Expression and characterization of the catalytic core of tryptophan hydroxylase

The aromatic amino acid hydroxylases: Structures, catalysis, and regulation of phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase

The aromatic amino acid hydroxylases phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase are non-heme iron enzymes that catalyze key physiological reactions. This review discusses the present understanding of the common catalytic mechanism of these enzymes and recent advances in understanding the relationship between their structures and their regulation.

Metabolic engineering of Escherichia coli for efficient production of L-5-hydroxytryptophan from glucose

Background

5-hydroxytryptophan (5-HTP), the direct biosynthetic precursor of the neurotransmitter 5-hydroxytryptamine, has been shown to have unique efficacy in the treatment of a variety of disorders, including depression, insomnia, and chronic headaches, and is one of the most commercially valuable amino acid derivatives. However, microbial fermentation for 5-HTP production continues to face many challenges, including low titer/yield and the presence of the intermediate L-tryptophan (L-Trp), owing to the complexity and low activity of heterologous expression in prokaryotes. Therefore, there is a need to construct an efficient microbial cell factory for 5-HTP production.

Results

We describe the systematic modular engineering of wild-type Escherichia coli for the efficient fermentation of 5-HTP from glucose. First, a xylose-induced T7 RNA polymerase-P_T7 promoter system was constructed to ensure the efficient expression of each key heterologous pathway in E. coli. Next, a new tryptophan hydroxylase mutant was used to construct an efficient tryptophan hydroxylation module, and the cofactor tetrahydrobiopterin synthesis and regeneration pathway was expressed in combination. The L-Trp synthesis module was constructed by modifying the key metabolic nodes of tryptophan biosynthesis, and the heterologous synthesis of 5-HTP was achieved. Finally, the NAD(P)H regeneration module was constructed by the moderate expression of the heterologous GDH_esi pathway, which successfully reduced the surplus of the intermediate L-Trp. The final engineered strain HTP11 was able to produce 8.58 g/L 5-HTP in a 5-L bioreactor with a yield of 0.095 g/g glucose and a maximum real-time productivity of 0.48 g/L/h, the highest values reported by microbial fermentation.

Conclusion

In this study, we demonstrate the successful design of a cell factory for high-level 5-HTP production, combined with simple processes that have potential for use in industrial applications in the future. Thus, this study provides a reference for the production of high-value amino acid derivatives using a systematic modular engineering strategy and a basis for an efficient engineered strain development of 5-HTP high-value derivatives.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01920-3.

Melatonin biosynthesis pathways in nature and its production in engineered microorganisms

Melatonin is a biogenic amine that can be found in plants, animals and microorganism. The metabolic pathway of melatonin is different in various organisms, and biosynthetic endogenous melatonin acts as a molecular signal and antioxidant protection against external stress. Microbial synthesis pathways of melatonin are similar to those of animals but different from those of plants. At present, the method of using microorganism fermentation to produce melatonin is gradually prevailing, and exploring the biosynthetic pathway of melatonin to modify microorganism is becoming the mainstream, which has more advantages than traditional chemical synthesis. Here, we review recent advances in the synthesis, optimization of melatonin pathway. l-tryptophan is one of the two crucial precursors for the synthesis of melatonin, which can be produced through a four-step reaction. Enzymes involved in melatonin synthesis have low specificity and catalytic efficiency. Site-directed mutation, directed evolution or promotion of cofactor synthesis can enhance enzyme activity and increase the metabolic flow to promote microbial melatonin production. On the whole, the status and bottleneck of melatonin biosynthesis can be improved to a higher level, providing an effective reference for future microbial modification.

Biosynthesis and Genetic Incorporation of 3,4-Dihydroxy-L-Phenylalanine into Proteins in Escherichia coli

While 20 canonical amino acids are used by most organisms for protein synthesis, the creation of cells that can use noncanonical amino acids (ncAAs) as additional protein building blocks holds great promise for preparing novel medicines and for studying complex questions in biological systems. However, only a small number of biosynthetic pathways for ncAAs have been reported to date, greatly restricting our ability to generate cells with ncAA building blocks. In this study, we report the creation of a completely autonomous bacterium that utilizes 3,4-dihydroxy-L-phenylalanine (DOPA) as its 21^st amino acid building block. Like canonical amino acids, DOPA can be biosynthesized without exogenous addition and can be genetically incorporated into proteins in a site-specific manner. Equally important, the protein production yield of DOPA-containing proteins from these autonomous cells is greater than that of cells exogenously fed with 9 mM DOPA. The unique catechol moiety of DOPA can be used as a versatile handle for site-specific protein functionalizations via either oxidative coupling or strain-promoted oxidation-controlled cyclooctyne-1,2-quinone (SPOCQ) cycloaddition reactions. We further demonstrate the use of these autonomous cells in preparing fluorophore-labeled anti-human epidermal growth factor 2 (HER2) antibodies for the detection of HER2 expression on cancer cells.

Advances in the Microbial Synthesis of 5-Hydroxytryptophan

5-Hydroxytryptophan (5-HTP) plays an important role in the regulation of emotion, behavior, sleep, pain, body temperature, and other physiological functions. It is used in the treatment of depression, insomnia, migraine, and other diseases. Due to a lack of effective biosynthesis methods, 5-HTP is mainly obtained by natural extraction, which has been unable to meet the needs of the market. Through the directed evolution of enzymes and the introduction of substrate supply pathways, 5-HTP biosynthesis and yield increase have been realized. This review provides examples that illustrate the production mode of 5-HTP and the latest progress in microbial synthesis.

Mutagenesis of an Active-Site Loop in Tryptophan Hydroxylase Dramatically Slows the Formation of an Early Intermediate in Catalysis

Solution studies of the aromatic amino acid hydroxylases are consistent with the Fe^IVO intermediate not forming until both the amino acid and tetrahydropterin substrates have bound. Structural studies have shown that the positions of active-site loops differs significantly between the free enzyme and the enzyme-amino acid-tetrahydropterin complex. In tryptophan hydroxylase (TrpH) these mobile loops contain residues 124-134 and 365-371, with a key interaction involving Ile366. The I366N mutation in TrpH results in decreases of 1-2 orders of magnitude in the k_cat and k_cat/ K_m values. Single turnover analyses establish that the limiting rate constant for turnover is product release for the wild-type enzyme but is formation of the first detectable intermediate I in catalysis in the mutant enzyme. The mutation does not alter the kinetics of NO binding to the ternary complex nor does it uncouple Fe^IVO formation from amino acid hydroxylation. The effects on the k_cat value of wild-type TrpH of changing viscosity are consistent with rate-limiting product release. While the effect of viscosity on the k_cat/ K_O2 value is small, consistent with reversible oxygen binding, the effects on the k_cat/ K_m values for tryptophan and the tetrahydropterin are large, with the latter value exceeding the expected limit and varying with the identity of the viscogen. In contrast, the kinetic parameters of I366N TrpH show small changes with viscosity. The results are consistent with binding of the amino acid and pterin substrate to form the ternary complex being directly coupled to closure of loops over the active site and formation of the reactive complex. The mutation destabilizes this initial event.

