ISBN: 978-83-60043-10-3 - eurobic9

Eurobic9, 2-6 September, 2008, Wrocław, Poland
P202. The Crystal Structure of Tryptophan Hydroxylase with Bound
Amino Acid Substrate
M.S. Windahl a , C.R. Petersen b , P. Harris b , H.E.M. Christensen b
a
Department of Natural Science, University of Copenhagen, Thorvaldsensvej 40, 1871, Frederiksberg C,
Denmark
e-mail:mskn@life.ku.dk
b
Department of Chemistry, Technical University of Denmark, Kemitorvet 207, 2800, Kgs. Lyngby, Denmark
Tryptophan hydroxylase (TPH) is part of the small enzyme family of tetrahydrobiopterin (BH4) dependent
aromatic amino acid hydroxylases [1]. The TPH catalysed formation of 5-hydroxytryptophan is the first and ratelimiting
step in the biosynthesis of the neurotransmitter and hormone serotonin (5-hydroxytryptamine). Although
serotonin has many physiological functions, it is mainly known as a neurotransmitter. Abnormalities in the
serotonergic neurons are implicated in a wide range of neuropsychiatric disorders such as depression, obsessivecompulsive
disorder and schizophrenia [2]. Two isoforms of TPH exist: isoform 1 (TPH1) is primarily found in
the mast cells, pineal gland and enterochromaffin cells, while isoform 2 (TPH2) appears mostly in the
serotonergic neurons of the brain [3]. TPH is a homotetrameric three domain enzyme; its three domains are an
N-terminal regulatory domain, a catalytic domain and a small C-terminal tetramerisation domain.
We have previously reported the expression, purification and crystallisation of the catalytic domain (∆1-
100/∆415-445) of chicken tryptophan hydroxylase isoform 1 [4]. We here present the 1.9 Å resolution crystal
structure in complex with the tryptophan substrate and an iron bound imidazole. The iron coordination can be
described as a distorted trigonal bipyramidal coordination with His273, His278 and Glu318 (partially bidentate)
and one imidazole as ligands. The tryptophan binding pocket is distinct from the BH4 binding pocket and the
tryptophan stacks against Pro269 with a distance of 3.9 Å between the iron and the tryptophan C5 that is
hydroxylated. The binding of tryptophan and imidazole have caused structural changes in the catalytic domain
compared to the structure of the human TPH1 with bound dihydrobiopterin [5]. The structure of chicken TPH1 is
more compact and the loops of residues Leu124-Asp139 and Ile367-Thr369 closes around the active site. The
same structural changes are seen in the catalytic domain of phenylalanine hydroxylase (PAH) upon binding of
substrate analogues norleucine and thienylalanine to the PAH•BH4 complex [6]. In fact the chicken TPH1•Trp
structure resembles the PAH•BH4•thienylalanine structure (r.m.s.d. of Cα atoms 0.94 Å) more than the human
TPH1 structure (r.m.s.d. 1.47 Å).
References:
[1] Fitzpatrick, P. F. (1999) Annu. Rev. Biochem. 68, 355-381.
[2] Lucki, I. (1998) Biol. Psychiatry 44, 151-162.
[3] Walther, D. J., and Bader, M. (2003) Biochem. Pharmacol. 66, 1673-1680.
[4] Nielsen, M. S., Petersen, C. R., Munch, A., Vendelboe, T. V., Boesen, J., Harris, P., and Christensen, H. E.
M. (2008) Prot. Expr. Purif. 57, 116-126.
[5] Wang, L., Erlandsen, H., Haavik, J., Knappskog, P. M., and Stevens, R. C. (2002) Biochemistry 41, 12569-
12574.
[6] Andersen, O. A., Stokka, A. J., Flatmark, T., and Hough, E. (2003) J. Mol. Biol. 333, 747-757.
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