Urea cycle

Urea Cycle

The Urea Cycle (pronounced: yoo-REE-uh CY-kul) is a series of biochemical reactions that produces urea from ammonia (NH3). This cycle occurs in the liver and is crucial for the removal of excess nitrogen from the body. The urea cycle was first described by Hans Adolf Krebs and Kurt Henseleit in 1932.

Etymology

The term "Urea Cycle" is derived from the compound "urea", which is produced during the cycle, and the word "cycle", referring to the series of reactions involved. Urea was first discovered in urine in 1773 by the French chemist Hilaire Rouelle. The word "urea" comes from the Greek word "ouron", meaning urine.

Process

The urea cycle consists of five main steps:

1. Carbamoyl phosphate synthetase I (CPS1) catalyzes the reaction of ammonia and bicarbonate to form carbamoyl phosphate.
2. Ornithine transcarbamylase (OTC) catalyzes the reaction of carbamoyl phosphate and ornithine to form citrulline.
3. Argininosuccinate synthetase (ASS) catalyzes the reaction of citrulline and aspartate to form argininosuccinate.
4. Argininosuccinate lyase (ASL) catalyzes the reaction of argininosuccinate to form arginine and fumarate.
5. Arginase (ARG) catalyzes the reaction of arginine to form urea and ornithine.

The cycle then repeats with the ornithine produced in the last step.

Related Terms

• Ammonia: A compound of nitrogen and hydrogen with the formula NH3.
• Urea: A compound with the chemical formula (NH2)2CO that is produced in the liver and excreted in urine.
• Ornithine: An amino acid that plays a role in the urea cycle.
• Citrulline: An amino acid that is an important intermediate in the urea cycle.
• Arginine: An amino acid that is a key component of the urea cycle.

Disorders

Disorders of the urea cycle, known as urea cycle disorders, can lead to an accumulation of ammonia in the body, which can be harmful. These disorders are usually caused by mutations in the genes that encode the enzymes involved in the urea cycle.

External links

• Medical encyclopedia article on Urea cycle
• Wikipedia's article - Urea cycle

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