenesis, where oxaloacetate serves as a starting point. Gluconeogenesis is the pathway that generates glucose, particularly significant during periods of fasting or intense activity. This ensures that the body maintains adequate glucose levels even when carbohydrates are low.

Gluconeogenesis

Gluconeogenesis is the metabolic pathway by which the liver produces glucose from non-carbohydrate sources. This is critical during times of fasting or intense exercise when dietary glucose is not available. The pathway begins with the carboxylation of pyruvate to oxaloacetate. From there, oxaloacetate undergoes several transformations to eventually become glucose. Key enzymes, including pyruvate carboxylase and phosphoenolpyruvate carboxykinase, play essential roles. Gluconeogenesis ensures that the body continues to have a steady supply of glucose, which is vital for organs and tissues, particularly the brain and muscles.

Acetyl-CoA Production

Acetyl-CoA is a central molecule in metabolism, bridging various pathways. It is primarily produced by the oxidative decarboxylation of pyruvate, catalyzed by pyruvate dehydrogenase. However, during fatty acid β-oxidation, acetyl-CoA is generated directly from the breakdown of fatty acids. This process happens in the mitochondria, where fatty acids are cleaved into two-carbon units of acetyl-CoA. The acetyl-CoA then serves as a substrate for the citric acid cycle. This is particularly important during periods of high energy demand, as it provides a rich source of ATP.

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a series of chemical reactions that generate energy through the oxidation of acetyl-CoA. This cycle takes place in the mitochondria and serves as a key metabolic hub. Acetyl-CoA combines with oxaloacetate to form citrate, which undergoes several transformations, leading to the regeneration of oxaloacetate and the production of ATP, NADH, and FADH2. These products are essential for the electron transport chain, which produces the majority of ATP in cellular respiration. Thus, the citric acid cycle plays a pivotal role in energy production.

Fermentation vs Oxidation

Fermentation and oxidation are two different metabolic processes that cells use to generate energy. Fermentation occurs in anaerobic conditions (absence of oxygen) and leads to the production of lactate or ethanol. It is less efficient than oxidative metabolism, producing only 2 ATP molecules per glucose molecule. In contrast, oxidation occurs in the presence of oxygen (aerobic conditions) and involves the citric acid cycle and electron transport chain. This process is much more efficient, generating up to 36 ATP molecules per glucose molecule. During fatty acid β-oxidation, the body relies on oxidative metabolism to maximize energy output.