Ornithine
Ornithine is a non-proteinogenic α-amino acid that plays a role in the urea cycle. It is not incorporated into proteins during translation. Ornithine is abnormally accumulated in the body in ornithine transcarbamylase deficiency, a disorder of the urea cycle. The moiety derived from ornithine is called ornithyl.
Role in urea cycle
L-Ornithine is one of the products of the action of the enzyme arginase on L-arginine, creating urea. Therefore, ornithine is a central component of the urea cycle, which enables the disposal of excess nitrogen. Ornithine itself is recycled and, in a sense, acts as a catalyst.First, ammonia is converted into carbamoyl phosphate by carbamoyl phosphate synthetase. Ornithine transcarbamylase then catalyzes the reaction between carbamoyl phosphate and ornithine to form citrulline and phosphate. Another amino group is contributed by aspartate, leading to the formation of arginine and the byproduct fumarate. The resulting arginine, a guanidinium compound, is subsequently hydrolyzed by arginase to regenerate ornithine and release urea.
The two nitrogen atoms in urea are derived from ammonia and aspartate, while the nitrogen atoms in ornithine remain unchanged.
Image:OTC reaction.png|thumb|left|458px|Reaction mechanism:. The side-chain amino group of ornithine nucleophilically attacks the carbonyl carbon of carbamoyl phosphate, left, forming a tetrahedral transition state, middle. Charge rearrangement then releases citrulline and phosphate, right.
Ornithine is not an amino acid directly coded for by DNA; that is, it is not a proteinogenic amino acid. However, in mammalian non-hepatic tissues, the primary role of the urea cycle is often the biosynthesis of arginine. As an intermediate in metabolic pathways, ornithine is thus quite important.
Other reactions
Ornithine, through the action of ornithine decarboxylase, serves as the starting point for the synthesis of polyamines such as putrescine.In bacteria such as E. coli, ornithine can be synthesized from L-glutamate.