Synthesis and antibacterial activity of mechanism-based inhibitors of KDO8P synthase and DAH7P synthase

KDO8PS (3-deoxy-D-manno-2-octulosonate-8-phosphate synthase) and DAH7PS (3-deoxy-D-arabino-2-heptulosonate-7-phosphate synthase) are attractive targets for the development of new anti-infectious agents. Both enzymes appear to proceed via a common mechanism involving the reaction of phosphoenolpyruvate (PEP) with arabinose 5-phosphate or erythrose-4-phosphate, to produce the corresponding ulosonic acids, KDO8P and DAH7P, respectively. The synthesis of new inhibitors closely related to the supposed tetrahedral intermediate substrates for the enzymes is described. The examination of the antibacterial activity of these derivatives is reported.

[Regulation of 3-desoxy-D-arabino-heptulosonate-7-phosphate synthetase in the bacteria Pseudomonas aurantiaca B-162]

The regulation of activity and synthesis of DAHP-synthase in Pseudomonas aurantiaca B-162 was studied. Analysis of partially purified preparations of the enzyme revealed two isoenzymes: DAHP-synthase [tyr] and DAHP-synthase [phe], each of them being regulated by a corresponding amino acid. DAHP-synthase [tyr] is a dominant isoenzyme presenting 78 % of the enzyme activity, 50 % inhibition of which is possible by 1,3 x 10(-5) M of tyrosine. DAHP-synthase [phe] is minor isoenzyme (sharing 22 % of enzyme activity) and is controlled by phenylalanine. In this case, 50% of inhibition of activity is possible by adding 5,5 10(-6) M of corresponding amino acid. Synthesis of DAHP-synthase is constitutive.

Allosteric inhibition of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase alters the coordination of both substrates

3-Deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS), the first enzyme of the aromatic biosynthetic pathway in microorganisms and plants, catalyzes the aldol-like condensation of phosphoenolpyruvate and D-erythrose-4-phosphate with the formation of 3-deoxy-D-arabino-heptulosonate-7-phosphate. In Escherichia coli, there are three isoforms of DAHPS, each specifically feedback-regulated by one of the three aromatic amino acid end products. The crystal structure of the phenylalanine-regulated DAHPS from E.coli in complex with its inhibitor, L-phenylalanine, phosphoenolpyruvate, and metal cofactor, Mn(2+), has been determined to 2.8A resolution. Phe binds in a cavity formed by residues of two adjacent subunits and is located about 20A from the closest active site. A model for the mechanism of allosteric inhibition has been derived from conformational differences between the Phe-bound and previously determined Phe-free structures. Two interrelated paths of conformational changes transmit the inhibitory signal from the Phe-binding site to the active site of DAHPS. The first path involves transmission within a single subunit due to the movement of adjacent segments of the protein. The second involves alterations in the contacts between subunits. The combination of these two paths changes the conformation of one of the active site loops significantly and shifts the other slightly. This alters the interaction of DAHPS with both of its substrates. Upon binding of Phe, the enzyme loses the ability to bind D-erythrose-4-phosphate and binds phosphoenolpyruvate in a flipped orientation.

Regulative fine-tuning of the two novel DAHP isoenzymes aroFp and aroGp of the filamentous fungus Aspergillus nidulans

Two novel genes, aroF and aroG, from the filamentous fungus Aspergillus nidulans were isolated and the regulative fine-tuning between the encoded, differentially regulated 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthases was analyzed. A wide range of DAHP synthase isoenzymes of various organisms are known, but only a few have been characterized further. DAHP synthases (EC 4.1.2.15) catalyze the first committed step of the shikimate pathway, which is a putative target for anti-weed drugs. The reaction is the condensation of erythrose-4-phosphate (E4P) and phosphoenolpyruvate (PEP) to yield DAHP. The two purified DAHP synthases showed different affinities for the substrates: 175 microM for PEP and 341 microM for E4P for the aroFp isoenzyme and weaker affinities of 239 microM (PEP) and 475 microM (E4P) for the aroGp isoenzyme. The enzymes are differentially regulated by tyrosine (aroFp) and phenylalanine (aroGp). The calculated kcat values are 7.0 s-1 for the tyrosine-inhibitable (aroFp) and 5.5 s-1 for the phenylalanine inhibitable (aroGp) enzyme. Tyrosine is a competitive inhibitor of the aroFp DAHP synthase in its reaction with PEP. Phenylalanine is a competitive inhibitor of the isoenzyme aroGp in its reaction with E4P. Both enzymes are inhibited by the chelating agent EDTA, which indicates a metal ion as cofactor.

