Enzymes: aromatic l-amino acid decarboxylase (EC 126.96.36.199); phenylalanine hydroxylase (phenylalanine-4-hydroxylase; EC 188.8.131.52); tryptophan hydroxylase (tryptophan-5-monooxygenase; EC 184.108.40.206); tyrosine hydroxylase (tyrosine-3-monooxygenase; EC 220.127.116.11).
Modeled ligand-protein complexes elucidate the origin of substrate specificity and provide insight into catalytic mechanisms of phenylalanine hydroxylase and tyrosine hydroxylase
Article first published online: 24 FEB 2003
European Journal of Biochemistry
Volume 270, Issue 6, pages 1065–1075, March 2003
How to Cite
Maaß, A., Scholz, J. and Moser, A. (2003), Modeled ligand-protein complexes elucidate the origin of substrate specificity and provide insight into catalytic mechanisms of phenylalanine hydroxylase and tyrosine hydroxylase. European Journal of Biochemistry, 270: 1065–1075. doi: 10.1046/j.1432-1033.2003.03429.x
- Issue published online: 24 FEB 2003
- Article first published online: 24 FEB 2003
- (Received 9 October 2002, revised 29 November 2002, accepted 16 December 2002)
- phenylalanine hydroxylase;
- tyrosine hydroxylase;
- substrate specificity;
- feedback inhibition
NMR spectroscopy and X-ray crystallography have provided important insight into structural features of phenylalanine hydroxylase (PAH) and tyrosine hydroxylase (TH). Nevertheless, significant problems such as the substrate specificity of PAH and the different susceptibility of TH to feedback inhibition by l-3,4-dihydroxyphenylalanine (l-DOPA) compared with dopamine (DA) remain unresolved. Based on the crystal structures 5pah for PAH and 2toh for TH (Protein Data Bank), we have used molecular docking to model the binding of 6(R)-l-erythro-5,6,7,8-tetrahydrobiopterin (BH4) and the substrates phenylalanine and tyrosine to the catalytic domains of PAH and TH. The amino acid substrates were placed in positions common to both enzymes. The productive position of tyrosine in TH·BH4 was stabilized by a hydrogen bond with BH4. Despite favorable energy scores, tyrosine in a position trans to PAH residue His290 or TH residue His336 interferes with the access of the essential cofactor dioxygen to the catalytic center, thereby blocking the enzymatic reaction. DA and l-DOPA were directly coordinated to the active site iron via the hydroxyl residues of their catechol groups. Two alternative conformations, rotated 180° around an imaginary iron–catecholamine axis, were found for DA and l-DOPA in PAH and for DA in TH. Electrostatic forces play a key role in hindering the bidentate binding of the immediate reaction product l-DOPA to TH, thereby saving the enzyme from direct feedback inhibition.