Tyrosine hydroxylase (TyrH) is the rate-limiting enzyme in the biosynthesis of catecholamines, including dopamine, epinephrine, and norepinephrine. It catalyzes the hydroxylation of tyrosine to L-DOPA using tetrahydrobiopterin and molecular oxygen. The enzyme's activity is regulated by various mechanisms, including phosphorylation by multiple kinases at four serine residues, dephosphorylation by phosphatases, and feedback inhibition by catecholamines. Dopamine binds to TyrH competitively with tetrahydrobiopterin and interacts with the R domain. TyrH activity is also modulated by protein-protein interactions with enzymes in the same pathway or the tetrahydrobiopterin pathway, structural proteins that act as chaperones, and the protein that transfers dopamine into secretory vesicles. TyrH is modified by nitration of tyrosine residues and glutathioneylation of cysteine residues in the presence of nitric oxide. TyrH is a member of a family of enzymes that also includes phenylalanine hydroxylase (PheH) and tryptophan hydroxylase (TrpH). All three enzymes perform hydroxylation of the aromatic ring of an amino acid. They share a similar active site structure, with a multi-domain structure consisting of an amino-terminal regulatory domain (R), a catalytic domain (C), and a coiled-coil domain. The R domain is involved in regulation, with phosphorylation at serine residues playing a key role in modulating enzyme activity. Phosphorylation by cAMP-dependent protein kinase (PKA) reduces feedback inhibition by catecholamines, while phosphorylation by other kinases such as CaMKII, ERK, and MAPKAPK-2 affects substrate binding and catalytic efficiency. TyrH forms complexes with various proteins, including 14-3-3 proteins, α-synuclein, PP2A, AADC, GTPCH, VMAT, and DJ-1. These interactions influence TyrH activity, localization, and regulation. 14-3-3 proteins bind to phosphorylated TyrH and may stabilize the enzyme or exclude phosphatases. α-synuclein can inhibit TyrH activity by excluding PP2A, while DJ-1 may regulate TyrH and DOPA levels through its oxidation state. The enzyme is also subject to nitration and glutathioneylation, which can inactivate it and are associated with Parkinson's disease. Nitration of TyrH occurs at specific tyrosine residues and is linked to oxidative stress and neuronal damage. These modifications highlight the complex regulatory mechanisms that control TyrH activity and its role in dopamine biosynthesis.Tyrosine hydroxylase (TyrH) is the rate-limiting enzyme in the biosynthesis of catecholamines, including dopamine, epinephrine, and norepinephrine. It catalyzes the hydroxylation of tyrosine to L-DOPA using tetrahydrobiopterin and molecular oxygen. The enzyme's activity is regulated by various mechanisms, including phosphorylation by multiple kinases at four serine residues, dephosphorylation by phosphatases, and feedback inhibition by catecholamines. Dopamine binds to TyrH competitively with tetrahydrobiopterin and interacts with the R domain. TyrH activity is also modulated by protein-protein interactions with enzymes in the same pathway or the tetrahydrobiopterin pathway, structural proteins that act as chaperones, and the protein that transfers dopamine into secretory vesicles. TyrH is modified by nitration of tyrosine residues and glutathioneylation of cysteine residues in the presence of nitric oxide. TyrH is a member of a family of enzymes that also includes phenylalanine hydroxylase (PheH) and tryptophan hydroxylase (TrpH). All three enzymes perform hydroxylation of the aromatic ring of an amino acid. They share a similar active site structure, with a multi-domain structure consisting of an amino-terminal regulatory domain (R), a catalytic domain (C), and a coiled-coil domain. The R domain is involved in regulation, with phosphorylation at serine residues playing a key role in modulating enzyme activity. Phosphorylation by cAMP-dependent protein kinase (PKA) reduces feedback inhibition by catecholamines, while phosphorylation by other kinases such as CaMKII, ERK, and MAPKAPK-2 affects substrate binding and catalytic efficiency. TyrH forms complexes with various proteins, including 14-3-3 proteins, α-synuclein, PP2A, AADC, GTPCH, VMAT, and DJ-1. These interactions influence TyrH activity, localization, and regulation. 14-3-3 proteins bind to phosphorylated TyrH and may stabilize the enzyme or exclude phosphatases. α-synuclein can inhibit TyrH activity by excluding PP2A, while DJ-1 may regulate TyrH and DOPA levels through its oxidation state. The enzyme is also subject to nitration and glutathioneylation, which can inactivate it and are associated with Parkinson's disease. Nitration of TyrH occurs at specific tyrosine residues and is linked to oxidative stress and neuronal damage. These modifications highlight the complex regulatory mechanisms that control TyrH activity and its role in dopamine biosynthesis.