Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter III. METABOLISM OF MAJOR CLASSES OF BIOMOLECULES

CHAPTER 17. AMINO ACID METABOLISM I. GENERAL PATHWAYS OF TRANSFORMATION

17.2. AMINO ACID TRANSAMINATION

Transamination reactions involve The transfer of an α-amino group from an amino acid to the α-carbon atom of an α-keto acid, which serves as the amino group acceptor (most commonly α-ketoglutarate). This reaction yields the α-keto analogue of the original Amino Acid and a new amino acid (L-glutamate when α-ketoglutarate is used as the acceptor):

Class="center">

Fig. 17.2. General scheme of transamination reactions.

The Enzymes catalyzing transamination reactions are aminotransferases (transaminases).

Aminotransferase Reactions

Human and Animal Tissues contain more than ten different aminotransferases that vary in their substrate Specificity. The most widespread aminotransferases include:

(1) Alanine aminotransferase (glutamate-Pyruvate transaminase — GPT):

(2) aspartate aminotransferase (glutamate-oxaloacetate transaminase — GOT):

(3) Tyrosine aminotransferase:

(4) leucine aminotransferase:

Transamination Reactions Catalyzed by aminotransferases proceed actively in many Organs, most intensely in the Liver, skeletal Muscles, myocardium, Brain, and Kidneys. Determining The activity of alanine aminotransferase (alanine transaminase — ALT) and aspartate aminotransferase (aspartate transaminase — AST) is widely used in medical practice to diagnose internal organ damage. Due to the leakage of these enzyme Proteins through damaged Cell membranes into the Blood, a significant increase in serum AST activity is observed in myocardial infarction, while viral and toxic liver damage lead to a sharp rise in ALT activity.

MECHANISM OF ACTION of Aminotransferases

Aminotransferases are complex protein enzymes whose prosthetic groups are Coenzyme forms of vitamin B₆ (pyridoxine, pyridoxol) — namely, Pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP), which is formed from PLP during the amino group transfer.

The formation of the coenzyme from dietary vitamin B₆ occurs via the phosphorylation of pyridoxol to pyridoxol phosphate (PLP) by an ATP-dependent kinase, followed by The oxidation of pyridoxol phosphate to PLP by a specific flavoprotein.

Within the aminotransferase enzyme, the coenzyme (PLP) is linked to the polypeptide chain through the formation of an aldimine bond (Schiff base) with the ε-amino group of a Lysine residue (Lys-258).

During the catalytic act of transamination, PLP undergoes cyclic conversion into PMP:

The process consists of two half-reactions:

1) interaction of an amino acid losing its amino group with pyridoxal phosphate, yielding a keto acid and pyridoxamine phosphate:

The reaction mechanism involves the interaction of The amino acid with PLP-E, where the aldimine bond within PLP-E is replaced by an aldimine bond between the coenzyme and the amino acid (1a); following an intramolecular rearrangement (1b), the ketimine undergoes Hydrolysis (1c) to yield the amino group-containing coenzyme form (PMP) and $\alpha$-keto acid₁:

Half-reaction of the transamination catalytic cycle.

2) interaction of the amino group-accepting $\alpha$-keto acid₂ with pyridoxamine phosphate, resulting in the formation of a new amino acid and the regeneration of pyridoxal phosphate:

The Mechanism of this half-reaction is analogous to that described for the first half-reaction (operating in reverse); the PLP regenerated In the second half-reaction re-associates via an aldimine bond with the protein moiety of the enzyme.

The transamination process and the enzymes catalyzing it were first described in 1937 by A.E. Braunstein and M.G. Kritzmann; later, A.E. Braunstein and M.M. Shemyakin proposed The Theory of pyridoxal catalysis. A.E. Braunstein was born in Kharkiv, graduated from the Kharkiv Medical Institute, and worked at the Institute of Biological and Medical Chemistry of the USSR Academy of Medical Sciences and the Institute of Molecular Biology of the USSR Academy of Sciences.

Fig. 17.3. Alexander O. Braunstein (1902–1986)— Academician of the USSR Academy of Sciences and Academy of Medical Sciences.

Biochemical significance of transamination reactions

As follows from the above, transamination reactions do not involve deamination, i.e., the release of ammonia, because the amino group cleaved from the $\alpha$-L-amino acid is accepted by a corresponding $\alpha$-keto acid, most commonly $\alpha$-ketoglutaric acid.

The biochemical significance of transamination varies significantly across different organs.

In the liver, The Role of transamination lies in its collector function, which is the pooling of amino groups from various Amino Acids predominantly into a single molecular form—L-glutamic acid. The biochemical rationale of this process is that L-glutamate serves as the primary substrate for deamination reactions, acting as the supplier of amino groups for the metabolic pathway leading to the formation of urea, the end product of nitrogen Catabolism.

In muscles, the directionality of transamination reactions leads to the formation of significant amounts of alanine (via Transamination of Amino acids with pyruvate), which is released into the bloodstream and taken up by hepatocytes; in the liver, alanine is converted back to pyruvate, which is utilized in Gluconeogenesis (the glucose-alanine cycle—see Chapter 12).