Histidine 268 in 3-deoxy-D-arabino-heptulosonic acid 7-phosphate synthase plays the same role as histidine 202 in 3-deoxy-D-manno-octulosonic acid 8-phosphate synthase

The enzyme 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAH 7-P) synthase (Phe) is inactivated by diethyl pyrocarbonate (DEPC). The inactivation is first order with respect to enzyme and DEPC concentrations with a pseudo-second order rate constant of inactivation by DEPC of 4.9 +/- 0.8 m(-1) s(-1) at pH 6.8 and 4 degrees C. The dependence of inactivation on pH and the spectral features of enzyme modified at specific pH values imply that both histidine and cysteine residues are modified, which is confirmed by site-directed mutagenesis. Analysis of the chemical modification data indicates that one histidine is essential for activity. DAH 7-P synthase (Phe) is protected against DEPC inactivation by phosphoenolpyruvate, whereas d-erythrose 4-phosphate offers only minimal protection. The conserved residues H-172, H-207, H-268, and H-304 were individually mutated to glycine. The H304G and H207G mutants retain some level of activity, whereas the H268G and H172G mutants are virtually inactive. A comparison of the circular dichroism spectra of wild-type enzyme and the various mutants demonstrates that H-172 may play a structural role. Comparison of the UV spectra of the H268G and wild-type enzymes saturated with Cu(2+) indicates that the metal-binding site of the H268G mutant resembles that of the wild-type enzyme. The residue H-268 may play a catalytic role based on the site-directed mutagenesis and spectroscopic studies. Cysteine 61 appears to influence the pK(a) of H-268 in the wild-type enzyme. The pK(a) of H-268 increases from 6.0 to 7.0 following mutation of C-61 to glycine.

Crystallization and preliminary X-ray analysis of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (tyrosine inhibitable) from Saccharomyces cerevisiae

3-Deoxy-D-arabino-heptulosonate-7-phosphate synthase (E.C. 4.1.2.15) catalyses the first step in the biosynthesis of aromatic amino acids: the condensation of phophoenolpyruvate and erythrose 4-phosphate to 3-deoxy-D-arabino-heptulosonate-7-phosphate. Diffraction-quality crystals of the tyrosine-inhibitable form of the enzyme from Saccharomyces cerevisiae have been obtained by the hanging-drop vapour-diffusion method in the presence of polyethylene glycol. The crystals belong to the triclinic space group P1, with unit-cell parameters a = 81.5, b = 94.0, c = 104.6 A, alpha = 65.5, beta = 85.2, gamma = 75.0 degrees, and can be flash-cooled using glycerol as a cryoprotectant. A data set to 2.3 A has been collected at 120 K.

Crystal structure of phenylalanine-regulated 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Escherichia coli

BACKGROUND

In microorganisms and plants the first step in the common pathway leading to the biosynthesis of aromatic compounds is the stereospecific condensation of phosphoenolpyruvate (PEP) and D-erythrose-4-phosphate (E4P) giving rise to 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP). This reaction is catalyzed by DAHP synthase (DAHPS), a metal-activated enzyme, which in microorganisms is the target for negative-feedback regulation by pathway intermediates or by end products. In Escherichia coli there are three DAHPS isoforms, each specifically inhibited by one of the three aromatic amino acids.

RESULTS

The crystal structure of the phenylalanine-regulated form of DAHPS complexed with PEP and Pb2+ (DAHPS(Phe)-PEP-Pb) was determined by multiple wavelength anomalous dispersion phasing utilizing the anomalous scattering of Pb2+. The tetramer consists of two tight dimers. The monomers of the tight dimer are coupled by extensive interactions including a pair of three-stranded, intersubunit beta sheets. The monomer (350 residues) is a (beta/alpha)8 barrel with several additional beta strands and alpha helices. The PEP and Pb2+ are at the C-ends of the beta strands of the barrel, as is SO4(2-), inferred to occupy the position of the phosphate of E4P. Mutations that reduce feedback inhibition cluster about a cavity near the twofold axis of the tight dimer and are centered approximately 15 A from the active site, indicating the location of a separate regulatory site.