Isoform-Specific Substrate Inhibition Mechanism of Human Tryptophan Hydroxylase

Tryptophan hydroxylase (TPH) catalyzes the initial and rate-limiting step in the biosynthesis of serotonin, which is associated with a variety of disorders such as depression and irritable bowel syndrome. TPH exists in two isoforms: TPH1 and TPH2. TPH1 catalyzes the initial step in the synthesis of serotonin in the peripheral tissues, while TPH2 catalyzes this step in the brain. In this study, the steady-state kinetic mechanism for the catalytic domain of human TPH1 has been determined. Varying substrate tryptophan (Trp) and tetrahydrobiopterin (BH4) results in a hybrid Ping Pong-ordered mechanism in which the reaction can either occur through a Ping Pong or a sequential mechanism depending on the concentration of tryptophan. The catalytic domain of TPH1 shares a sequence identity of 81% with TPH2. Despite the high sequence identity, differences in the kinetic parameters of the isoforms have been identified; i.e., only TPH1 displays substrate tryptophan inhibition. This study demonstrates that the difference can be traced to an active site loop which displays different properties in the TPH isoforms. Steady-state kinetic results of the isoforms, and variants with point mutations in a loop lining the active site, show that the kinetic parameters of only TPH1 are significantly changed upon mutations. Mutations in the active site loop of TPH1 result in an increase in the substrate inhibition constant, Ki, and therefore turnover rate. Molecular dynamics simulations reveal that this substrate inhibition mechanism occurs through a closure of the cosubstrate, BH4, binding pocket, which is induced by Trp binding.

Stabilization of tryptophan hydroxylase 2 by l-phenylalanine-induced dimerization

Tryptophan hydroxylase 2 (TPH2) catalyses the initial and rate‐limiting step in the biosynthesis of serotonin, which is associated with a variety of disorders such as depression, obsessive compulsive disorder, and schizophrenia. Full‐length TPH2 is poorly characterized due to low purification quantities caused by its inherent instability. Three truncated variants of human TPH2 (rchTPH2; regulatory and catalytic domain, NΔ47‐rchTPH2; truncation of 47 residues in the N terminus of rchTPH2, and chTPH2; catalytic domain) were expressed, purified, and examined for changes in transition temperature, inactivation rate, and oligomeric state. chTPH2 displayed 14‐ and 11‐fold higher half‐lives compared to rchTPH2 and NΔ47‐rchTPH2, respectively. Differential scanning calorimetry experiments demonstrated that this is caused by premature unfolding of the less stable regulatory domain. By differential scanning fluorimetry, the unfolding transitions of rchTPH2 and NΔ47‐rchTPH2 are found to shift from polyphasic to apparent two‐state by the addition of l‐Trp or l‐Phe. Analytical gel filtration revealed that rchTPH2 and NΔ47‐rchTPH2 reside in a monomer–dimer equilibrium which is significantly shifted toward dimer in the presence of l‐Phe. The dimerizing effect induced by l‐Phe is accompanied by a stabilizing effect, which resulted in a threefold increase in half‐lives of rchTPH2 and NΔ47‐rchTPH2. Addition of l‐Phe to the purification buffer significantly increases the purification yields, which will facilitate characterization of hTPH2.

The regulatory domain of human tryptophan hydroxylase 1 forms a stable dimer

The three eukaryotic aromatic amino acid hydroxylases phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase have essentially identical catalytic domains and discrete regulatory domains. The regulatory domains of phenylalanine hydroxylase form ACT domain dimers when phenylalanine is bound to an allosteric site. In contrast the regulatory domains of tyrosine hydroxylase form a stable ACT dimer that does not bind the amino acid substrate. The regulatory domain of isoform 1 of human tryptophan hydroxylase was expressed and purified; mutagenesis of Cys64 was required to prevent formation of disulfide-linked dimers. The resulting protein behaved as a dimer upon gel filtration and in analytical ultracentrifugation. The sw value of the protein was unchanged from 2.7 to 35 μM, a concentration range over which the regulatory domain of phenylalanine hydroxylase forms both monomers and dimers, consistent with the regulatory domain of tryptophan hydroxylase 1 forming a stable dimer stable that does not undergo a monomer-dimer equilibrium. Addition of phenylalanine, a good substrate for the enzyme, had no effect on the sw value, consistent with there being no allosteric site for the amino acid substrate.

Glucose-based microbial production of the hormone melatonin in yeast Saccharomyces cerevisiae

Melatonin is a natural mammalian hormone that plays an important role in regulating the circadian cycle in humans. It is a clinically effective drug exhibiting positive effects as a sleep aid and a powerful antioxidant used as a dietary supplement. Commercial melatonin production is predominantly performed by complex chemical synthesis. In this study, we demonstrate microbial production of melatonin and related compounds, such as serotonin and N‐acetylserotonin. We generated Saccharomyces cerevisiae strains that comprise heterologous genes encoding one or more variants of an L‐tryptophan hydroxylase, a 5‐hydroxy‐L‐tryptophan decarboxylase, a serotonin acetyltransferase, an acetylserotonin O‐methyltransferase, and means for providing the cofactor tetrahydrobiopterin via heterologous biosynthesis and recycling pathways. We thereby achieved de novo melatonin biosynthesis from glucose. We furthermore accomplished increased product titers by altering expression levels of selected pathway enzymes and boosting co‐factor supply. The final yeast strain produced melatonin at a titer of 14.50 ± 0.57 mg L⁻¹ in a 76h fermentation using simulated fed‐batch medium with glucose as sole carbon source. Our study lays the basis for further developing a yeast cell factory for biological production of melatonin.

