Synthesis of L-tyrosine from pyruvate, ammonia and phenol by crystalline tyrosine phenol lyase

Research overview of L-DOPA production using a bacterial enzyme, tyrosine phenol-lyase

L-DOPA is an amino acid that is used as a treatment for Parkinson's disease. A simple enzymatic synthesis method of L-DOPA had been developed using bacterial L-tyrosine phenol-lyase (Tpl). This review describes research on screening of bacterial strains, culture conditions, properties of the enzyme, reaction mechanism of the enzyme, and the reaction conditions for the production of L-DOPA. Furthermore, molecular bleeding of constitutively Tpl-overproducing strains is described, which were developed based on mutations in a DNA binding protein, TyrR, which controls the induction of tpl gene expression.

M379A Mutant Tyrosine Phenol-lyase from Citrobacter freundii Has Altered Conformational Dynamics

The M379A mutant of Citrobacter freundii tyrosine phenol‐lyase (TPL) has been prepared. M379A TPL is a robust catalyst to prepare a number of tyrosines substituted at the 3‐position with bulky groups that cannot be made with wild type TPL. The three dimensional structures of M379A TPL complexed with L‐methionine and 3‐bromo‐dl‐phenylalanine have been determined by X‐ray crystallography. Methionine is bound as a quinonoid complex in a closed active site in 3 of 4 chains of homotetrameric M379A TPL. M379A TPL reacts with l‐methionine about 8‐fold slower than wild type TPL. The temperature dependence shows that the slower reaction is due to less positive activation entropy. The structure of the M379A TPL complex of 3‐bromo‐DL‐phenylalanine has a quinonoid complex in two subunits, with an open active site conformation. The effects of the M379A mutation on TPL suggest that the mutant enzyme has altered the conformational dynamics of the active site.

Engineering Translation Components for Genetic Code Expansion

The expansion of the genetic code consisting of four bases and 20 amino acids into diverse building blocks has been an exciting topic in synthetic biology. Many biochemical components are involved in gene expression; therefore, adding a new component to the genetic code requires engineering many other components that interact with it. Genetic code expansion has advanced significantly for the last two decades with the engineering of several components involved in protein synthesis. These components include tRNA/aminoacyl-tRNA synthetase, new codons, ribosomes, and elongation factor Tu. In addition, biosynthesis and enhanced uptake of non-canonical amino acids have been attempted and have made meaningful progress. This review discusses the efforts to engineer these translation components, to improve the genetic code expansion technology.

Rapid Structural Analysis of a Synthetic Non-canonical Amino Acid by Microcrystal Electron Diffraction

Structural characterization of small molecules is a crucial component of organic synthesis. In this work, we applied microcrystal electron diffraction (MicroED) to analyze the structure of the product of an enzymatic reaction that was intended to produce the unnatural amino acid 2,4-dihydroxyphenylalanine (24DHF). Characterization of our isolated product with nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS) suggested that an isomer of 24DHF had been formed. Microcrystals present in the isolated product were then used to determine its structure to 0.62 Å resolution, which confirmed its identity as 2-amino-2-(2,4-dihydroxyphenyl)propanoic acid (24DHPA). Moreover, the MicroED structural model indicated that both enantiomeric forms of 24DHPA were present in the asymmetric unit. Notably, the entire structure determination process including setup, data collection, and refinement was completed in ~1 h. The MicroED data not only bolstered previous results obtained using NMR and MS but also immediately provided information about the stereoisomers present in the product, which is difficult to achieve using NMR and MS alone. Our results therefore demonstrate that MicroED methods can provide useful structural information on timescales that are similar to many commonly used analytical methods and can be added to the existing suite of small molecule structure determination tools in future studies.

Genetic incorporation of l-dihydroxyphenylalanine (DOPA) biosynthesized by a tyrosine phenol-lyase

l-Dihydroxyphenylalanine (DOPA) was biosynthesized by a tyrosine-phenol lyase from catechol, pyruvate, and ammonia in Escherichia coli, and the biosynthesized amino acid was directly incorporated into proteins. Three biochemical experiments with mutant proteins containing DOPA confirmed the genetic incorporation of biosynthesized DOPA, and revealed its potential for various biochemical applications.