CONCLUSIONS

The crystal structure of DAHPS(Phe)-PEP-Pb reveals the active site of this key enzyme of aromatic biosynthesis and indicates the probable site of inhibitor binding. This is the first reported structure of a DAHPS; the structure of its two paralogs and of a variety of orthologs should now be readily determined by molecular replacement.

Steady-state kinetics and inhibitor binding of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (tryptophan sensitive) from Escherichia coli

The tryptophan-inhibited 3-deoxy-d-arabino-heptulosonate-7-phosphate synthase [DAHPS(Trp)] of Escherichia coli was analyzed with respect to steady-state kinetics and tryptophan binding. DAHPS(Trp) is one of three differentially regulated isoforms that catalyze the first step of aromatic biosynthesis, the condensation of phosphoenolpyruvate and erythrose-4-phosphate to form 3-deoxy-D-arabino-heptulosonate-7-phosphate. The DAHP synthase isozymes are metalloproteins, being activated in vitro by a variety of divalent metals. Both catalytic activity and substrate affinity are dependent on the species of activating metal ion. We report here kinetic and binding studies of metal-homogeneous (Mn2+-activated) DAHPS(Trp). The homodimeric enzyme had an apparent kcat of 21 s-1 and displayed sigmoidal kinetics with respect to both substrates. The S0.5 was 35 microM for erythrose-4-phosphate and 5.3 microM for phosphoenolpyruvate. Equilibrium binding studies with radiolabeled tryptophan demonstrated two independent inhibitor binding sites per enzyme dimer, with KdTrp of 1 microM. L-Tryptophan binding decreased kcat, increased affinity for both substrates, decreased positive homotropic cooperativity for both substrates and activated the enzyme at low concentrations of erythrose-4-phosphate. The results suggest an inhibition mechanism analogous to system C5 hyperbolic mixed-type inhibition with respect to erythrose-4-phosphate and partial noncompetitive inhibition with respect to phosphoenolpyruvate.

Overexpression, purification, and characterization of tyrosine-sensitive 3-deoxy-D-arabino-heptulosonic acid 7-phosphate synthase from Escherichia coli

An overexpression system (pCR105) for DAHP synthase (Tyr) was constructed by cloning the aroF gene at the NdeI site of the pET-22b(+) translation vector, a plasmid expression vector that contains the T7 lac promoter. The enzyme was overexpressed, purified to > 90% purity (by SDS-polyacrylamide gel electrophoresis), and characterized. The protein was overexpressed at a level of 58% the total soluble cell protein (based on enzymatic activities). About 244 mg of pure enzyme was obtained from a 2-liter cell culture. So far, this is the highest yield reported for the isozyme DAHP synthase (Tyr). The enzyme showed a bell-shaped pH-activity profile, with a pH optimum at pH 7.0-7.5 and pK values of 6.10 and 8.92. Inhibition of the enzyme by tyrosine was specific with 50% inhibition observed at 9 microM tyrosine, pH 7.0. The specific activity of the enzyme increased with added metal and metal sensitivity increased with purity of the enzyme. Only substoichiometric amounts of Cu, Fe, and Zn were found in the pure enzyme and this result is consistent with sensitivity of the enzyme to added metal. Although treatment with EDTA inactivated the enzyme almost completely, the activity of the apoenzyme was restored to differing extents by a variety of metals including Mn2+, Cd2+, Co2+, Fe2+, Cu2+, Mg2+, and Zn2+. Both Fe2+ and Cu2+ only partially reactivated EDTA-treated enzyme. Reconstitution of EDTA-treated enzyme with either Cd2+ or Mn2+ gave 1 mol of metal per mole of enzyme monomer. KCN inactivated the enzyme to only 80% and added metals reactivated the CN-treated enzyme only to a small extent. These results confirm the importance of the metal in the enzymatic reaction.