Mechanisms of tryptophan and tyrosine hydroxylase

The aromatic amino acid hydroxylases tryptophan hydroxylase and tyrosine hydroxylase are responsible for the initial steps in the formation of serotonin and the catecholamine neurotransmitters, respectively. Both enzymes are nonheme iron-dependent monooxygenases that catalyze the insertion of one atom of molecular oxygen onto the aromatic ring of their amino acid substrates, using a tetrahydropterin as a two electron donor to reduce the second oxygen atom to water. This review discusses the current understanding of the catalytic mechanism of these two enzymes. The reaction occurs as two sequential half reactions: a reaction between the active site iron, oxygen, and the tetrahydropterin to form a reactive Fe(IV) O intermediate and hydroxylation of the amino acid by the Fe(IV) O. The mechanism of formation of the Fe(IV) O is unclear; however, considerable evidence suggests the formation of an Fe(II) -peroxypterin intermediate. The amino acid is hydroxylated by the Fe(IV) O intermediate in an electrophilic aromatic substitution mechanism.

Kinetic mechanism of phenylalanine hydroxylase: intrinsic binding and rate constants from single-turnover experiments

Phenylalanine hydroxylase (PheH) catalyzes the key step in the catabolism of dietary phenylalanine, its hydroxylation to tyrosine using tetrahydrobiopterin (BH(4)) and O(2). A complete kinetic mechanism for PheH was determined by global analysis of single-turnover data in the reaction of PheHΔ117, a truncated form of the enzyme lacking the N-terminal regulatory domain. Formation of the productive PheHΔ117-BH(4)-phenylalanine complex begins with the rapid binding of BH(4) (K(d) = 65 μM). Subsequent addition of phenylalanine to the binary complex to form the productive ternary complex (K(d) = 130 μM) is approximately 10-fold slower. Both substrates can also bind to the free enzyme to form inhibitory binary complexes. O(2) rapidly binds to the productive ternary complex; this is followed by formation of an unidentified intermediate, which can be detected as a decrease in absorbance at 340 nm, with a rate constant of 140 s(-1). Formation of the 4a-hydroxypterin and Fe(IV)O intermediates is 10-fold slower and is followed by the rapid hydroxylation of the amino acid. Product release is the rate-determining step and largely determines k(cat). Similar reactions using 6-methyltetrahydropterin indicate a preference for the physiological pterin during hydroxylation.

Formation of the iron-oxo hydroxylating species in the catalytic cycle of aromatic amino acid hydroxylases

The first part of the catalytic cycle of the pterin-dependent, dioxygen-using nonheme-iron aromatic amino acid hydroxylases, leading to the Fe(IV)=O hydroxylating intermediate, has been investigated by means of density functional theory. The starting structure in the present investigation is the water-free Fe-O(2) complex cluster model that represents the catalytically competent form of the enzymes. A model for this structure was obtained in a previous study of water-ligand dissociation from the hexacoordinate model complex of the X-ray crystal structure of the catalytic domain of phenylalanine hydroxylase in complex with the cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)) (PAH-Fe(II)-BH(4)). The O-O bond rupture and two-electron oxidation of the cofactor are found to take place via a Fe-O-O-BH(4) bridge structure that is formed in consecutive radical reactions involving a superoxide ion, O(2)(-). The overall effective free-energy barrier to formation of the Fe(IV)=O species is calculated to be 13.9 kcal  mol(-1), less than 2 kcal  mol(-1) lower than that derived from experiment. The rate-limiting step is associated with a one-electron transfer from the cofactor to dioxygen, whereas the spin inversion needed to arrive at the quintet state in which the O-O bond cleavage is finalized, essentially proceeds without activation.

Direct evidence for a phenylalanine site in the regulatory domain of phenylalanine hydroxylase

The hydroxylation of phenylalanine to tyrosine by the liver enzyme phenylalanine hydroxylase is regulated by the level of phenylalanine. Whether there is a distinct allosteric binding site for phenylalanine outside of the active site has been unclear. The enzyme contains an N-terminal regulatory domain that extends through Thr117. The regulatory domain of rat phenylalanine hydroxylase was expressed in Escherichia coli. The purified protein behaves as a dimer on a gel filtration column. In the presence of phenylalanine, the protein elutes earlier from the column, consistent with a conformational change in the presence of the amino acid. No change in elution is seen in the presence of the non-activating amino acid proline. ¹H-¹⁵N HSQC NMR spectra were obtained of the ¹⁵N-labeled protein alone and in the presence of phenylalanine or proline. A subset of the peaks in the spectrum exhibits chemical shift perturbation in the presence of phenylalanine, consistent with binding of phenylalanine at a specific site. No change in the NMR spectrum is seen in the presence of proline. These results establish that the regulatory domain of phenylalanine hydroxylase can bind phenylalanine, consistent with the presence of an allosteric site for the amino acid.

Single turnover kinetics of tryptophan hydroxylase: evidence for a new intermediate in the reaction of the aromatic amino acid hydroxylases

Tryptophan hydroxylase (TrpH) uses a non-heme mononuclear iron center to catalyze the tetrahydropterin-dependent hydroxylation of tryptophan to 5-hydroxytryptophan. The reactions of the TrpH.Fe(II), TrpH.Fe(II).tryptophan, TrpH.Fe(II).6MePH(4).tryptophan, and TrpH.Fe(II).6MePH(4).phenylalanine complexes with O(2) were monitored by stopped-flow absorbance spectroscopy and rapid quench methods. The second-order rate constant for the oxidation of TrpH.Fe(II) has a value of 104 M(-1) s(-1) irrespective of the presence of tryptophan. Stopped-flow absorbance analyses of the reaction of the TrpH.Fe(II).6MePH(4).tryptophan complex with oxygen are consistent with the initial step being reversible binding of oxygen, followed by the formation with a rate constant of 65 s(-1) of an intermediate I that has maximal absorbance at 420 nm. The rate constant for decay of I, 4.4 s(-1), matches that for formation of the 4a-hydroxypterin product monitored at 248 nm. Chemical-quench analyses show that 5-hydroxytryptophan forms with a rate constant of 1.3 s(-1) and that overall turnover is limited by a subsequent slow step, presumably product release, with a rate constant of 0.2 s(-1). All of the data with tryptophan as substrate can be described by a five-step mechanism. In contrast, with phenylalanine as substrate, the reaction can be described by three steps: a second-order reaction with oxygen to form I, decay of I as tyrosine forms, and slow product release.

