Urea cycle

The urea cycle is a series of reactions, mainly in the liver, that converts toxic ammonia into urea for excretion. It links nitrogen disposal with amino acid metabolism.

Carbamoyl phosphate synthetase I: A mitochondrial enzyme that combines ammonia, bicarbonate, and ATP to form carbamoyl phosphate. It initiates the cycle by fixing free ammonia into a usable intermediate.
Ammonia: A nitrogen-containing compound produced during amino-acid breakdown that can disrupt cellular function at high concentrations. The cycle removes ammonia before its accumulation becomes toxic.
Ornithine transcarbamylase deficiency: An X-linked urea-cycle disorder caused by deficient ornithine transcarbamylase activity. It is the most common urea-cycle disorder and often causes elevated ammonia and orotic acid.
Uric acid: A nitrogen-containing waste product formed during purine breakdown and excreted by many animals. Unlike urea, it is produced from purine metabolism and is less water-soluble.
Ornithine transcarbamylase: A mitochondrial enzyme that transfers a carbamoyl group from carbamoyl phosphate to ornithine, producing citrulline. It creates citrulline, which carries cycle intermediates from mitochondria into the cytosol.
Aspartate: An amino acid that donates nitrogen in biosynthesis and participates in energy metabolism. It supplies one of the two nitrogen atoms incorporated into urea.
Carbamoyl phosphate synthetase I deficiency: A rare inherited disorder in which deficient carbamoyl phosphate synthetase I impairs ammonia disposal. Failure at the cycle’s first reaction can cause severe hyperammonemia.
Ammonotelism: An excretory strategy in which animals eliminate most nitrogen directly as ammonia. It avoids the urea cycle’s conversion steps but requires abundant water for ammonia excretion.
Citrulline: A nonprotein amino acid formed in mitochondria during the urea cycle and used to build arginine. Its movement into the cytosol marks the cycle’s transition between cellular compartments.
Glutamate: An amino acid that collects and transfers nitrogen through reactions including transamination and deamination. It channels amino-group nitrogen toward ammonia and aspartate, feeding the cycle.

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