A single Ser-180 mutation desensitizes feedback inhibition of the phenylalanine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthetase in Escherichia coli

The Escherichia coli phenylalanine-sensitive DAHP (3-deoxy-D-arabino-heptulosonate 7-phosphate) synthetase (aroG product) is one of the DAHP synthetase isozymes that catalyze the first committed step in the biosynthesis of aromatic amino acids and vitamins. Through target-directed mutagenesis of the cloned aroG on a plasmid vector, followed by screening of phenylalanine-resistant colonies, we isolated a clone (pG908) showing feedback-insensitive mutation of DAHP synthetase. The mutations were identified as a T-->A mutation at nucleotide 22, and a C-->T mutation at nucleotide 539, causing a Leu-8 to Ile-8 mutation and a Ser-180 to Phe-180 substitution, respectively. The resulting enzyme exhibited comparable enzymatic activity to the wild type, but the degree of feedback inhibition had declined from approximately 60% to less than 10% in the presence of 20 mM phenylalanine in the assay medium. Replacement of Ile-8 by Leu, and substitution of Phe-180 with Ser, Asn and Cys, using site-directed mutagenesis, demonstrated that Ser-180 is a critical residue in the feedback inhibition of AroG. The result suggests that the major role played by Ser-180 may not involve a simple charge-size effect in the Phe-binding site of the enzyme molecule, but rather may involve more complicated molecular interactions occurring in the feedback inhibition mechanisms.

Purification and characterization of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Streptomyces rimosus

A purification scheme for 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (EC 4.1.2.15) from Streptomyces rimosus is presented. A four-step procedure involving conventional chromatography and FPLC led to a 2700-fold enrichment of the enzyme activity present in crude extracts. Enzyme prepared by this procedure was judged to be 97% homogeneous by densitometry of a Coomassie-blue-stained polyacrylamide gel. A novel, sensitive, coupled assay was used to follow the purification. The Michaelis constants of the purified enzyme are 6.7 microM and 2.6 microM for phospho enol pyruvate and erythrose 4-phosphate, respectively. These values are one to two orders of magnitude lower than the Km values reported for partially purified enzyme from other Streptomycetes. A number of potential metabolic effectors were tested for their ability to inhibit the purified enzyme. Tryptophan was found to be a partial inhibitor and appeared to bind to the enzyme cooperatively.

Cloning, primary structure and regulation of the ARO4 gene, encoding the tyrosine-inhibited 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Saccharomyces cerevisiae

In Saccharomyces cerevisiae, two differently regulated 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase (DAHPS; EC 4.1.2.15) isoenzymes carry out the first step in the shikimate pathway. Mutations in both genes are necessary to cause aromatic amino acid (aa) auxotrophy, since one isoenzyme alone is sufficient to produce enough DAHP for normal growth of the cells. The phenylalanine-inhibited DAHPS is encoded by the previously isolated and characterized ARO3 gene. Here, we report the cloning and characterization of the ARO4 gene, encoding the second DAHPS, which is inhibited by tyrosine. The aa sequence of the ARO4 gene product reveals 76% similarity to the ARO3-encoded isoenzyme and 66 and 73% to the three DAHPS isoenzymes from Escherichia coli. ARO4 gene expression is regulated by the general control system of aa biosynthesis. As in the case of the ARO3 gene, a single GCN4-recognition element in the promoter is responsible for derepression of the ARO4 gene under aa starvation conditions. However, in contrast to the situation in the isogene, ARO3, GCN4 does not contribute to the basal level of ARO4 transcription under nonderepressing conditions.