Regulation of phenylalanine hydroxylase: conformational changes upon phenylalanine binding detected by hydrogen/deuterium exchange and mass spectrometry

Phenylalanine acts as an allosteric activator of the tetrahydropterin-dependent enzyme phenylalanine hydroxylase. Hydrogen/deuterium exchange monitored by mass spectrometry has been used to gain insight into local conformational changes accompanying activation of rat phenylalanine hydroxylase by phenylalanine. Peptides in the regulatory and catalytic domains that lie in the interface between these two domains show large increases in the extent of deuterium incorporation from solvent in the presence of phenylalanine. In contrast, the effects of phenylalanine on the exchange kinetics of a mutant enzyme lacking the regulatory domain are limited to peptides surrounding the binding site for the amino acid substrate. These results support a model in which the N-terminus of the protein acts as an inhibitory peptide, with phenylalanine binding causing a conformational change in the regulatory domain that alters the interaction between the catalytic and regulatory domains.

Mechanism of Inhibition of Novel Tryptophan Hydroxylase Inhibitors Revealed by Co-crystal Structures and Kinetic Analysis

Trytophan Hydroxylase Type I (TPH1), most abundantly expressed in the gastrointestinal tract, initiates the synthesis of serotonin by catalyzing hydroxylation of tryptophan in the presence of biopterin and oxygen. We have previously described three series of novel, periphery-specific TPH1 inhibitors that selectively deplete serotonin in the gastrointestinal tract. We have now determined co-crystal structures of TPH1 with three of these inhibitors at high resolution. Analysis of the structural data showed that each of the three inhibitors fills the tryptophan binding pocket of TPH1 without reaching into the binding site of the cofactor pterin, and induces major conformational changes of the enzyme. The enzyme-inhibitor complexes assume a compact conformation that is similar to the one in tryptophan complex. Kinetic analysis showed that all three inhibitors are competitive versus the substrate tryptophan, consistent with the structural data that the compounds occupy the tryptophan binding site. On the other hand, all three inhibitors appear to be uncompetitive versus the cofactor 6-methyltetrahydropterin, which is not only consistent with the structural data but also indicate that the hydroxylation reaction follows an ordered binding mechanism in which a productive complex is formed only if tryptophan binds only after pterin, similar to the kinetic mechanisms of tyrosine and phenylalanine hydroxylase.

Identification by hydrogen/deuterium exchange of structural changes in tyrosine hydroxylase associated with regulation

The activity of tyrosine hydroxylase is regulated by reversible phosphorylation of serine residues in an N-terminal regulatory domain and catecholamine inhibition at the active site. Catecholamines such as dopamine bind very tightly to the resting enzyme; phosphorylation of Ser40 decreases the affinity for catecholamines by 3 orders of magnitude. The effects of dopamine binding and phosphorylation of Ser40 on the kinetics of deuterium incorporation into peptide bonds were examined by mass spectrometry. When dopamine is bound, three peptic peptides show significantly slower deuterium incorporation, 35-41 and 42-71 in the regulatory domain and 295-299 in the catalytic domain. In the phosphorylated enzyme, peptide 295-299 shows more rapid incorporation of deuterium, while 35-41 and 42-71 can not be detected. These results are consistent with tyrosine hydroxylase existing in two different conformations. In the closed conformation, the regulatory domain lies across the active site loop containing residues 295-298; this is stabilized when dopamine is bound in the active site. In the open conformation, the regulatory domain has moved out of the active site, allowing substrate access; this conformation is favored by phosphorylation of Ser40.

Demonstration of a peroxide shunt in the tetrahydropterin-dependent aromatic amino acid monooxygenases

The nonheme iron enzyme phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase catalyze the hydroxylation of their aromatic amino acid substrates using a tetrahydropterin as the source of electrons. The hydroxylating intermediate is proposed to be an Fe(IV)O species. We report here that all three enzymes will catalyze hydroxylation reactions using H(2)O(2) in place of tetrahydropterin and oxygen, forming tyrosine and 3-hydroxyphenylalanine from phenylalanine, 4-HOCH(2)-phenylalanine from 4-CH(3)-phenylalanine, and hydroxycyclohexylalanine from 3-cyclohexylalanine. No peroxide-dependent reaction is seen with active site mutants of TyrH and PheH in which the stability or reactivity of the iron center is compromised. These results provide further support for an Fe(IV)O hydroxylating intermediate.

Crystal structure of tryptophan hydroxylase with bound amino acid substrate

Tryptophan hydroxylase (TPH) is a mononuclear non-heme iron enzyme, which catalyzes the reaction between tryptophan, O 2, and tetrahydrobiopterin (BH 4) to produce 5-hydroxytryptophan and 4a-hydroxytetrahydrobiopterin. This is the first and rate-limiting step in the biosynthesis of the neurotransmitter and hormone serotonin (5-hydroxytryptamine). We have determined the 1.9 A resolution crystal structure of the catalytic domain (Delta1-100/Delta415-445) of chicken TPH isoform 1 (TPH1) in complex with the tryptophan substrate and an iron-bound imidazole. This is the first structure of any aromatic amino acid hydroxylase with bound natural amino acid substrate. The iron coordination can be described as distorted trigonal bipyramidal coordination with His273, His278, and Glu318 (partially bidentate) and one imidazole as ligands. The tryptophan stacks against Pro269 with a distance of 3.9 A between the iron and the tryptophan Czeta3 atom that is hydroxylated. The binding of tryptophan and maybe the imidazole has caused the structural changes in the catalytic domain compared to the structure of the human TPH1 without tryptophan. The structure of chicken TPH1 is more compact, and the loops of residues Leu124-Asp139 and Ile367-Thr369 close around the active site. Similar structural changes are seen in the catalytic domain of phenylalanine hydroxylase (PAH) upon binding of substrate analogues norleucine and thienylalanine to the PAH.BH 4 complex. In fact, the chicken TPH1.Trp.imidazole structure resembles the PAH.BH 4.thienylalanine structure more (root-mean-square deviation for Calpha atoms of 0.90 A) than the human TPH1 structure (root-mean-square deviation of 1.47 A).

