Urea Cycle

Converts toxic ammonia to urea for excretion.

Overview

The urea cycle detoxifies ammonia generated from amino acid catabolism by converting it to urea, which is excreted by the kidneys. The cycle spans both mitochondria and cytoplasm of hepatocytes. Carbamoyl phosphate synthetase I catalyzes the committed step in the mitochondria, and the cycle proceeds through citrulline, argininosuccinate, and arginine before arginase releases urea and regenerates ornithine.

Cellular Location

Mitochondria and cytoplasm (liver)

Clinical Significance

Essential for nitrogen waste elimination; defects cause hyperammonemia and urea cycle disorders; N-acetylglutamate is the essential allosteric activator.

Key Molecules

NH₃ (ammonia)CO₂ Carbamoyl phosphate Ornithine Citrulline Argininosuccinate Arginine Urea N-acetylglutamate

Key Enzymes

Carbamoyl phosphate synthetase I (CPS1)Ornithine transcarbamylase (OTC)Argininosuccinate synthetase Argininosuccinate lyase Arginase

Related Pathways

Amino Acid Catabolism

Degradation of amino acids to metabolic intermediates and ammonia.

Citric Acid Cycle (Krebs Cycle)

Oxidizes acetyl-CoA to CO₂, generating GTP, NADH, and FADH₂.