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

Section III. METABOLISM OF MAJOR CLASSES OF BIOMOLECULES

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

17.3. DEAMINATION OF AMINO ACIDS

In Human and Animal Cells, L-glutamic acid undergoes the most active deamination; this process occurs via The Mechanism of Oxidative Deamination, According to the equation:

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The first step—The formation of α-iminoglutarate—is catalyzed by the NAD-dependent enzyme Glutamate dehydrogenase, which is localized in the Cell/35.html">Mitochondria; the second step—the formation of α-ketoglutarate—is non-enzymatic. The resulting α-ketoglutarate is oxidized in The Tricarboxylic Acid Cycle, while ammonia is utilized by the urea synthesis enzymatic system.

The reverse process—the reductive amination of α-ketoglutarate to L-glutamate—can take place in the Cytosol with the participation of cytosolic NADP-dependent glutamate dehydrogenase, serving as an auxiliary mechanism for ammonia binding.

Indirect deamination of L-Amino Acids

Since L-glutamate is The amino acid formed during the Transamination of most L-amino acids, the course of the glutamate dehydrogenase reaction serves as the central biochemical mechanism through which most free Amino acids are deaminated and the bulk of ammonia is generated in the animal Organism.

Such deamination of free L-amino acids via the Mechanism of coupling transamination reactions with α-ketoglutarate and the oxidative deamination of L-glutamate is termed indirect deamination:

Fig. 17.4. Scheme of indirect deamination of L-amino acids.

Indirect amino acid deamination involving glutamate dehydrogenase is most active in the Liver, where the released ammonia enters The Urea Cycle.

L- and D-amino acid oxidases

The Liver and Kidneys contain Enzymes known as amino acid oxidases. These are Flavoproteins (FP) that catalyze the oxidative deamination of L- and D-amino acids with the formation of hydrogen peroxide:

L-amino acid oxidases are FMN-dependent, whereas D-amino acid oxidases are FAD-dependent enzymes. Hydrogen peroxide is a toxic metabolite that initiates Lipid Peroxidation of cell membranes. The detoxification of hydrogen peroxide is achieved through its Cleavage by peroxisomal catalase:

Under physiological conditions, L- and D-amino acid oxidases have low activity, and their exact biochemical significance remains unclear.