Analysis of the metal requirement of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Escherichia coli

The three isozymes of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Escherichia coli were overproduced, purified, and characterized with respect to their requirement for metal cofactor. The isolated isozymes contained 0.2-0.3 mol of iron/mol of enzyme monomer, variable amounts of zinc, and traces of copper. Enzymatic activity of the native enzymes was stimulated 3-4-fold by the addition of Fe2+ ions to the reaction mixture and was eliminated by treatment of the enzymes with EDTA. The chelated enzymes were reactivated by a variety of divalent metal ions, including Ca2+, Cd2+, Co2+, Cu2+, Fe2+, Mn2+, Ni2+, and Zn2+. The specific activities of the reactivated enzymes varied widely with the different metals as follows: Mn2+ greater than Cd2+, Fe2+ greater than Co2+ greater than Ni2+, Cu2+, Zn2+ much greater than Ca2+. Steady state kinetic analysis of the Mn2+, Fe2+, Co2+, and Zn2+ forms of the phenylalanine-sensitive isozyme (DAHPS(Phe)) revealed that metal variation significantly affected the apparent affinity for the substrate, erythrose 4-phosphate, but not for the second substrate, phosphoenolpyruvate, or for the feedback inhibitor, L-phenylalanine. The tetrameric DAHPS(Phe) exhibited positive homotropic cooperativity with respect to erythrose 4-phosphate, phophoenolpyruvate, and phenylalanine in the presence of all metals tested.

[Regulation of the activity and synthesis of 3-deoxy-D-arabinoheptuloso-7-phosphate synthase in the obligate methylotroph Methylobacillus flagellatum KT]

The activity of the first enzyme of aromatic path 3-deoxy-D-arabino-heptuloso-7-phosphate-synthase (DAHP-synthase) is regulated by retro-inhibition and is a subject of repression. Analysis of partially purified preparations of the enzyme has revealed three isoenzymes: DAHP-synthase-Tyr, DAHP-synthase-Trp and DAHP-synthase-Phe, each of them being regulated by a corresponding amino acid. DAHP-synthase-Phe is a dominant isoenzyme presenting 70% of the enzyme activity, 30% inhibition of which is possible by 7.0 mkM of phenylalanine. DAHP-synthase-Tyr and DAHP-synthase-Trp are minor isoenzymes (sharing 15% of enzyme activity each) and are controlled by tyrosine and tryptophane correspondingly. 50% of inhibition of activity is possible by adding 0.7 and 0.8 mkM of corresponding amino acid. Regulation of the enzyme synthesis was studied in the Trp-, Phe- and Tyr- mutants. The enzyme activity was registered under the conditions of limiting and surplus of each aromatic amino acid. The synthesis of DAHP-synthase in M. flagellatum KT is repressed by tryptophane and tyrosine decreasing the synthesis 18.8 and 15.6 fold.

Purification and properties of tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Escherichia coli

The aroH gene of Escherichia coli, which encodes the tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase isoenzyme of the common aromatic biosynthetic pathway, was cloned behind the tac promoter in expression plasmid pKK223-3. The enzyme was overexpressed, purified to homogeneity, and characterized. The native enzyme was found to be a dimeric metalloprotein containing 0.3 mol of iron per mol of subunit and variable amounts of zinc. The activity of the native enzyme was stimulated two- to threefold when assayed in the presence of Fe2+ ions. Pretreatment of the enzyme with Fe2+ also resulted in activation, accompanied by an equivalent increase in iron content. Treatment of the enzyme with chelating agents led to inactivation, which was fully reversed by the presence of Fe2+ in the assay mixture. The native enzyme exhibited a unique absorption profile, having a shoulder of absorbance on the aromatic band with a maximum around 350 nm and a broad, weak band with a maximum around 500 nm. Treatment of the enzyme with Fe2+ enhanced the absorbance at 350 nm and eliminated the band at 500 nm. Treatment with reducing agents caused the disappearance of both bands and destabilized the enzyme. Feedback regulation of the activity of the enzyme was specific for tryptophan, with maximum inhibition at about 70%.

Purification and properties of the 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (phenylalanine-inhibitable) of Saccharomyces cerevisiae

The phenylalanine-inhibitable 3-deoxy-D-arabino-heptulosonate-7-phosphate (dHp1P) synthase from Saccharomyces cerevisiae has been purified to apparent homogeneity by a 1250-fold enrichment of the enzyme activity present in wild-type crude extracts, employing an overproducing strain. The estimated molecular mass of 42 kDa corresponds to the calculated molecular mass of 42.13 kDa deduced from the previously determined primary sequence. Gel filtration indicates that the active enzyme is a monomer. The enzyme is an Fe protein and is inactivated by EDTA in a reaction which is reversible by several bivalent metal ions. The Michaelis constant of the enzyme is 18 microM for phosphoenolpyruvate (P-pyruvate) and 130 microM for erythrose 4-phosphate (Ery4P) and the rate constant was calculated as 10 s-1. Inhibition by phenylalanine is competitive with respect to erythrose 4-phosphate and non-competitive to phosphoenolpyruvate, with a Ki of 10 microM.