Breeding of a cyclic imide-assimilating bacterium, Pseudomonas putida s52, for high efficiency production of pyruvate

A succinimide-assimilating bacterium, Pseudomonas putida s52, was found to be a potent producer of pyruvate from fumarate. Using washed cells from P. putida s52 as catalyst, 400 mM pyruvate was produced from 500 mM fumarate in a 36-h reaction. Bromopyruvate, a malic enzyme inhibitor, was used for the selection of mutants with higher pyruvate productivity. A bromopyruvate-resistant mutant, P. putida 15160, was found to be an effective catalyst for pyruvate production. Moreover, under batch bioreactor conditions, 767 mM of pyruvate was successfully produced from 1,000 mM fumarate in a 72-h reaction with washed cells from P. putida 15160 as catalyst.

Microbial production of isoquinoline alkaloids as plant secondary metabolites based on metabolic engineering research

Plants produce a variety of secondary metabolites that possess strong physiological activities. Unfortunately, however, their production can suffer from a variety of serious problems, including low levels of productivity and heterogeneous quality, as well as difficulty in raw material supply. In contrast, microorganisms can be used to produce their primary and some of their secondary metabolites in a controlled environment, thus assuring high levels of efficiency and uniform quality. In an attempt to overcome the problems associated with secondary metabolite production in plants, we developed a microbial platform for the production of plant isoquinoline alkaloids involving the unification of the microbial and plant metabolic pathways into a single system. The potential applications of this system have also been discussed.

Crystallographic snapshots of tyrosine phenol-lyase show that substrate strain plays a role in C-C bond cleavage

The key step in the enzymatic reaction catalyzed by tyrosine phenol-lyase (TPL) is reversible cleavage of the Cβ-Cγ bond of L-tyrosine. Here, we present X-ray structures for two enzymatic states that form just before and after the cleavage of the carbon-carbon bond. As for most other pyridoxal 5'-phosphate-dependent enzymes, the first state, a quinonoid intermediate, is central for the catalysis. We captured this relatively unstable intermediate in the crystalline state by introducing substitutions Y71F or F448H in Citrobacter freundii TPL and briefly soaking crystals of the mutant enzymes with a substrate 3-fluoro-L-tyrosine followed by flash-cooling. The X-ray structures, determined at ~2.0 Å resolution, reveal two quinonoid geometries: "relaxed" in the open and "tense" in the closed state of the active site. The "tense" state is characterized by changes in enzyme contacts made with the substrate's phenolic moiety, which result in significantly strained conformation at Cβ and Cγ positions. We also captured, at 2.25 Å resolution, the X-ray structure for the state just after the substrate's Cβ-Cγ bond cleavage by preparing the ternary complex between TPL, alanine quinonoid and pyridine N-oxide, which mimics the α-aminoacrylate intermediate with bound phenol. In this state, the enzyme-ligand contacts remain almost exactly the same as in the "tense" quinonoid, indicating that the strain induced by the closure of the active site facilitates elimination of phenol. Taken together, structural observations demonstrate that the enzyme serves not only to stabilize the transition state but also to destabilize the ground state.

Effect of Culture Conditions on the Production of Tyrosine Phenol-Lyase by Erwinia herbicola

The effect of environmental parameters on the growth and the tyrosine phenol-lyase content of Erwinia herbicola was investigated. On mineral medium containing glycerol, l-tyrosine increased the enzyme content 23-fold. When the l-tyrosine was also the carbon source, bacterial growth was 300 times greater than the basal level. On a rich medium, tyrosine phenol-lyase production was strongly dependent on pH and aeration. Catabolite repression and induction both probably control enzyme content.

