How Does the Body Break Down Amino Acids?

Approximately 50% of the body's free amino acids are metabolized in the liver at any given time, highlighting its central role in protein homeostasis. Excess amino acids are not stored but are broken down through a process known as catabolism, which answers the question of how does the body break down amino acids.

Quick Summary

Amino acids are broken down through transamination and deamination to separate the nitrogen group from the carbon skeleton. The liver converts the toxic ammonia from this process into urea for excretion via the kidneys. The remaining carbon skeletons can be repurposed for energy, converted to glucose, or stored as fat.

Key Points

• Catabolism Process: The breakdown of excess amino acids involves two main stages: the removal of the nitrogen-containing amino group and the metabolism of the remaining carbon skeleton.

• Transamination: This process transfers an amino group from an amino acid to an alpha-keto acid, often involving glutamate as an intermediary to pool nitrogen.

• Deamination: This is the removal of the amino group as free ammonia, a highly toxic byproduct that must be detoxified quickly.

• Urea Cycle: The liver converts toxic ammonia into less harmful urea through a series of five enzymatic reactions, which is then excreted by the kidneys.

• Carbon Skeleton Fates: After the amino group is removed, the remaining carbon skeleton can be used for energy, converted into glucose (glucogenic), or synthesized into ketone bodies or fats (ketogenic).

• Metabolic Linkage: The processes of amino acid breakdown, the urea cycle, and energy production are closely linked, allowing the body to maintain homeostasis and respond to different energy demands.

The intricate process of amino acid breakdown, also known as amino acid catabolism, is vital for maintaining metabolic balance within the body. When dietary protein is consumed, it is first digested into individual amino acids and small peptides. These are then absorbed into the bloodstream and transported to the liver and other tissues. Unlike carbohydrates and fats, the body has no mechanism to store excess amino acids. Therefore, any surplus must be broken down to prevent toxic build-up, especially of nitrogen. This catabolic process involves two primary steps: the removal of the nitrogen-containing amino group and the metabolism of the remaining carbon skeleton.

The Initial Steps: Transamination and Deamination

The degradation of amino acids begins with the separation of the alpha-amino group from the carbon skeleton. This is primarily achieved through two closely related enzymatic reactions: transamination and deamination.

Transamination

Transamination is the exchange of an amino group from one amino acid to an alpha-keto acid, creating a new amino acid and a new alpha-keto acid. This process is crucial for redistributing amino groups and synthesizing non-essential amino acids as needed. The enzymes that catalyze this are called aminotransferases, and they require pyridoxal phosphate (a derivative of vitamin B6) as a coenzyme. In many reactions, alpha-ketoglutarate serves as the primary acceptor for the amino group, forming glutamate.

Deamination

Deamination is the direct removal of the amino group, typically releasing it as free ammonia ($NH_3$). While most amino acids first undergo transamination, glutamate is the only amino acid that undergoes significant oxidative deamination to liberate free ammonia for the urea cycle. The enzyme glutamate dehydrogenase catalyzes this reaction, linking glutamate metabolism with the Citric Acid Cycle. The resulting ammonia is highly toxic and must be detoxified immediately.

The Urea Cycle: Detoxifying Ammonia

Because ammonia is so toxic, particularly to the central nervous system, the body must quickly convert it into a safer, more excretable compound. This happens in the liver through the urea cycle, also known as the ornithine cycle.

1. Step 1: Carbamoyl Phosphate Synthesis: In the mitochondria of liver cells, ammonia ($NH_4^+$) and bicarbonate ($HCO_3^−$) are combined with the help of the enzyme carbamoyl phosphate synthetase I to form carbamoyl phosphate. This step requires two ATP molecules.
2. Step 2: Citrulline Formation: Carbamoyl phosphate then reacts with ornithine to form citrulline. This occurs in the mitochondria.
3. Step 3: Argininosuccinate Synthesis: Citrulline is transported to the cytoplasm, where it combines with aspartate (which provides the second nitrogen atom for urea) to form argininosuccinate.
4. Step 4: Arginine and Fumarate Production: Argininosuccinate is cleaved to produce arginine and fumarate.
5. Step 5: Urea Formation: Finally, arginine is hydrolyzed to produce urea and ornithine. The ornithine is then recycled back into the mitochondria to continue the cycle.

