Question

Name and draw the structure of the \(a\)-keto acid resulting when each of the four amino acids listed undergoes transamination with \(a\) ketoglutarate: (a) aspartate, (b) glutamate, (c) alanine, (d) phenylalanine.

Short answer

(a) Oxaloacetate, (b) α-Ketoglutarate, (c) Pyruvate, (d) Phenylpyruvate.

Step-by-step solution

Understanding Transamination

Transamination is the process where an amino acid transfers its amino group to a keto acid, typically α-ketoglutarate, resulting in a new keto acid and a new amino acid. The amino group from the original amino acid is transferred to the α-ketoglutarate, transforming it into glutamate.

Structure of Aspartate and its α-Keto Acid

Aspartate undergoes transamination by losing its amino group, leading to the formation of oxaloacetate. The structure of oxaloacetate is similar to aspartate but it has a keto group (=O) replacing the amine group (NH₂) at the alpha position.

Structure of Glutamate and its α-Keto Acid

Glutamate loses its amino group during transamination, resulting in the α-keto acid called α-ketoglutarate. In this reaction, α-ketoglutarate acts as both the donor and recipient molecule, so it stays unchanged in its structure.

Structure of Alanine and its α-Keto Acid

Alanine, when transaminated, loses its amino group and forms pyruvate as the α-keto acid. Pyruvate has a keto group at the alpha position instead of the amino group present in alanine.

Structure of Phenylalanine and its α-Keto Acid

Phenylalanine undergoes transamination and forms phenylpyruvate. In phenylpyruvate, the amino group of phenylalanine is replaced with a keto group at the alpha position, bonding to a benzene ring.

Key concepts

Amino Acids

Amino acids are the building blocks of proteins, which play critical roles in various biological processes. Each amino acid contains an amino group ( H₂N ), a carboxyl group (COOH), and a unique side chain or R-group that determines its specific properties. These molecules are essential for the synthesis of proteins, enzymes, neurotransmitters, and other crucial compounds in the body.

In the context of biochemical transamination, amino acids are the source of amino groups that get transferred to keto acids. The process helps in synthesizing non-essential amino acids, which the body can produce on its own. For example, during transamination, alanine can lose its amino group to form pyruvate, while aspartate becomes oxaloacetate.

Keto Acids

Keto acids are organic compounds that contain both a carboxyl group and a keto group. These acids are key intermediates in many metabolic pathways.

In transamination reactions, keto acids act as acceptors of amino groups from amino acids. When an amino acid donates its amino group, it converts into a corresponding keto acid. For instance, when alanine donates its amino group, it becomes pyruvate, a keto acid with the structure CH₃C(=O)COOH . These reactions are crucial for the catabolism and anabolism of amino acids.

α-Ketoglutarate

α-Ketoglutarate is a pivotal compound in transamination reactions and the Krebs cycle (TCA cycle). Structurally, it is a 5-carbon keto acid with the formula C₅H₆O₅ .

Its role in transamination is significant because it often serves as the amino group acceptor. When it accepts an amino group, it is converted to glutamate, another amino acid. This transformation exemplifies the crucial function of α-ketoglutarate as both a linker and recycler of nitrogen in the cell. Moreover, its conversion and regeneration ensure the balance between amino acid and keto acid pools within cells.

Enzymatic Reactions

Enzymatic reactions are biochemical processes catalyzed by enzymes, which are proteins that speed up chemical reactions in the body. In the context of transamination, enzymes called aminotransferases catalyze the transfer of amino groups between molecules.

Aminotransferases are specific for different pairs of amino acids and keto acids. They facilitate the conversion of, for instance, aspartate to oxaloacetate and alanine to pyruvate. The presence of an enzyme ensures the reaction is efficient and occurs under physiological conditions, which otherwise might be too slow. Enzyme specificity and function are vital for maintaining metabolic pathways and responding to cellular demands dynamically.