Insights into the catalytic mechanism of tyrosine phenol-lyase from X-ray structures of quinonoid intermediates

Amino acid transformations catalyzed by a number of pyridoxal 5'-phosphate (PLP)-dependent enzymes involve abstraction of the Calpha proton from an external aldimine formed between a substrate and the cofactor leading to the formation of a quinonoid intermediate. Despite the key role played by the quinonoid intermediates in the catalysis by PLP-dependent enzymes, limited accurate information is available about their structures. We trapped the quinonoid intermediates of Citrobacter freundii tyrosine phenol-lyase with L-alanine and L-methionine in the crystalline state and determined their structures at 1.9- and 1.95-A resolution, respectively, by cryo-crystallography. The data reveal a network of protein-PLP-substrate interactions that stabilize the planar geometry of the quinonoid intermediate. In both structures the protein subunits are found in two conformations, open and closed, uncovering the mechanism by which binding of the substrate and restructuring of the active site during its closure protect the quinonoid intermediate from the solvent and bring catalytically important residues into positions suitable for the abstraction of phenol during the beta-elimination of L-tyrosine. In addition, the structural data indicate a mechanism for alanine racemization involving two bases, Lys-257 and a water molecule. These two bases are connected by a hydrogen bonding system allowing internal transfer of the Calpha proton.

Effective production of 3,4-dihydroxyphenyl-L-alanine (L-DOPA) with Erwinia herbicola cells carrying a mutant transcriptional regulator TyrR

The enzymatic production of 3,4-dihydroxyphenyl-L-alanine (L-DOPA) using Erwinia herbicola cells involves the action of tyrosine phenol-lyase (Tpl, EC 4.1.99.2). Since Tpl is only synthesized under L-tyrosine-induced conditions, the addition of L-tyrosine to the medium is unavoidable when preparing cells (the enzyme source), but severely impedes the pure preparation of the final product L-DOPA. We circumvented this problem by using recombinant E. herbicola cells carrying a mutant transcriptional regulator TyrR, which is capable of activating the tpl promoter in the absence of L-tyrosine.

Evaluation of the mass spectrometric fragmentation of codeine and morphine after 13C-isotope biosynthetic labeling

All major fragment ions of codeine and morphine were elucidated using LC-electrospray MS/MS and high resolution FT-ICR-MS combined with an IRMPD system. Nanogram quantities of labeled codeine were isolated and purified from Papaver somniferum seedlings, which were grown for up to 9 days in the presence of [ring-13C6]-l-tyrosine, [ring-13C6]-tyramine and [1,2-13C2], [6-O-methyl 13C]-(R,S)-coclaurine. The labeling degree of codeine up to 57% into morphinans was observed.

Cloning and random mutagenesis of the Erwinia herbicola tyrR gene for high-level expression of tyrosine phenol-lyase

Tyrosine phenol-lyase (Tpl), which can synthesize 3, 4-dihydroxyphenylalanine from pyruvate, ammonia, and catechol, is a tyrosine-inducible enzyme. Previous studies demonstrated that the tpl promoter of Erwinia herbicola is activated by the TyrR protein of Escherichia coli. In an attempt to create a high-Tpl-expressing strain, we cloned the tyrR gene of E. herbicola and then randomly mutagenized it. Mutant TyrR proteins with enhanced ability to activate tpl were screened for by use of the lac reporter system in E. coli. The most increased transcription of tpl was observed for the strain with the mutant tyrR allele involving amino acid substitutions of alanine, cysteine, and glycine for valine-67, tyrosine-72, and glutamate-201, respectively. A tyrR-deficient derivative of E. herbicola was constructed and transformed with a plasmid carrying the mutant tyrR allele (V67A Y72C E201G substitutions). The resultant strain expressed Tpl without the addition of tyrosine to the medium and produced as much of it as was produced by the wild-type strain grown under tyrosine-induced conditions. The regulatory properties of the mutant TyrR(V67A), TyrR(Y72C), TyrR(E201G), and TyrR(V67A Y72C E201G) proteins were examined in vivo. Interestingly, as opposed to the wild-type TyrR protein, the mutant TyrR(V67A) protein had a repressive effect on the tyrP promoter in the presence of phenylalanine as the coeffector.