The urea is then released into the bloodstream, travels to the kidneys, and is excreted in urine.

The Fate of the Carbon Skeleton

After the amino group is removed, the remaining carbon skeleton (an alpha-keto acid) is a valuable metabolic resource. The fate of this carbon skeleton depends on which amino acid it came from, with all 20 amino acids being categorized into one or both of two groups.

Glucogenic vs. Ketogenic Amino Acids

Feature	Glucogenic Amino Acids	Ketogenic Amino Acids
Carbon Skeleton Fate	Converted into pyruvate or intermediates of the Citric Acid Cycle (e.g., oxaloacetate, alpha-ketoglutarate).	Degraded directly into acetyl-CoA or acetoacetyl-CoA.
End Product	Can be converted into glucose via gluconeogenesis, primarily during fasting or low glucose availability.	Cannot be converted to glucose. Used to form ketone bodies or fatty acids.
Exclusively	Cysteine, arginine, alanine, asparagine, aspartic acid, glutamine, glycine, histidine, methionine, proline, serine, and valine are examples.	Leucine and lysine are the only two exclusively ketogenic amino acids.
Both (Mixed)	Isoleucine, phenylalanine, threonine, tryptophan, and tyrosine can be both glucogenic and ketogenic.	Isoleucine, phenylalanine, threonine, tryptophan, and tyrosine can be both ketogenic and glucogenic.

Conclusion

The catabolism of amino acids is a multi-step, tightly regulated biochemical process essential for energy production, metabolic flexibility, and detoxification. Through transamination and deamination, the body efficiently separates the nitrogen from the carbon, neutralizing toxic ammonia via the urea cycle. The resulting carbon skeletons are then repurposed as an energy source, converted to glucose, or used for fatty acid synthesis, demonstrating the body's remarkable ability to adapt and utilize nutrients based on its immediate needs. Understanding how the body breaks down amino acids reveals a key mechanism of metabolic homeostasis and adaptation to dietary protein intake.

Frequently Asked Questions

What happens to the excess nitrogen from amino acids?

Excess nitrogen from amino acid breakdown is removed as an amino group, which is converted into highly toxic ammonia. The liver then processes this ammonia into urea via the urea cycle, which is safely excreted by the kidneys in urine.

Where does the breakdown of amino acids primarily occur?

While some breakdown occurs in other tissues like the small intestine, the liver is the principal site for amino acid metabolism. The liver handles the detoxification of ammonia and determines the metabolic fate of the carbon skeletons.

Are amino acids stored for later use?

Unlike glucose or fatty acids, the body has no dedicated storage mechanism for excess amino acids. Any surplus is quickly catabolized, and their components are either used for energy or converted into glucose or fat for storage.

What is the difference between glucogenic and ketogenic amino acids?

Glucogenic amino acids can be converted into glucose, while ketogenic amino acids can be converted into ketone bodies or fatty acids. Some amino acids fall into both categories.

What is the urea cycle and why is it important?

The urea cycle is a metabolic pathway that occurs primarily in the liver. It is crucial for converting toxic ammonia, a byproduct of amino acid breakdown, into urea, a much less toxic compound that the body can safely excrete.

What is transamination?

Transamination is a reversible chemical reaction where an amino group is transferred from an amino acid to an alpha-keto acid. This process is used to redistribute amino groups and synthesize non-essential amino acids.

Can amino acids be used as an energy source?

Yes, amino acids can be used for energy. When carbohydrates and fats are in short supply (e.g., during starvation or prolonged exercise), the body can break down proteins and use the carbon skeletons from amino acids to produce energy through the Citric Acid Cycle.