Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-requiring processes
Metabolism of nitrogen-containing compounds
Excretion of ammonia from the organism

During amino acid degradation, which takes place in the Liver in mammals, ammonia is released. In addition, significant amounts of ammonia are produced during The breakdown of Purines and Pyrimidines. Some of this ammonia is immediately utilized for the Synthesis of Other Amino Acids and nitrogenous bases, while the remaining portion must be rapidly inactivated or excreted from the Organism, as this compound is a potent cellular toxin.

In organisms at lower evolutionary stages, as well as in aquatic animals, ammonia is excreted directly from Cells, or through the gills in fish, for example. Such animals are termed ammonotelic. In most terrestrial vertebrates, including mammals and humans, ammonia is converted into urea. Organisms that eliminate the bulk of their ammonia in the form of urea are called ureotelic. Finally, there are uricotelic organisms (birds and reptiles), which convert ammonia into uric acid and excrete it in a solid state.

The properties of urea as a substance through which ammonia nitrogen is excreted in ureotelic organisms include being a neutral, non-toxic, low-molecular-weight compound that easily crosses membrane barriers via Passive Diffusion and is readily soluble in Water. Urea is easily transported by the Blood and excreted in the urine.

Urea is synthesized in liver cells through a closed sequence of reactions known as The Urea Cycle.

The urea cycle. One of the nitrogen atoms in the urea molecule is derived from ammonia, and the second from aspartate. The carbon atom of urea originates from a CO₂ molecule.

In the first step, free ammonia and carbon dioxide form a molecule of carbamoyl phosphate in an ATP-dependent reaction. This molecule contains an anhydride bond and features a high carbamoyl-group transfer potential. The carbamoyl residue is transferred to Ornithine with the participation of ornithine transcarbamylase, yielding citrulline (Fig. 16.13).

In the next step, mediated by argininosuccinate synthase, the second amino group—contributed by aspartate—enters the cycle. The Condensation of citrulline with aspartate is also coupled with ATP Hydrolysis and leads to The formation of argininosuccinate.

In The final stage, involving argininosuccinate lyase, the substrate is cleaved into Arginine and fumarate. Arginine is then hydrolyzed by arginase into urea and ornithine, which can once again accept a carbamoyl residue. Fumarate can be converted into oxaloacetate (via TCA cycle reactions), which is subsequently transformed into aspartate through Transamination involving glutamate. Aspartate can then re-enter the urea cycle by condensing with citrulline (Fig. 16.13).

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Fig. 16.13. The urea cycle. Enzyme names are enclosed in circles beneath the arrows: 1 — ornithine transcarbamylase; 2 — argininosuccinate synthase; 3 — argininosuccinate lyase; 4 — arginase. Abbreviations: Glu — glutamate; a-KG — a-ketoglutarate

The compartmentalization of the urea cycle spans the Cell/35.html">Mitochondria and Cytosol: the reactions of carbamoyl phosphate formation, its incorporation into citrulline, and The conversion of fumarate to aspartate occur in the mitochondrial matrix. The remaining three reactions (the formation of argininosuccinate, arginine and fumarate, and ornithine and urea) take place in the cytosol.

The synthesis of a single urea molecule consumes four high-energy ATP bonds, meaning that The process of ammonia detoxification incurs a high energy cost for The Cell.

The Regulation of the urea cycle rate is governed by its first reaction. The enzyme catalyzing this step, carbamoyl phosphate synthetase I, is allosterically activated by N-acetylglutamate, a precursor of ornithine, and is practically inactive in its absence.

The urea cycle is the primary pathway for ammonia detoxification. However, even ureotelic organisms are capable of excreting a small fraction of ammonia directly via the hydrolytic deamidation of glutamine and the oxidative deamidation of glutamate in Kidney cells. The released ammonia diffuses across the cell membranes of the renal tubules into the tubular lumen (urine), where it combines with protons to form ammonium ions. In this form, ammonia can no longer cross the membrane barrier to re-enter the cell and is thus completely excreted in the urine.

Several inherited disorders are known that involve Impairment of the urea cycle due to defects in its enzymatic machinery. These conditions are invariably associated with elevated concentrations of ammonia in Tissues and blood (hyperammonemia), which can even be fatal. In such diseases, the systems most sensitive to ATP depletion—specifically Nerve Cells and the Brain—suffer first. This can be explained by the fact that high concentrations of ammonia shift the equilibrium of the reversible Glutamate dehydrogenase reaction toward glutamate formation, which inevitably depletes a-ketoglutarate reserves. However, a-ketoglutarate is an intermediate of the TCA cycle, and its excessive consumption leads to a slowdown in The Citric Acid Cycle—the primary supplier of reducing equivalents for the Respiratory Chain. Consequently, The rate of ATP synthesis decreases.