Arginine biosynthesis starts with glutamate as the precursor (the called "glutamate family") from where also proline and ornithine come from. The precursor of Arginine biosynthesis from glutamate - proline biosynthesis pathway is an intermediate compound, the glutamate-5-semialdehyde that is transaminated by the enzyme ornithine-delta-aminotransferase to form an ornithine molecule. On the other hand, glutamate can form ornithine through another way, first, the formation of N-Acetylglutamate (catalyzed by the enzyme N-acetylglutamate synthase) that is phosphorylated by ATP, catalyzed by the acetylglutamate kinase enzyme, to form N-Acetylglutamate-5-phosphate. This intermediate form N-Acetylglutamate-5-semialdehyde by the enzyme N-acetyl-gamma-glutamyl phosphate dehydrogenase in the presence of NAD(P)H. The next step is the formation of N-Acetylornithine catalyzed by the enzyme N-acetylornithine-delta-aminotransferase. Finally, ornithine is formed by the catalysis of the enzyme acetylornithine deacetylase. In both cases, an ornithine compound that goes to the urea cycle, that occurs partially in the mitochondrion and partially in the cytosol. In the urea cycle, ornithine is transported across the mitochondrial membrane by a specific transport system and converted to citrulline by an ornithine transcarbamoylase. Citrulline leaves the mitochondrion by a specific transport through the mitochondrial membrane and converted into Argininosuccinate by the enzyme argininosuccinate synthase. The latter compound is transformed into Arginineby the loss of fumarate molecule and catalyzed by the enzyme arginase.
