Question

Direct synthesis. Which of the 20 amino acids can be synthesized directly from a common metabolic intermediate by a transamination reaction?

Short answer

Alanine, aspartate, and glutamate can be synthesized by direct transamination from pyruvate, oxaloacetate, and α-ketoglutarate, respectively.

Step-by-step solution

Understanding Amino Acid Synthesis

Amino acids can be synthesized through various biochemical pathways, one of which is transamination. Transamination involves transferring an amino group to a precursor molecule.

Identifying Transamination Reactions

In transamination, a common keto acid intermediate can be converted into an amino acid. This process often involves intermediates from central metabolic pathways such as glycolysis or the citric acid cycle.

Reviewing Common Metabolic Intermediates

Common metabolic intermediates include pyruvate, oxaloacetate, and α-ketoglutarate. These intermediates can accept amino groups to form specific amino acids through transamination.

Matching Intermediates with Amino Acids

- Pyruvate is transaminated to form alanine. - Oxaloacetate is transaminated to form aspartate. - α-ketoglutarate is transaminated to form glutamate.

Conclusion

The amino acids alanine, aspartate, and glutamate can be synthesized directly from the common metabolic intermediates pyruvate, oxaloacetate, and α-ketoglutarate, respectively, via transamination reactions.

Key concepts

Transamination

Transamination is a crucial process in amino acid synthesis where an amino group is transferred from one molecule to another. This is usually facilitated by enzymes called transaminases or aminotransferases. The donor molecule typically loses its amino group, becoming a keto acid, while the recipient becomes an amino acid.
Transamination reactions are reversible and play a significant role in the nitrogen economy of the cell, allowing for the creation of essential amino acids from existing nitrogen sources. This adaptability is vital for cellular survival, especially under nutrient-scarce conditions.

Metabolic Intermediates

Metabolic intermediates are molecules that are part of the metabolic pathways which lead to the synthesis of various cellular components, including amino acids.
These intermediates are often by-products or transforming products of major nutrient breakdown pathways like glycolysis and the citric acid cycle.

• They serve as vital links connecting various biochemical processes.
• Some intermediates directly undergo transamination to partake in amino acid synthesis.

They thus contribute to the seamless integration of metabolic pathways within cells, ensuring efficient energy flow and resource use.

Keto Acids

Keto acids are a fundamental part of the transamination process. They are acids that contain a ketone group and act as precursors for amino acid synthesis. In transamination, a keto acid receives an amino group, transforming into an amino acid.
Examples of keto acids include:

• Pyruvate - which can form alanine
• Oxaloacetate - which can form aspartate
• α-Ketoglutarate - which can form glutamate

Such transformations are crucial for providing the necessary building blocks for protein synthesis and cellular function.

Glycolysis

Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, generating energy in the form of ATP. It is the first step in cellular respiration and takes place in the cytoplasm of the cell.
Throughout glycolysis, several intermediates are produced, which have roles in transamination. For instance:

• Pyruvate, an end product of glycolysis, is a keto acid that participates in forming alanine through transamination processes.

This link highlights the interconnectedness between energy metabolism and amino acid synthesis.

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle, plays a central role in energy production and provides intermediates for biosynthesis, including amino acids.
It takes place in the mitochondria, processing acetyl-CoA into CO₂ and capturing high-energy electrons.

• Prominent intermediates like α-ketoglutarate and oxaloacetate participate in transamination.
• These intermediates contribute to forming amino acids such as glutamate and aspartate, underscoring the cycle's integrative role in metabolism.

The citric acid cycle thus operates at the heart of both energy and nitrogen metabolism, balancing the cell's needs for energy and amino acids.