The recent evolutionary origin of the phenylalanine-sensitive isozyme of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase in the enteric lineage of bacteria

Evolutionary events that generated the three regulatory isozymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase present in contemporary strains of Escherichia coli have been proposed recently [Ahmad et al. (1986) J Bacteriol 165:146-154]. The phylogenetic subdivision of gram-negative prokaryotes studied (Superfamily B) includes enteric bacteria, an Oceanospirillum cluster, pseudomonad Group I (e.g., Pseudomonas aeruginosa), pseudomonad Group V (e.g., Xanthomonas), and the Acinetobacter grouping. DAHP synthase-phe, a regulatory isozyme subject to allosteric control by L-phenylalanine, was the last member of the isozyme family to evolve. Thus, DAHP synthase-phe is absent throughout Superfamily B except within the enteric lineage. Bacteria that make up the enteric lineage (Escherichia, Klebsiella, Erwinia, Serratia, Proteus, Aeromonas, and Alteromonas) were examined in detail; DAHP synthase-phe was present in each of these organisms. Therefore, the isozyme originated between the separation of the enteric and Oceanospirillum lineages, prior to the divergence of Alteromonas putrefaciens (44% homology with E. coli by DNA:rRNA hybridization) from the rest of the enteric lineage. DAHP synthase-tyr and DAHP synthase-trp were uniformly present within the enteric lineage, although it was often necessary to derepress DAHP synthase-trp by physiological manipulation in order to demonstrate its presence.

[Repression of the synthesis and allosteric inhibition of 3-deoxy-D-arabinoheptulosonate-7-phosphate synthase in facultative methylotrophic bacterium Pseudomonas sp. M]

The synthesis of 3-deoxy-D-arabinoheptulosonate-7-phosphate-synthase has been shown to be repressed by tyrosine and phenylalanine in the cells of facultative methylotrophic bacteria Pseudomonas sp. M. Activity of the enzyme is subjected to allosteric inhibition by tyrosine, tryptophane, anthranylate and phenylpyruvate.

A pair of regulatory isozymes for 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase is conserved within group I pseudomonads

Two closely related subgroups of group I pseudomonads, which differ from one another in the overall enzymatic makeup of aromatic amino acid biosynthesis, possess in common the recently characterized major (tyrosine-sensitive) and minor (tryptophan-sensitive) isozymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase of Pseudomonas aeruginosa (17). Since these characterizations were made for strains whose phylogenetic positions have been determined by oligonucleotide cataloging, an initial perception of the evolution of aromatic pathway construction and regulation is emerging.

Regulation of the aromatic pathway in the cyanobacterium Synechococcus sp. strain Pcc6301 (Anacystis nidulans)

A pattern of allosteric control for aromatic biosynthesis in cyanobacteria relies upon early-pathway regulation as the major control point for the entire branched pathway. In Synechococcus sp. strain PCC6301 (Anacystis nidulans), two enzymes which form precursors for L-phenylalanine biosynthesis are subject to control by feedback inhibition. 3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase (first pathway enzyme) is feedback inhibited by L-tyrosine, whereas prephenate dehydratase (enzyme step 9) is feedback inhibited by L-phenylalanine and allosterically activated by L-tyrosine. Mutants lacking feedback inhibition of prephenate dehydratase excreted relatively modest quantities of L-phenylalanine. In contrast, mutants deregulated in allosteric control of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase excreted large quantities of L-phenylalanine (in addition to even greater quantities of L-tyrosine). Clearly, in the latter mutants, the elevated levels of prephenate must overwhelm the inhibition of prephenate dehydratase by L-phenylalanine, an effect assisted by increased intracellular L-tyrosine, an allosteric activator. The results show that early-pathway flow regulated in vivo by 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase is the dominating influence upon metabolite flow-through to L-phenylalanine. L-Tyrosine biosynthesis exemplifies such early-pathway control even more simply, since 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase is the sole regulatory enzyme subject to end-product control by L-tyrosine.

