Tyrosine is an amino acid that forms proteins. It has ionized side chains of aromatic rings and is hydrophilic. Tyrosine is produced by the hydroxylation of phenylalanine in humans and animals, so it is a nonessential amino acid when phenylalanine is adequately nourished. The catabolism of tyrosine begins with the conversion of tyrosine into hydroxyphenylpyruvate catalyzed by Chemicalbook ammonase, which requires pyridoxal phosphate as a coenzyme. Through the action of p-hydroxyphenylpyruvate hydroxylase, p-hydroxyphenylpyruvate simultaneously causes oxidative decarboxylation and transfer of side chain pyruvate and ortho hydroxylation of benzene ring to produce urine black acid (dihydroxyphenylacetic acid).