Urea Cycle

The urea cycle is the pathway, located mainly in the liver, that converts toxic ammonia derived from amino acid breakdown into urea, a soluble and far less toxic compound that the kidneys excrete. It was the first cyclic metabolic pathway to be described and remains the body's principal route for disposing of surplus nitrogen.

Definition

The urea cycle is a partly mitochondrial, partly cytosolic series of reactions that combines ammonia and aspartate-derived nitrogen with carbon dioxide to synthesize urea, regenerating the carrier ornithine with each turn.

Scope

This entry covers the five core enzymatic steps of the cycle, its partitioning between mitochondrion and cytosol, the two nitrogen sources it incorporates, and how it is regulated. The fate of amino groups before they enter the cycle is covered in the catabolism entry, and broader ammonia handling in the nitrogen entry.

Core questions

• Where do the two nitrogen atoms of urea come from?
• Which steps occur in the mitochondrion and which in the cytosol?
• How is flux through the cycle matched to the nitrogen load?

Key concepts

• Carbamoyl phosphate synthetase I
• Ornithine transcarbamylase
• Argininosuccinate synthetase and lyase
• Arginase and regeneration of ornithine
• N-acetylglutamate as an allosteric activator
• Mitochondrial-cytosolic compartmentation

Mechanisms

Within the mitochondrion, ammonia and bicarbonate are condensed into carbamoyl phosphate by carbamoyl phosphate synthetase I, an enzyme that requires the allosteric activator N-acetylglutamate. Ornithine transcarbamylase then joins carbamoyl phosphate to ornithine to form citrulline, which is exported to the cytosol. There, argininosuccinate synthetase incorporates the second nitrogen from aspartate to make argininosuccinate, which argininosuccinate lyase cleaves into arginine and fumarate. Arginase finally hydrolyzes arginine to urea and ornithine, and the regenerated ornithine re-enters the mitochondrion to begin another turn. Thus one nitrogen of urea comes from free ammonia and the other from aspartate, while the carbon comes from bicarbonate. Flux is tuned both by the supply of N-acetylglutamate, which reflects the overall nitrogen load, and by longer-term changes in enzyme amounts.

Clinical relevance

Inherited deficiencies of urea cycle enzymes impair nitrogen disposal and can lead to ammonia accumulation, and the cycle is central to how the body remains protected from ammonia toxicity. This entry describes the pathway and how it is studied; diagnosis and management of urea cycle disorders follow specialist consensus guidelines and are not addressed as individual advice here.

Epidemiology

Urea cycle disorders are individually rare inborn errors of metabolism; consensus guidelines summarize their combined frequency and clinical recognition, while detailed estimates belong to dedicated clinical sources.

Evidence & guidelines

The biochemistry is established textbook knowledge; for the clinical disorders of the cycle, international consensus guidelines have been published and revised (Haeberle et al., 2012; 2019), which the present entry references only to indicate where clinical standards reside.

History

Hans Krebs and Kurt Henseleit described the ornithine cycle of urea formation in 1932, the first metabolic cycle to be recognized. Subsequent work, notably by Sarah Ratner, characterized the argininosuccinate steps and completed the enzymatic map of the pathway.

Key figures

• Hans Krebs
• Kurt Henseleit
• Sarah Ratner

Seminal works

• morris-2002
• haeberle-2019

Frequently asked questions

Why does the body convert ammonia to urea?

Ammonia is toxic, especially to the nervous system, so converting it to urea, which is soluble and far less toxic, allows surplus nitrogen to be transported in the blood and excreted safely by the kidneys.

Where in the cell does the urea cycle take place?

It is split between two compartments: the first steps occur in the mitochondrial matrix of liver cells, and the remaining steps occur in the cytosol, with intermediates shuttled between the two.