Ornithine

Ornithine is a nonproteinogenic amino acid that functions as an intermediate in the urea cycle and a precursor to polyamines.

Urea cycle: The metabolic pathway that converts toxic ammonia into urea for excretion. Ornithine is regenerated within this cycle as nitrogen is routed toward urea.
Amino acid: An organic molecule containing amino and carboxyl groups, often serving as a protein building block or metabolic intermediate. Ornithine belongs to this chemical family but is not incorporated into proteins.
Ornithine transcarbamylase deficiency: An inherited urea-cycle disorder caused by deficient ornithine transcarbamylase activity. The blocked use of ornithine impairs ammonia disposal and can cause hyperammonemia.
Polyamine: A small organic compound containing multiple amino groups, involved in cell growth and other cellular processes. Ornithine supplies the carbon skeleton for putrescine, the entry point to polyamine synthesis.
Citrulline: A nonproteinogenic amino acid formed from ornithine and carbamoyl phosphate in the urea cycle. Ornithine combines with carbamoyl phosphate to produce citrulline.
Arginine: A proteinogenic amino acid and urea-cycle intermediate that can be cleaved to form ornithine. Arginase generates ornithine from arginine in the final urea-cycle reaction.
Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome: A rare metabolic disorder caused by defective mitochondrial ornithine transport. Impaired ornithine entry into mitochondria disrupts the urea cycle.
Arginine metabolism: The network of reactions that synthesize, use, and recycle arginine in cells. Arginine breakdown is a major source of ornithine in mammals.
Argininosuccinate: A urea-cycle intermediate formed from citrulline and aspartate. Its breakdown produces arginine, which is then cleaved to regenerate ornithine.
Carbamoyl phosphate: An activated carbamoyl-group donor used in the urea cycle and pyrimidine synthesis. It donates the carbamoyl group that converts ornithine into citrulline.

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Tropane alkaloidRelated: Plant tropane biosynthesis draws on ornithine-derived intermediates to build the nitrogenous ring.