Kinetic isotope effects on aromatic and benzylic hydroxylation by Chromobacterium violaceum phenylalanine hydroxylase as probes of chemical mechanism and reactivity

Phenylalanine hydroxylase from Chromobacterium violaceum (CvPheH) is a non-heme iron monooxygenase that catalyzes the hydroxylation of phenylalanine to tyrosine. In this study, we used deuterium kinetic isotope effects to probe the chemical mechanisms of aromatic and benzylic hydroxylation to compare the reactivities of bacterial and eukaryotic aromatic amino acid hydroxylases. The (D) k cat value for the reaction of CvPheH with [(2)H 5]phenylalanine is 1.2 with 6-methyltetrahydropterin and 1.4 with 6,7-dimethyltetrahydropterin. With the mutant enzyme I234D, the (D) k cat value decreases to 0.9 with the latter pterin; this is likely to be the intrinsic effect for addition of oxygen to the amino acid. The isotope effect on the subsequent tautomerization of a dienone intermediate was determined to be 5.1 by measuring the retention of deuterium in tyrosine produced from partially deuterated phenylalanine; this large isotope effect is responsible for the normal effect on k cat. The isotope effect for hydroxylation of the methyl group of 4-CH 3-phenylalanine, obtained from the partitioning of benzylic and aromatic hydroxylation products, is 10. The temperature dependence of this isotope effect establishes the contribution of hydrogen tunneling to benzylic hydroxylation by this enzyme. The results presented here provide evidence that the reactivities of the prokaryotic and eukaryotic hydroxylases are similar and further define the reactivity of the iron center for the family of aromatic amino acid hydroxylases.

Characterization of metal ligand mutants of phenylalanine hydroxylase: Insights into the plasticity of a 2-histidine-1-carboxylate triad

The iron atom in the nonheme iron monooxygenase phenylalanine hydroxylase is bound on one face by His285, His290, and Glu330. This arrangement of metal ligands is conserved in the other aromatic amino acid hydroxylases, tyrosine hydroxylase and tryptophan hydroxylase. A similar 2-His-1-carboxylate facial triad of two histidines and an acidic residue are the ligands to the iron in other nonheme iron enzymes, including the alpha-ketoglutarate-dependent hydroxylases and the extradiol dioxygenases. Previous studies of the effects of conservative mutations of the iron ligands in tyrosine hydroxylase established that there is some plasticity in the nature of the ligands and that the three ligands differ in their sensitivity to mutagenesis. To determine the generality of this finding for enzymes containing a 2-His-1-carboxylate facial triad, the His285, His290, and Glu330 in rat phenylalanine hydroxylase were mutated to glutamine, glutamate, and histidine. All of the mutant proteins had low but measurable activities for tyrosine formation. In general, mutation of Glu330 had the greatest effect on activity and mutation of His290 the least. All of the mutations resulted in an excess of tetrahydropterin oxidized relative to tyrosine formation, with mutation of His285 having the greatest effect on the coupling of the two partial reactions. The H285Q enzyme had the highest activity as tetrahydropterin oxidase at 20% the wild-type value. All of the mutations greatly decreased the affinity for iron, with mutation of Glu330 the most deleterious. The results complement previous results with tyrosine hydroxylase in establishing the plasticity of the individual iron ligands in this enzyme family.

Insights into the catalytic mechanisms of phenylalanine and tryptophan hydroxylase from kinetic isotope effects on aromatic hydroxylation

Phenylalanine hydroxylase (PheH) and tryptophan hydroxylase (TrpH) catalyze the aromatic hydroxylation of phenylalanine and tryptophan, forming tyrosine and 5-hydroxytryptophan, respectively. The reactions of PheH and TrpH have been investigated with [4-(2)H]-, [3,5-(2)H(2)]-, and (2)H(5)-phenylalanine as substrates. All (D)k(cat) values are normal with Delta117PheH, the catalytic core of rat phenylalanine hydroxylase, ranging from 1.12-1.41. In contrast, for Delta117PheH V379D, a mutant protein in which the stoichiometry between tetrahydropterin oxidation and amino acid hydroxylation is altered, the (D)k(cat) value with [4-(2)H]-phenylalanine is 0.92 but is normal with [3,5-(2)H(2)]-phenylalanine. The ratio of tetrahydropterin oxidation to amino acid hydroxylation for Delta117PheH V379D shows a similar inverse isotope effect with [4-(2)H]-phenylalanine. Intramolecular isotope effects, determined from the deuterium contents of the tyrosine formed from [4-(2)H]-and [3,5(2)H(2)]-phenylalanine, are identical for Delta117PheH and Delta117PheH V379D, suggesting that steps subsequent to oxygen addition are unaffected in the mutant protein. The inverse effects are consistent with the reaction of an activated ferryl-oxo species at the para position of the side chain of the amino acid to form a cationic intermediate. The normal effects on the (D)k(cat) value for the wild-type enzyme are attributed to an isotope effect of 5.1 on the tautomerization of a dienone intermediate to tyrosine with a rate constant 6- to7-fold that for hydroxylation. In addition, there is a slight ( approximately 34%) preference for the loss of the hydrogen originally at C4 of phenylalanine. With (2)H(5)-indole-tryptophan as a substrate for Delta117PheH, the (D)k(cat) value is 0.89, consistent with hydroxylation being rate-limiting in this case. When deuterated phenylalanines are used as substrates for TrpH, the (D)k(cat) values are within error of those for Delta117PheH V379D. Overall, these results are consistent with the aromatic amino acid hydroxylases all sharing the same chemical mechanism, but with the isotope effect for hydroxylation by PheH being masked by tautomerization of an enedione intermediate to tyrosine.

Effects of ligands on the mobility of an active-site loop in tyrosine hydroxylase as monitored by fluorescence anisotropy

Fluorescence anisotropy has been used to monitor the effect of ligands on a mobile loop over the active site of tyrosine hydroxylase. Phe184 in the center of the loop was mutated to tryptophan, and the three native tryptophan residues were mutated to phenylalanine to form an enzyme with a single tryptophan residue in the mobile loop. The addition of 6-methyl-5-deazatetrahydropterin to the enzyme resulted in a significant increase in the fluorescence anisotropy. The addition of phenylalanine did not result in a significant change in the anisotropy in the presence or absence of the deazapterin. The K(d) value for the deazapterin was unaffected by the presence of phenylalanine. Qualitatively similar results were obtained with apoenzyme, except that the addition of phenylalanine led to a slight decrease in anisotropy. Frequency-domain lifetime measurements showed that the distribution of lifetimes was unaffected by both the amino acid and deazapterin. Frequency-domain anisotropy analyses were consistent with a decrease in the motion of the sole tryptophan in the presence of the deazapterin. This could be modeled as a decrease in the cone angle for the indole ring of about 12 degrees . The data are consistent with a model in which binding of a tetrahydropterin results in a change in the conformation of the surface loop required for proper formation of the amino acid binding site.