The role of glutamic acid-69 in the activation of Citrobacter freundii tyrosine phenol-lyase by monovalent cations

Tyrosine phenol-lyase (TPL) from Citrobacter freundii is activated about 30-fold by monovalent cations, the most effective being K(+), NH(4)(+), and Rb(+). Previous X-ray crystal structure analysis has demonstrated that the monovalent cation binding site is located at the interface between subunits, with ligands contributed by the carbonyl oxygens of Gly52 and Asn262 from one chain and monodentate ligation by one of the epsilon-oxygens of Glu69 from another chain [Antson, A. A., Demidkina, T. V., Gollnick, P., Dauter, Z., Von Tersch, R. L., Long, J., Berezhnoy, S. N., Phillips, R. S., Harutyunyan, E. H., and Wilson, K. S. (1993) Biochemistry 32, 4195]. We have studied the effect of mutation of Glu69 to glutamine (E69Q) and aspartate (E69D) to determine the role of Glu69 in the activation of TPL. E69Q TPL is activated by K(+), NH(4)(+), and Rb(+), with K(D) values similar to wild-type TPL, indicating that the negative charge on Glu69 is not necessary for cation binding and activation. In contrast, E69D TPL exhibits very low basal activity and only weak activation by monovalent cations, even though monovalent cations are capable of binding, indicating that the geometry of the monovalent cation binding site is critical for activation. Rapid-scanning stopped-flow kinetic studies of wild-type TPL show that the activating effect of the cation is seen in an acceleration of rates of quinonoid intermediate formation (30-50-fold) and of phenol elimination. Similar rapid-scanning stopped-flow results were obtained with E69Q TPL; however, E69D TPL shows only a 4-fold increase in the rate of quinonoid intermediate formation with K(+). Preincubation of TPL with monovalent cations is necessary to observe the rate acceleration in stopped flow kinetic experiments, suggesting that the activation of TPL by monovalent cations is a slow process. In agreement with this conclusion, a slow increase (k < 0.5 s(-)(1)) in fluorescence intensity (lambda(ex) = 420 nm, lambda(em) = 505 nm) is observed when wild-type and E69Q TPL are mixed with K(+), Rb(+), and NH(4)(+) but not Li(+) or Na(+). E69D TPL shows no change in fluorescence under these conditions. High concentrations (>100 mM) of all monovalent cations result in inhibition of wild-type TPL. This inhibition is probably due to cation binding to the ES complex to form a complex that releases pyruvate slowly.

Citrobacter freundii tyrosine phenol-lyase: the role of asparagine 185 in modulating enzyme function through stabilization of a quinonoid intermediate

Asn185 is an invariant residue in all known sequences of TPL and of closely related tryptophanase and it may be aligned with the Asn194 in aspartate aminotransferase. According to X-ray data, in the holoenzyme and in the Michaelis complex Asn185 does not interact with the cofactor pyridoxal 5'-phosphate, but in the external aldimine a conformational change occurs which is accompanied by formation of a hydrogen bond between Asn185 and the oxygen atom in position 3 of the cofactor. The substitution of Asn185 in TPL by alanine results in a mutant N185A TPL of moderate residual activity (2%) with respect to adequate substrates, L-tyrosine and 3-fluoro-L-tyrosine. The affinities of the mutant enzyme for various amino acid substrates and inhibitors, studied by both steady-state and rapid kinetic techniques, were lower than for the wild-type TPL. This effect mainly results from destabilization of the quinonoid intermediate, and it is therefore concluded that the hydrogen bond between Asn185 and the oxygen at the C-3 position of the cofactor is maintained in the quinonoid intermediate. The relative destabilization of the quinonoid intermediate and external aldimine leads to the formation of large amounts of gem-diamine in reactions of N185A TPL with 3-fluoro-L-tyrosine and L-phenylalanine. For the reaction with 3-fluoro-L-tyrosine it was first possible to determine kinetic parameters of gem-diamine formation by the stopped-flow method. For the reactions of N185A TPL with substrates bearing good leaving groups the observed values of k(cat) could be accounted for by taking into consideration two effects: the decrease in the quinonoid content under steady-state conditions and the increase in the quinonoid reactivity in a beta-elimination reaction. Both effects are due to destabilization of the quinonoid and they counterbalance each other. Multiple kinetic isotope effect studies on the reactions of N185A TPL with suitable substrates, L-tyrosine and 3-fluoro-L-tyrosine, show that the principal mechanism of catalysis, suggested previously for the wild-type enzyme, does not change. In the framework of this mechanism the observed considerable decrease in k(cat) values for reactions of N185A TPL with L-tyrosine and 3-fluoro-L-tyrosine may be ascribed to participation of Asn185 in additional stabilization of the keto quinonoid intermediate.

