This model describes the larger, tetrameric form of phenylalanine-4-hydroxylase, as found in metazoans. The enzyme irreversibly converts phenylalanine to tryosine and is known to be the rate-limiting step in phenylalanine catabolism in some systems. It is closely related to metazoan tyrosine 3-monooxygenase and tryptophan 5-monoxygenase, and more distantly to monomeric phenylalanine-4-hydroxylases of some Gram-negative bacteria. The member of this family from Drosophila has been described as having both phenylalanine-4-hydroxylase and tryptophan 5-monoxygenase activity (. However, a Drosophila member of the tryptophan 5-monoxygenase clade has subsequently been discovered.