A comparison of kinetic and regulatory properties of the tetrameric and dimeric forms of wild-type and Thr427→Pro mutant human phenylalanine hydroxylase -: Contribution of the flexible hinge region Asp425-Gln429 to the tetramerization and cooperative substrate binding

Recombinant human phenylalanine hydroxylase (hPAH, phenylalanine 4-monooxygenase EC 1.14.16.1) is catalytically active both as a tetramer and a dimer [Knappskog, P.M., Flatmark, T., Aarden, J.M., Haavik, J. and Martinet, A. (1996) Eur. J. Biochem. 242, 813-821]. In the present study we have further characterized the differences in kinetic and regulatory properties of the two oligomeric forms when expressed in Escherichia coli. The positive cooperativity of L-Phe binding to the tetrameric form both in enzyme kinetic studies (h = 1.6) and intrinsic tryptophan fluorescence measurements (h = 2.3) was abolished in the dimer, which also revealed a catalytic efficiency (V-max/[S](0.5)) of only 35% of the tetramer. Whereas the catalytic activity of the tetramer was activated fivefold to sixfold by preincubation with L-Phe, the dimer revealed only a 1.6-fold activation. The crystal structure has identified a five-residue flexible hinge region (Asp425-Gln429) that links the beta -strand T beta2 (Ile421-Leu424) and the 24 residue amphipathic alpha -helix Tal (Gln428-Lys452) at the C-terminus which forms an antiparallel colied-coli structure in the center of the tetramer [Fusetti, F. Erlandsen, H., Flatmark, T. & Stevens, R.C. (1998) J. Biol. Chem. 273, 16962-16967]. The potential role of this flexible hinge in the tetramerization and the conformational transition of wt-hPAH ion the cooperative binding of L-Phe was examined by site-specific mutagenesis. Substitution of Thr427 by a Pro (as in tyrosine hydroxylase) resulted in a mutant protein which was isolated mainly (about 95%) as a dimer. The isolated tetramer of T427P revealed no kinetic cooperativity of L-Phe binding, the catalytic efficiency (V-max/[S](0.5)) was decreased to about 39% of the wild-type tetramer and it was not activated by L-Phe preincubation. The dimeric forms of T427P and wt-hPAH revealed rather similar kinetic properties. The lack of kinetic cooperativity of the T427P tetramer was associated with a corresponding change in the binding isotherm for L-Phe as studied by intrinsic tryptophan fluorescence measurements. Protein stability was also reduced both for the E. coli expressed and the in vitro synthesized mutant enzyme. Collectively, these results indicate that the positive cooperativity of L-Phe binding to wt-hPAH requires a tetrameric enzyme with a C-terminal flexibel hinge region (Asp425-Gln429) which has a structural role in the formation of the enzyme tetramer. Furthermore, this hinge region represents a motif in the PAH structure that is involved in the conformational change transmitted through the protein on the cooperative binding of L-Phe to tetrameric wt-hPAH. This conclusion is further supported by studies on two disease (phenylketonuria)-associated mutant forms.