Metabolic flux analysis for efficient pyruvate fermentation using vitamin-auxotrophic yeast of Torulopsis glabrata

The metabolism of a vitamin-auxotrophic pyruvate-producing microorganism, Torulopsis glabrata IFO 0005, was investigated by metabolic flux analysis. Particular attention was focused on the effect of culture conditions, such as dissolved oxygen (DO) concentration and thiamine concentration, on specific pathway activities. The results of metabolic flux analysis indicate that the thiamine concentration significantly affected pyruvate dehydrogenase and pyruvate decarboxylase activities, and plays an important role in cell growth and pyruvate production. Metabolic flux analysis was also utilized to clarify the metabolism of this strain during pyruvate fermentation under different oxygen supply conditions, and the reason for the enhanced pyruvate production under conditions of 30-40% DO concentration was clarified from the viewpoint of intracellular flux distributions. Based on the analysis of the effect of thiamine concentration on the metabolic fluxes, we conducted a fed-batch experiment where the initial thiamine concentration was reduced to 30 microg/l and thiamine was added at 10 microg/l during fermentation when the cell growth rate decreased to 0.2 h(-1). With separate addition of thiamine, the overall pyruvate yield could be improved by 15% due to the decrease of ethanol production.

The crystal structure of Citrobacter freundii tyrosine phenol-lyase complexed with 3-(4'-hydroxyphenyl)propionic acid, together with site-directed mutagenesis and kinetic analysis, demonstrates that arginine 381 is required for substrate specificity

The X-ray structure of tyrosine phenol-lyase (TPL) complexed with a substrate analog, 3-(4'-hydroxyphenyl)propionic acid, shows that Arg 381 is located in the substrate binding site, with the side-chain NH1 4.1 A from the 4'-OH of the analog. The structure has been deduced at 2.5 A resolution using crystals that belong to the P2(1)2(1)2 space group with a = 135.07 A, b = 143.91 A, and c = 59.80 A. To evaluate the role of Arg 381 in TPL catalysis, we prepared mutant proteins replacing arginine with alanine (R381A), with isoleucine (R381I), and with valine (R381V). The beta-elimination activity of R381A TPL has been reduced by 10(-4)-fold compared to wild type, whereas R381I and R381V TPL exhibit no detectable beta-elimination activity with L-tyrosine as substrate. However, R381A, R381I, and R381V TPL react with S-(o-nitrophenyl)-L-cysteine, beta-chloro-L-alanine, O-benzoyl-L-serine, and S-methyl-L-cysteine and exhibit k(cat) and k(cat)/Km values comparable to those of wild-type TPL. Furthermore, the Ki values for competitive inhibition by L-tryptophan and L-phenylalanine are similar for wild-type, R381A, and R381I TPL. Rapid-scanning-stopped flow spectroscopic analyses also show that wild-type and mutant proteins can bind L-tyrosine and form quinonoid complexes with similar rate constants. The binding of 3-(4'-hydroxyphenyl)propionic acid to wild-type TPL decreases at high pH values with a pKa of 8.4 and is thus dependent on an acidic group, possibly Arg404, which forms an ion pair with the analog carboxylate, or the pyridoxal 5'-phosphate Schiff base. R381A TPL shows only a small decrease in k(cat)/Km for tyrosine at lower pH, in contrast to wild-type TPL, which shows two basic pKas with an average value of about 7.8. Thus, it is possible that Arg 381 is one of the catalytic bases previously observed in the pH dependence of k(cat)/Km of TPL with L-tyrosine [Kiick, D. M., & Phillips. R. S. (1988) Biochemistry 27, 7333-7338], and hence Arg 381 is at least partially responsible for the substrate specificity of TPL.