Intrinsic isotope effects on benzylic hydroxylation by the aromatic amino acid hydroxylases: evidence for hydrogen tunneling, coupled motion, and similar reactivities

Deuterium kinetic isotope effects for hydroxylation of the methyl group of 4-methylphenylalanine have been used as a probe of the relative reactivities of the hydroxylating intermediates in the aromatic amino acid hydroxylases phenylalanine, tyrosine, and tryptophan hydroxylase. When there are three deuterium atoms in the methyl group, all three enzymes exhibit an intrinsic isotope effect of about 13. The temperature dependence of the isotope effect is consistent with moderate tunneling, with the extent of tunneling identical for all three enzymes. In the case of phenylalanine hydroxylase, the presence of the regulatory domain has no effect on the values. The intrinsic primary and secondary isotope effects were determined using 4-methylphenylalanine containing one or two deuterium atoms in the methyl group. With one deuterium atom, the intrinsic primary and secondary effects have average values of 10 and 1.1, respectively. With two deuterium atoms, the primary effects decrease to 7.4 and the secondary effect increases to 1.3, consistent with coupled motion of the primary and secondary hydrogens. The results with all three enzymes are consistent with a hydrogen abstraction mechanism. The similarities of the isotope effects and extent of tunneling establish that the reactivities of the hydroxylating intermediates in the three enzymes are essentially identical.

Reduction and oxidation of the active site iron in tyrosine hydroxylase: kinetics and specificity

Tyrosine hydroxylase (TyrH) is a pterin-dependent enzyme that catalyzes the hydroxylation of tyrosine to form dihydroxyphenylalanine. The oxidation state of the active site iron atom plays a central role in the regulation of the enzyme. The kinetics of reduction of ferric TyrH by several reductants were determined by anaerobic stopped-flow spectroscopy. Anaerobic rapid freeze-quench EPR confirmed that the change in the near-UV absorbance of TyrH upon adding reductant corresponded to iron reduction. Tetrahydrobiopterin reduces wild-type TyrH following a simple second-order mechanism with a rate constant of 2.8 +/- 0.1 mM(-)(1) s(-)(1). 6-Methyltetrahydropterin reduces the ferric enzyme with a second-order rate constant of 6.1 +/- 0.1 mM(-)(1) s(-)(1) and exhibits saturation kinetics. No EPR signal for a radical intermediate was detected. Ascorbate, glutathione, and 1,4-benzoquinone all reduce ferric TyrH, but much more slowly than tetrahydrobiopterin, suggesting that the pterin is a physiological reductant. E332A TyrH, which has an elevated K(m) for tetrahydropterin in the catalytic reaction, is reduced by tetrahydropterins with the same kinetic parameters as those of the wild-type enzyme, suggesting that BH(4) does not bind in the catalytic conformation during the reduction. Oxidation of ferrous TyrH by molecular oxygen can be described as a single-step second-order reaction, with a rate constant of 210 mM(-)(1) s(-)(1). S40E TyrH, which mimics the phosphorylated state of the enzyme, has oxidation and reduction kinetics similar to those of the wild-type enzyme, suggesting that phosphorylation does not directly regulate the interconversion of the ferric and ferrous forms.

Expression and purification of human tryptophan hydroxylase from Escherichia coli and Pichia pastoris

Tryptophan hydroxylase (TPH) from several mammalian species has previously been cloned and expressed in bacteria. However, due to the instability of wild type TPH, most successful attempts have been limited to the truncated forms of this enzyme. We have expressed full-length human TPH in large amounts in Escherichia coli and Pichia pastoris and purified the enzyme using new purification protocols. When expressed as a fusion protein in E. coli, the maltose-binding protein-TPH (MBP-TPH) fusion protein was more soluble than native TPH and the other fusion proteins and had a 3-fold higher specific activity than the His-Patch-thioredoxin-TPH and 6xHis-TPH fusion proteins. The purified MBP-TPH had a V(max) of 296 nmol/min/mg and a K(m) for L-tryptophan of 7.5+/-0.7 microM, compared to 18+/-5 microM for the partially purified enzyme from P. pastoris. To overcome the unfavorable properties of TPH, the stabilizing effect of different agents was investigated. Both tryptophan and glycerol had a stabilizing effect, whereas dithiothreitol, (6R)-5,6,7,8,-tetrahydrobiopterin, and Fe(2+) inactivated the enzyme. Irrespective of expression conditions, both native TPH expressed in bacteria or yeast, or TPH fusion proteins expressed in bacteria exhibited a strong tendency to aggregate and precipitate during purification, indicating that this is an intrinsic property of this enzyme. This supports previous observations that the enzyme in vivo may be stabilized by additional interactions.

A unique central tryptophan hydroxylase isoform

Serotonin (5-hydroxytryptophan, 5-HT) is a neurotransmitter synthesized in the raphe nuclei of the brain stem and involved in the central control of food intake, sleep, and mood. Accordingly, dysfunction of the serotonin system has been implicated in the pathogenesis of psychiatric diseases. At the same time, serotonin is a peripheral hormone produced mainly by enterochromaffin cells in the intestine and stored in platelets, where it is involved in vasoconstriction, haemostasis, and the control of immune responses. Moreover, serotonin is a precursor for melatonin and is therefore synthesized in high amounts in the pineal gland. Tryptophan hydroxylase (TPH) catalyzes the rate limiting step in 5-HT synthesis. Until recently, only one gene encoding TPH was described for vertebrates. By gene targeting, we functionally ablated this gene in mice. To our surprise, the resulting animals, although being deficient for serotonin in the periphery and in the pineal gland, exhibited close to normal levels of 5-HT in the brain stem. This led us to the detection of a second TPH gene in the genome of humans, mice, and rats, called TPH2. This gene is predominantly expressed in the brain stem, while the classical TPH gene, now called TPH1, is expressed in the gut, pineal gland, spleen, and thymus. These findings clarify puzzling data, which have been collected over the last decades about partially purified TPH proteins with different characteristics and justify a new concept of the serotonin system. In fact, there are two serotonin systems in vertebrates, independently regulated and with distinct functions.

