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

Section III. METABOLISM OF THE MAIN CLASSES OF BIOMOLECULES

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

17.4. AMINO ACID DECARBOXYLATION

The amino acid decarboxylation reaction involves the Cleavage of carbon dioxide from an amino acid molecule to form amines (biogenic amines), many of which exhibit high physiological activity as Hormones or Neurotransmitters, or serve as their precursors and metabolites:

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This reaction is catalyzed by Enzymes—amino acid Decarboxylases—whose coenzyme is Pyridoxal phosphate. During the catalytic act, pyridoxal phosphate forms Schiff bases with Amino Acids, similar to those observed in Transamination reactions. Amino acid decarboxylases are stereospecific enzymes that act exclusively on L-stereoisomers.

Both acyclic and cyclic Amino acids can undergo decarboxylation; about forty biogenic amines have been identified in human urine.

Physiological Significance of Amino Acid Decarboxylation

1. Formation of physiologically active compounds—hormones and neurotransmitters.

Examples of physiologically active compounds produced via amino acid decarboxylation include the following reactions:

(1) Formation of γ-(gamma)-aminobutyric acid (GABA, γ-aminobutyrate) from L-glutamate:

(2) formation of histamine from L-Histidine:

2. Amino Acid Catabolism during intestinal protein putrefaction.

Decarboxylation processes of amino acids occur intensively in the lumen of the Large Intestine under the action of microbial enzymes that constitute the normal microflora of the Human digestive tract ("bacterial putrefaction of Proteins in the intestine").

Examples of intestinal amino acid decarboxylation reactions include The formation of ptomaines ("corpse poisons") from Diaminomonocarboxylic Acids:

(a, δ-diaminovaleric acid) (tetramethylenediamine)

(a, ε-diaminocaproic acid) (pentamethylenediamine)

Oxidation of Biogenic Amines

The accumulation of biogenic amines in the body leads to adverse pathophysiological changes in The Cardiovascular system, intestines, and other smooth-Muscle Organs. The detoxification (neutralization) of physiologically active amines takes place in Liver Cells involving mitochondrial monoamine oxidase, a FAD-dependent enzyme that catalyzes the Oxidative Deamination of amines into aldehydes:

Aldehydes, the products of biogenic amine deamination, are oxidized to their corresponding acids and undergo further oxidative degradation or are excreted from the body in the urine. Ammonia enters the urea synthesis pathway.