Effects of tyrosine ring fluorination on rates and equilibria of formation of intermediates in the reactions of carbon-carbon lyases

The interactions of ring fluorinated analogs of tyrosine with tyrosine phenol-lyase and tryptophan indole-lyase (tryptophanase) were studied by rapid-scanning stopped-flow spectrophotometry. The reaction of L-tyrosine with tyrosine phenol-lyase resulted in rapid formation of a small absorbance peak at 500 nm, attributed to a quinonoid intermediate. The reaction of 3-fluoro-L-tyrosine with tyrosine phenol-lyase resulted in a peak at 500 nm with much higher absorbance, as did the reaction of 3,5-difluoro-L-tyrosine, due to increased accumulation of quinonoid intermediates. In constrast, complexes with 2-fluoro-L-tyrosine, 2,3-difluoro-L-tyrosine, 2,5-difluoro-L-tyrosine, and 2,6-difluoro-L-tyrosine exhibited much lower absorbance intensity at 500 nm. The rate constant for quinonoid intermediate formation from 3-fluoro-L-tyrosine was comparable to that for L-tyrosine. However, 3,5-difluoro-L-tyrosine reacted to form a quinonoid intermediate at about half the rate of L-tyrosine, while 2,3-difluoro-L-tyrosine reacted at twice the rate of L-tyrosine. In addition, the 2-substituted difluorotyrosines exhibited an intermediate, which was formed rapidly, absorbing strongly at about 340 nm, which is likely due to a gem-diamine intermediate. Tyrosine is not a substrate for tryptophan indole-lyase; the reaction of tryptophan indole-lyase with L-tyrosine resulted in formation of external aldimine, which absorbed at 420 nm, and a very small absorbance peak at 500 nm. 3-Fluoro-L-tyrosine reacted with tryptophan indole-lyase to produce a prominent quinonoid absorbance peak at 500 nm, whereas L-tyrosine, 2-fluoro-L-tyrosine, and all difluoro-L-tyrosines, had a much reduced intensity for this peak. Thus, the presence of ring fluorine substituents in L-tyrosine that are remote from the site of the chemical transformation has significant effects on the rates and equilibria of intermediate formation in the reactions with both tyrosine phenol-lyase and tryptophan indole-lyase. Although it is commonly thought that fluorine substitution will not result in any significant steric effects, our results suggest that the effects of fluorine substitution in the reactions of fluorinated tyrosines with tyrosine phenol-lyase and tryptophan indole-lyase are due to a combination of steric and electronic effects.

Binding of phenol and analogues to alanine complexes of tyrosine phenol-lyase from Citrobacter freundii: implications for the mechanisms of alpha,beta-elimination and alanine racemization

We have examined the interaction of Citrobacter freundii tyrosine phenol-lyase with both L- and D-alanine. This enzyme catalyzes the racemization of alanine as a side reaction, in addition to the physiological beta-elimination of L-tyrosine to give phenol and ammonium pyruvate. The steady-state kinetic parameters for alanine racemization, kcat and Km, for D-alanine are 0.008 S-1 and 32 mM, respectively, while those for L-alanine are 0.03 S-1 and 11 mM. Incubation of tyrosine phenol-lyase with either L- or D-alanine forms a quinonoid complex that exhibits a strong peak at 500 nm. The presence of K+ increases the intensity of the 500-nm absorption with L-alanine, but decreases the intensity of the peak with D-alanine. Rate constants for the formation of these quinonoid intermediates and the effects of phenol and analogues on the reaction with either L- or D-alanine have been studied by rapid-scanning and single-wavelength stopped-flow spectrophotometry. Phenol binds to all the intermediates of tyrosine phenol-lyase with L- and D-alanine, but most strongly to the external aldimine complex, resulting in a decrease in the absorbance at 500 nm at equilibrium. Pyridine N-oxide binds selectively to the quinonoid complex of alanine, and thus causes an increase in the absorbance at 500 nm at equilibrium. 4-Hydroxypyridine causes a decrease in absorbance at 500 nm during the fast phase, but an increase in absorbance at 502 nm in a subsequent slow relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Reactions of O-acyl-L-serines with tryptophanase, tyrosine phenol-lyase, and tryptophan synthase