Role of tryptophan hydroxylase phe313 in determining substrate specificity

The active site residue phenylalanine 313 is conserved in the sequences of all known tryptophan hydroxylases. The tryptophan hydroxylase F313W mutant protein no longer shows a preference for tryptophan over phenylalanine as a substrate, consistent with a role of this residue in substrate specificity. A tryptophan residue occupies the homologous position in tyrosine hydroxylase. The tyrosine hydroxylase W372F mutant enzyme does not show an increased preference for tryptophan over tyrosine or phenylalanine, so that this residue cannot be considered the dominant factor in substrate specificity in this family of enzymes.

Conformation of the substrate and pterin cofactor bound to human tryptophan hydroxylase. Important role of Phe313 in substrate specificity

Tryptophan hydroxylase (TPH) carries out the 5-hydroxylation of L-Trp, which is the rate-limiting step in the synthesis of serotonin. We have prepared and characterized a stable N-terminally truncated form of human TPH that includes the catalytic domain (Delta90TPH). We have also determined the conformation and distances to the catalytic non-heme iron of both L-Trp and the tetrahydrobiopterin cofactor analogue L-erythro-7,8-dihydrobiopterin (BH2) bound to Delta90TPH by using 1H NMR spectroscopy. The bound conformers of the substrate and the pterin were then docked into the modeled three-dimensional structure of TPH. The resulting ternary TPH-BH2-L-Trp structure is very similar to that previously determined by the same methods for the complex of phenylalanine hydroxylase (PAH) with BH2 and L-Phe [Teigen, K., et al. (1999) J. Mol. Biol. 294, 807-823]. In the model, L-Trp binds to the enzyme through interactions with Arg257, Ser336, His272, Phe318, and Phe313, and the ring of BH2 interacts mainly with Phe241 and Glu273. The distances between the hydroxylation sites at C5 in L-Trp and C4a in the pterin, i.e., 6.1 +/- 0.4 A, and from each of these sites to the iron, i.e., 4.1 +/- 0.3 and 4.4 +/- 0.3 A, respectively, are also in agreement with the formation of a transient iron-4a-peroxytetrahydropterin in the reaction, as proposed for the other hydroxylases. The different conformation of the dihydroxypropyl chain of BH2 in PAH and TPH seems to be related to the presence of nonconserved residues, i.e., Tyr235 and Pro238 in TPH, at the cofactor binding site. Moreover, Phe313, which seems to interact with the substrate through ring stacking, corresponds to a Trp residue in both tyrosine hydroxylase and PAH (Trp326) and appears to be an important residue for influencing the substrate specificity in this family of enzymes. We show that the W326F mutation in PAH increases the relative preference for L-Trp as the substrate, while the F313W mutation in TPH increases the preference for L-Phe, possibly by a conserved active site volume effect.

A novel neurodevelopmental syndrome responsive to 5-hydroxytryptophan and carbidopa

Tryptophan hydroxylase (TPH; EC 1.14.16.4) catalyzes the first rate-limiting step of serotonin biosynthesis by converting l-tryptophan to 5-hydroxytryptophan. Serotonin controls multiple vegetative functions and modulates sensory and alpha-motor neurons at the spinal level. We report on five boys with floppiness in infancy followed by motor delay, development of a hypotonic-ataxic syndrome, learning disability, and short attention span. Cerebrospinal fluid (CSF) analysis showed a 51 to 65% reduction of the serotonin end-metabolite 5-hydroxyindoleacetic acid (5HIAA) compared to age-matched median values. In one out of five patients a low CSF 5-methyltetrahydrofolate (MTHF) was present probably due to the common C677T heterozygous mutation of the methylenetetrahydrofolate reductase (MTHFR) gene. Baseline 24-h urinary excretion showed diminished 5HIAA values, not changing after a single oral load with l-tryptophan (50-70 mg/kg), but normalizing after 5-hydroxytryptophan administration (1 mg/kg). Treatment with 5-hydroxytryptophan (4-6 mg/kg) and carbidopa (0.5-1.0 mg/kg) resulted in clinical amelioration and normalization of 5HIAA levels in CSF and urine. In the patient with additional MTHFR heterozygosity, a heterozygous missense mutation within exon 6 (G529A) of the TPH gene caused an exchange of valine by isoleucine at codon 177 (V177I). This has been interpreted as a rare DNA variant because the pedigree analysis did not provide any genotype-phenotype correlation. In the other four patients the TPH gene analysis was normal. In conclusion, this new neurodevelopmental syndrome responsive to treatment with 5-hydroxytryptophan and carbidopa might result from an overall reduced capacity of serotonin production due to a TPH gene regulatory defect, unknown factors inactivating the TPH enzyme, or selective loss of serotonergic neurons.

Identification of substrate orienting and phosphorylation sites within tryptophan hydroxylase using homology-based molecular modeling

Tryptophan hydroxylase (TPH) is the initial and rate-limiting enzyme in the biosynthesis of serotonin. The inherent instability of TPH has prevented a crystallographic structure from being resolved. For this reason, multiple sequence alignment-based molecular modeling was utilized to generate a full-length model of human TPH. Previously determined crystal coordinates of two highly homologous proteins, phenylalanine hydroxylase and tyrosine hydroxylase, were used as templates. Analysis of the model aided rational mutagenesis studies to further dissect the regulation and catalysis of TPH. Using rational site-directed mutagenesis, it was determined that Tyr235 (Y235), within the active site of TPH, appears to be involved as a tryptophan substrate orienting residue. The mutants Y235A and Y235L displayed reduced specific activity compared to wild-type TPH ( approximately 5 % residual activity). The K(m) of tryptophan for the Y235A (564 microM) and Y235L (96 microM) mutant was significantly increased compared to wild-type TPH (42 microM). In addition, kinetic analyses were performed on wild-type TPH and a deletion construct that lacks the amino terminal autoregulatory sequence (TPH NDelta15). This sequence in phenylalanine hydroxylase (residues 19 to 33) has previously been proposed to act as a steric regulator of substrate accessibility to the active site. Changes in the steady-state kinetics for tetrahydrobiopterin (BH(4)) and tryptophan for TPH NDelta15 were not observed. Finally, it was demonstrated that both Ser58 and Ser260 are substrates for Ca(2+)/calmodulin-dependent protein kinase II. Additional analysis of this model will aid in deciphering the regulation and substrate specificity of TPH, as well as providing a basis to understand as yet to be identified polymorphisms.

