Structure and function of the aromatic amino acid hydroxylases

Effect of mutations at Cys237 on the activation state and activity of human phenylalanine hydroxylase

Wild-type recombinant human phenylalanine hydroxylase (wt-hPAH) is activated about 1.5-fold by exposure to alkaline pH (pH 8.5-9.0). In order to study whereas this activation might be related to the activation of the rat enzyme by N-ethylamaleimide-modification of Cys237 [Gibbs and Benkovic (1991) Biochemistry 30, 67951, mutant proteins of hPAH with Cys237 changed to Ser (S) or Glu (D) have been prepared. The mutant forms have high specific activity at pH 7.0 and high affinity for L-Phe, notably for hPAH-C237D, which shows a 3-fold higher activity than L-Phe-activated wt-hPAH and it is not further activated by pre-incubation with L-Phe. Moreover, the emission maximum of the intrinsic fluorescence of hPAH-C237D (lambda(maxem) = 347 nm) resembles that of activated forms of wt-hPAH. However, the activity of this mutant at neutral pH is further activated by exposure to alkaline pH, indicating that activation of wt-hPAH by alkaline pH is not restricted to ionization of Cys237.

Regulation and crystallization of phosphorylated and dephosphorylated forms of truncated dimeric phenylalanine hydroxylase

Phenylalanine hydroxylase is regulated in a complex manner, including activation by phosphorylation. It is normally found as an equilibrium of dimeric and tetrameric species, with the tetramer thought to be the active form. We converted the protein to the dimeric form by deleting the C-terminal 24 residues and show that the truncated protein remains active and regulated by phosphorylation. This indicates that changes in the tetrameric quaternary structure of phenylalanine hydroxylase are not required for enzyme activation. Truncation also facilitates crystallization of both phosphorylated and dephosphorylated forms of the enzyme.

Identification of an Fe(III)-dihydroxyphenylalanine site in recombinant phosphomannose isomerase from Candida albicans

Candida albicans phosphomannose isomerase (PMI) is a zinc metalloprotein of known crystal structure. When heterologously overexpressed in Escherichia coli, a blue protein that contains up to 0.5 iron atoms/PMI molecule could be isolated, with absorption maxima at 420 nm and 680 nm. These bands are reminiscent of ferric catecholate complexes, an assignment that has been confirmed by resonance Raman spectroscopy, and by reaction with Arnow's reagent, which is specific for the presence of 3,4-dihydroxyphenylalanine (Dopa). After enzymatic digestion of blue PMI, a peptide with the sequence DPHAXISG was isolated corresponding to residues Asp283-Gly290 in the amino acid sequence of C. albicans PMI, where the unidentified residue X287 is encoded by a tyrosine codon. It is proposed that iron and oxygen bring about hydroxylation of Tyr287 in PMI and that Fe(III) subsequently chelates the Dopa residue to give the characteristic absorption spectrum. The EPR spectrum of the blue protein suggests three iron environments in the protein, two in axial environments with E/D values approximately equal to 0.06 and 0.12 and one rhombic species. The nature of the iron co-ordination sites is discussed with the help of model systems and by comparison with other blue non-heme iron proteins.

Structure/function relationships in human phenylalanine hydroxylase. Effect of terminal deletions on the oligomerization, activation and cooperativity of substrate binding to the enzyme

Amino-terminal and carboxy-terminal deletion mutagenesis have been used to identify structurally and functionally critical regions of recombinant wild-type human phenylalanine hydroxylase (wt-hPAH; Ser2-Lys452). The wild-type form consisted of dimeric and tetrameric forms in equilibrium, and only the isolated tetrameric form showed positive cooperativity of substrate (L-Phe) binding (Hill coefficient h = 2.2, S0.5 = 154 microM). The deletion mutants lacking the carboxy-terminal 24 amino acids hPAH (Ser2-Gln428) and hPAH(Gly103-Gln428) formed catalytically active dimers, and incubation with L-Phe did not promote the formation of tetramers, a characteristic property of dimeric wt-hPAH. The carboxyterminus thus seems to contain a motif required for dimer-dimer interaction in wt-hPAH. The deletion mutants hPAH(Asp112-Lys452), hPAH(Ser2-Gln428) and hPAH(Gly103-Gln428) were all activated by prior incubation with L-Phe, but did not reveal any positive cooperativity of substrate binding (h = 1.0). The activation by L-Phe was accompanied by a measurable conformational change (as probed by intrinsic fluorescence spectroscopy) only in the enzyme forms containing the amino-terminal sequence. i.e. wt-hPAH and the Ser2-Gln428 mutant. The amino-terminal deletion mutants hPAH(Asp112-Lys452) and hPAH(Gly103-Gln428) revealed high specific activity, increased apparent affinity for L-Phe (S0.5 = 60 microM) and a tryptophan fluorescence emission spectrum similar to that of the L-Phe-activated wt-hPAH. Moreover, prior incubation of the enzyme forms with lysophosphatidylcholine, a commonly used activator of the PAH, only increased the activity of those forms containing the wt-hPAH amino-terminal sequence. Our results are compatible with a model in which incubation of wt-hPAH with L-Phe induces both a conformational change (with cooperativity in the tetrameric enzyme) which relieves the inhibition imposed by the amino-terminal domain to the high-affinity binding of L-Phe, and an additional activation, as observed for the truncated forms lacking the amino-terminal.

Conformational properties and stability of tyrosine hydroxylase studied by infrared spectroscopy. Effect of iron/catecholamine binding and phosphorylation

The conformation and stability of recombinant tetrameric human tyrosine hydroxylase isoenzyme 1 (hTH1) was studied by infrared spectroscopy and by limited tryptic proteolysis. Its secondary structure was estimated to be 42% alpha-helix, 35% beta-extended structures (including beta-sheet), 14% beta-turns, and 10% nonstructured conformations. Addition of Fe(II) or Fe(II) plus dopamine to the apoenzyme did not significantly modify its secondary structure. However, an increased thermal stability and resistance to proteolysis, as well as a decreased cooperativity in the thermal denaturation transition, was observed for the ligand-bound forms. Thus, as compared with the apoenzyme, the ligand-bound subunits of hTH1 showed a more compact tertiary structure but weaker intersubunit contacts within the protein tetramer. Phosphorylation of the apoenzyme by cyclic AMP-dependent protein kinase did not change its overall conformation but allowed on iron binding a conformational change characterized by an increase (about 10%) in alpha-helix and protein stability. Our results suggest that the conformational events involved in TH inhibition by catecholamines are mainly related to modifications of tertiary and quaternary structural features. However, the combined effect of iron binding and phosphorylation, which activates the enzyme, also involves modifications of the protein secondary structure.