The reactions of tryptophanase, tyrosine phenol-lyase, and tryptophan synthase with a new class of substrates, the O-acyl-L-serines, have been examined. A method for preparation of O-benzoyl-L-serine in high yield from tert.-butyloxycarbonyl (tBoc)-L-serine has been developed. Reaction of the cesium salt of tBoc-L-serine with benzyl bromide in dimethylformamide gives tBoc-L-serine benzyl ester in excellent yield. Acylation with benzoyl chloride and triethylamine in acetonitrile followed by hydrogenolysis with 10% palladium on carbon in trifluoroacetic acid gives O-benzoyl-L-serine, isolated as the hydrochloride salt. O-Benzoyl-L-serine is a good substrate for beta-elimination or beta-substitution reactions catalyzed by both tryptophanase and tyrosine phenol-lyase, with Vmax values 5- to 6-fold those of the physiological substrates and comparable to that of S-(o-nitrophenyl)-L-cysteine. Unexpectedly, O-acetyl-L-serine is a very poor substrate for these enzymes, with Vmax values about 5% of those of the physiological substrates. Both O-acyl-L-serines are poor substrates for tryptophan synthase, measured either by the synthesis of 5-fluoro-L-tryptophan from 5-fluoroindole and L-serine catalyzed by the intact alpha 2 beta 2 subunit or by the beta-elimination reaction catalyzed by the isolated beta 2 subunit. With all three enzymes, the elimination of benzoate appears to be irreversible. These results suggest that the binding energy from the aromatic ring of O-benzoyl-L-serine is used to lower the transition-state barrier for the elimination reactions catalyzed by tryptophanase and tyrosine phenol-lyase. Our findings support the suggestion (M. N. Kazarinoff and E. E. Snell (1980) J. Biol. Chem. 255, 6228-6233) that tryptophanase undergoes a conformational change during catalysis and suggest that tyrosine phenol-lyase also may undergo a conformational change during catalysis.

Enzymatic transformations of 3-chloroalanine into useful amino acids

The investigation of the combination of enzymatic and chemical synthetic processes for the production of useful compounds has been carried out. This review focuses on the enzymatic transformation of chemically synthesized 3-chloroalanine into useful amino acids.

Syntheses of L-tyrosine-related amino acids by tyrosine phenol-lyase of Citrobacter intermedius

Degradation of tyrosine to phenol, pyruvate and ammonia by tyrosine phenol-lyase from Citrobacter intermedius (formerly named Escherichia intermedia) is readily reversible at high concentrations of pyruvate and ammonia. Spectrophotometric studies indicate that ammonia is the first substrate which interacts with bound pyridoxal 5'-phosphate. Kinetic results show that pyruvate is the second substrate bound, hence phenol must be the third. When an appropriate phenol derivative is substituted for phenol, the corresponding tyrosine analogue can be synthesized. 3-Fluoro-, 2-fluoro-, 3-chloro-, 2-chloro-, 3-bromo-, 2-bromo-, 2-iodo-, 3-methyl-, 2-methyl- and 2-methoxy-L-tyrosines have been synthesized by this reaction. By using various phenol derivatives or tyrosine analogues as substrates, the substrate specificity of tyrosine phenol-lyase is investigated and the situation of its active site is discussed.