Intersubunit binding domains within tyrosine hydroxylase and tryptophan hydroxylase

Tryptophan hydroxylase (TPH), the rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin (5-HT) belongs to the aromatic amino acid hydroxylase superfamily, which includes phenylalanine hydroxylase (PAH) and tyrosine hydroxylase (TH). The crystal structures for both PAH and TH have been reported, but a crystallographic model of TPH remains elusive. For this reason, we have utilized the information presented in the TH crystal structure in combination with primary sequence alignments to design point mutations in potential structural domains of the TPH protein. Mutation of a TH salt bridge (K170E) was sufficient to alter enzyme macromolecular assembly. We found that the disruption of the cognate intersubunit dimerization salt bridge (K111-E223) in TPH, however, did not affect the macromolecular assembly of TPH. Enzyme peaks representing only tetramers were observed with size exclusion chromatography. By contrast, a single-point mutation within the tetramerization domain of TPH (L435A) was sufficient to disrupt the normal homotetrameric assembly of TPH. These studies indicate that, although the proposed salt bridge dimerization interface of TH is conserved in TPH, this hypothetical TPH intersubunit binding domain, K111-E223, is not required for the proper macromolecular assembly of the protein. However, leucine 435 within the tetramerization domain is necessary for the proper macromolecular assembly of TPH.

Tetrahydropterin-dependent amino acid hydroxylases

Phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase constitute a small family of monooxygenases that utilize tetrahydropterins as substrates. When from eukaryotic sources, these enzymes are composed of a homologous catalytic domain to which are attached discrete N-terminal regulatory domains and short C-terminal tetramerization domains, whereas the bacterial enzymes lack the N-terminal and C-terminal domains. Each enzyme contains a single ferrous iron atom bound to two histidines and a glutamate. Recent mechanistic studies have begun to provide insights into the mechanisms of oxygen activation and hydroxylation. Although the hydroxylating intermediate in these enzymes has not been identified, the iron is likely to be involved. Reversible phosphorylation of serine residues in the regulatory domains affects the activities of all three enzymes. In addition, phenylalanine hydroxylase is allosterically regulated by its substrates, phenylalanine and tetrahydrobiopterin.

The aromatic amino acid hydroxylases

The enzymes phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase constitute the family of pterin-dependent aromatic amino acid hydroxylases. Each enzyme catalyzes the hydroxylation of the aromatic side chain of its respective amino acid substrate using molecular oxygen and a tetrahydropterin as substrates. Recent advances have provided insights into the structures, mechanisms, and regulation of these enzymes. The eukaryotic enzymes are homotetramers comprised of homologous catalytic domains and discrete regulatory domains. The ligands to the active site iron atom as well as residues involved in substrate binding have been identified from a combination of structural studies and site-directed mutagenesis. Mechanistic studies with nonphysiological and isotopically substituted substrates have provided details of the mechanism of hydroxylation. While the complex regulatory properties of phenylalanine and tyrosine hydroxylase are still not fully understood, effects of regulation on key kinetic parameters have been identified. Phenylalanine hydroxylase is regulated by an interaction between phosphorylation and allosteric regulation by substrates. Tyrosine hydroxylase is regulated by phosphorylation and feedback inhibition by catecholamines.

Influence of steric bulk and electrostatics on the hydroxylation regiospecificity of tryptophan hydroxylase: characterization of methyltryptophans and azatryptophans as substrates

Tryptophan hydroxylase is a pterin-dependent amino acid hydroxylase that catalyzes the incorporation of one atom of molecular oxygen into tryptophan to form 5-hydroxytryptophan. The substrate specificity and hydroxylation regiospecificity of tryptophan hydroxylase have been investigated using tryptophan analogues that have methyl substituents or nitrogens incorporated into the indole ring. The products of the reactions show that the regiospecificity of tryptophan hydroxylase is stringent. Hydroxylation does not occur at the 4 or 6 carbon in response to changes in substrate topology or atomic charge. 5-Hydroxymethyltryptophan and 5-hydroxy-4-methyltryptophan are the products from 5-methyltryptophan. These products establish that the hydroxylating intermediate is sufficiently potent to hydroxylate benzylic carbons and that the direction of the NIH shift in tryptophan hydroxylase is from carbon 5 to carbon 4. The effects on the V/K values for the amino acids indicate that the enzyme is most sensitive to changes at position 5 of the indole ring. The V(max) values for amino acid hydroxylation differ at most by a factor of 3 from that observed for tryptophan, while the efficiencies of hydroxylation with respect to tetrahydropterin consumption vary 6-fold, consistent with oxygen transfer to the amino acid being partially or fully rate limiting in productive catalysis.

Cloning and expression of recombinant human pineal tryptophan hydroxylase in Escherichia coli: purification and characterization of the cloned enzyme

The first step in the biosynthesis of melatonin in the pineal gland is the hydroxylation of tryptophan to 5-hydroxytryptophan. A cDNA of human tryptophan hydroxylase (TPH) was cloned from a library of human pineal gland and expressed in Escherichia coli. This cDNA sequence is identical to the cDNA sequence published from the human carcinoid tissue [1]. This human pineal hydroxylase gene encodes a protein of 444 amino acids and a molecular mass of 51 kDa estimated for the purified enzyme. Tryptophan hydroxylase from human brainstem exhibits high sequence homology (93% identity) with the human pineal hydroxylase. The recombinant tryptophan hydroxylase exists in solution as tetramers. The expressed human pineal tryptophan hydroxylase has a specific activity of 600 nmol/min/mg when measured in the presence of tetrahydrobiopterin and L-tryptophan. The enzyme catalyzes the hydroxylation of tryptophan and phenylalanine at comparable rates. Phosphorylation of the hydroxylase by protein kinase A or calmodulin-dependent kinase II results in the incorporation of 1 mol of phosphate/mol of subunit, but this degree of phosphorylation leads to only a modest (30%) increase in BH(4)-dependent activity when assayed in the presence of 14-3-3. Rapid scanning ultraviolet spectroscopy has revealed the formation of the transient intermediate compound, 4alpha-hydroxytetrahydrobiopterin, during the hydroxylation of either tryptophan or phenylalanine catalyzed by the recombinant pineal TPH.