Created: 29 July 1998
Last modified: 2 February 1999

Aromatic amino acid hydroxylases

Prosthetic group features
Aromatic amino acid hydroxylase (AAAOH) reaction
AAAOH in enzyme databases
AAAOH in motif databases
AAAOH in alignment databases
AAAOH in 3D databases
References

Iron centre	Iron ligands	Formal iron oxidation/spin states
Fe(N_His)₂O_Glu·2H₂O	2×N_His; ¹O_Glu; 2×H₂O	Fe^II (S=2); Fe^III (S=5/2)

Tyrosine hydroxylase (TyrOH), phenylalanine hydroxylase (PheOH) and tryptophan hydroxylase (TrpOH) are members of the aromatic amino acid hydroxylase family [1, 2]. They all are homotetrameric, contain a mononuclear iron and utilise dioxygen and tetrahydrobiopterin as substrates in a hydroxylase reaction (1) (under some conditions, oxidation of the tetrahydrobiopterin can be uncoupled from the hydroxylation of the amino acid [3]).

A	+	Tetrahydrobiopterin	+	O₂		A-OH	+	Dihydrobiopterin	+	H₂O	(1)
A = Phe, Tyr or Trp

The purified TyrOH from mammalian tissues contains a highspin Fe^III centre and has a characteristic bluegreen colour ( lambda

_max ~ 700 nm). This colour has been shown to be due to a bidentate catecholamine-Fe^III complex in the enzyme which gives rise to the chargetransfer transitions. The catecholaminebound form is substantially inhibited. The purified recombinant enzyme does not contain bound catecholamines and has a light green colour due to the broad absorbance from the ultraviolet to about 500 nm which is attributed to histidinetoFe^III chargetransfer interactions. Catalytically active TyrOH, however, contains Fe^II. This suggests that the iron is reduced before or during the catalytic cycle. Tetrahydrobiopterin is a likely physiological reductant. Once reduced, the predominant form of the enzyme during turnover remains ferrous, though a small fraction of the enzyme is oxidised to the ferric form, possibly with O₂ [4].

The hydroxylases are organised into three domains: Nterminal regulatory domain, catalytic domain, and Cterminal oligomerisation domain [2]. Proteolysis experiments on rat PheOH and TyrOH have shown that deletion of the regulatory domain results in active truncated enzymes which show low binding specificity for either amino acid. The attachment of either regulatory domain enhances the substrate specificity displayed by the catalytic domain [5].

The crystal structures of tetramers of Cterminal fragments of rat TyrOH (residues 156-498) and human PheOH (residues 118-452) have been determined [6, 7]. The truncated forms of enzymes contain catalytic and tetramerisation domains. The overall fold of the monomer is a basketlike arrangement of helices and loops. The tetramerisation domain (residues 457-498 in TyrOH and 408-452 in PheOH) consists of two ßstrands, forming a ßribbon, and a 40 Å long helix ( alpha 14) which contains a hydrophobic heptad repeat. Helices alpha 14 from each of the four subunits form an antiparallel coiled coil with 222 crystallographic symmetry spanning the full width of a tetramer.

The catalytic domain (residues 156-456 in TyrOH; residues 118-407 in PheOH) consists of 13 alpha helices, six ßstrands and a number of long loops. The active site is located in a deep cleft in the core of each monomer. The iron is 10 Å below the enzyme surface within the active site cleft. The iron is coordinated by two histidines and one glutamine (His331/His285, His336/His290, Glu376/Glu330 in rat TyrOH/human PheOH, respectively) as well as two water molecules. The coordination geometry is square pyramidal with His331/His285 as the axial ligand and the remaining ligands in the equatorial positions.

AAAOH in enzyme databases

ENZYME	LIGAND	BRENDA	Official name	Alternative names
1.14.16.1	1.14.16.1	1.14.6.1	Phenylalanine 4monooxygenase	Phenylalanine 4hydroxylase; phenylalaninase
1.14.16.2	1.14.16.2	1.14.6.2	Tyrosine 3monooxygenase	Tyrosine 3hydroxylase
1.14.16.4	1.14.16.4	1.14.6.4	Tryptophan 5monooxygenase	Tryptophan 5hydroxylase

AAAOH in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
FYWHYDRXLASE	PR00372	BIOPTERIN_HYDROXYL	PS00367	BL00367

AAAOH in alignment databases

Protein Superfamily	Pfam	LPFC 3D alignment
00197; phenylalanine 4monooxygenase	PF00351; biopterin_H	-

AAAOH in 3D databases

AAAOH contain a mononuclear iron centre.

PDB scop BSM RELI
Base Header MMS Abstract ¹

1pah - 1pah - Phenylalanine hydroxylase (residues 117-424; catalytic domain); human (recombinant) -
1toh - 1toh 1toh Tyrosine hydroxylase (residues 156-498; catalytic and tetramerization domains) (ferric); rat (recombinant) -

PDB	scop	BSM	RELI Base	Header	¹
1pah	-	1pah	-	Phenylalanine hydroxylase (residues 117-424; catalytic domain); human (recombinant)	-
1toh	-	1toh	1toh	Tyrosine hydroxylase (residues 156-498; catalytic and tetramerization domains) (ferric); rat (recombinant)	-

¹ Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

Harayama, S., Kok, M. and Neidle, E.L. (1992) Functional and evolutionary relationships among diverse oxygenases. Annu. Rev. Microbiol. 46, 565-601.
Hufton, S.E., Jennings, I.G. and Cotton, R.G.H. (1995) Structure and function of the aromatic amino acid hydroxylases. Biochem. J. 311, 353-366.
Hillas, P.J. and Fitzpatrick, P.F. (1996) A mechanism for hydroxylation by tyrosine hydroxylase based on partitioning of substituted phenylalanines. Biochemistry 35, 6969-6975.
Ramsey, A.J., Hillas, P.J. and Fitzpatrick, P.F. (1996) Characterization of the active site iron in tyrosine hydroxylase. Redox states of the iron. J. Biol. Chem. 271, 24395-24400.
Daubner, S.C., Hillas, P.J. and Fitzpatrick, P.F. (1997) Characterization of chimeric pterindependent hydroxylases: Contributions of the regulatory domains of tyrosine and phenylalanine hydroxylase to substrate specificity. Biochemistry 36, 11574-11582.
Goodwill, K.E., Sabatier, C., Marks, C., Raag, R., Fitzpatrick, P.F. and Stevens, R.C. (1997) Crystal structure of tyrosine hydroxylase at 2.3 Å and its implications for inherited neurodegenerative diseases. Nature Struct. Biol. 4, 578-585.
Fusetti, F., Erlandsen, H., Flatmark, T. and Stevens, R.C. (1998) Structure of tetrameric human phenylalanine hydroxylase and its implications for phenylketonuria. J. Biol. Chem. 273, 16962-16967.