Some kinetic properties of the tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase from Neurospora crassa

The steady-state kinetic properties of purified tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase from Neurospora crassa were examined. The results suggest that the enzyme obeys a Rapid-Equilibrium Ordered mechanism, in which phosphoenolpyruvate is the first substrate to bind and 3-deoxy-D-arabino-heptulosonate 7-phosphate is the second product to be released, rather than a Ping Pong mechanism as has been reported previously. The inhibition by tryptophan was found to be parabolic competitive with respect to D-erythrose 4-phosphate and parabolic non-competitive with respect to phosphoenolpyruvate. The enzyme was inactivated by EDTA, and could be protected against this inactivation by phosphoenolpyruvate or 3-deoxy-D-arabino-heptulosonate 7-phosphate but not by D-erythrose 4-phosphate, tryptophan or Pi. This suggests that the enzyme may be a metalloenzyme.

The purification and molecular properties of the tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase from Neurospora crassa

Neurospora crassa contains three isoenzymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase, which are inhibited by tyrosine, tryptophan and phenylalanine respectively, and it was estimated that the relative proportions of the total activity were 54%, 14% and 32% respectively. The tryptophan-sensitive isoenzyme was purified to homogeneity as judged by polyacrylamide-gel electrophoresis and ultracentrifugation. The tyrosine-sensitive and phenylalanine-sensitive isoenzymes were only partially purified. The three isoenzymes were completely separated from each other, however, and can be distinguished by (NH4)2SO4 fractionation, chromatography on DEAE-cellulose and Ultrogel AcA-34 and polyacrylamide-gel electrophoresis. Polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate indicated that the tryptophan-sensitive isoenzyme contained one type of subunit of molecular weight 52000. The molecular weight of the native enzyme was found to be 200000 by sedimentation-equilibrium centrifugation, indicating that the enzyme is a tetramer, and the results of cross-linking and gel-filtration studies were in agreement with this conclusion.

Purification of the tyrosine inhibitable 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Schizosaccharomyces pombe

A method is described for the purification of the tyrosine inhibitable isoenzyme 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (7-phospho-2-keto-3-deoxy-D-arabino-heptonate D-erythrose-4-phosphate-lyase(pyruvate phosphorylating), EC 4.1.2.15) to homogeneity as judged by polyacrylamide gel electrophoresis.

Properties of tyrosine-inhibitable 3-deoxy-d-arabinoheptulosonic acid-7-phosphate synthase from Salmonella

Tyrosine-inhibitable 3-deoxy-D-arabinoheptulosonic acid-7-phosphate (DAHP) synthase was purified to homogeneity without significant loss of sensitivity to inhibition by tyrosine from an operator-constitutive strain (tyrOc) of Salmonella. The enzyme had an apparent molecular weight of 76,000 by gel filtration and a subunit molecular weight of 40,000 by sodium dodecyl sulfate-gel electrophoresis and by reaction with dimethyl suberimidate. It had an isoelectric point of 4.68. Inhibition by L-tyrosine showed a Hill coefficient of 1.8 at pH 7.0, suggesting cooperative interaction between tyrosine-binding sites, and was competitive with phosphoenol pyruvate and noncompetitive with erythrose-4-phosphate.

A nonspecific inhibitory effect of tRNA on the activity of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Saccharomyces cerevisiae

The inhibitory effect of tRNA on yeast 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase (EC 4.1.2.15) has been reinvestigated. From earlier studies the inhibition by tRNAPhe appeared to be quite specific. This study shows that tRNAPhe is indeed a potent inhibitor but so is unfractionated tRNA, as well as ribosomal RNA and heparin. Complete digestion to mononucleotides relieves the inhibition. Since the enzyme requires a metal ion (Co2+) we suggest that the RNA and heparin are inhibitory by virtue of their capacity to chelate the Co2+.

Aromatic amino acid biosynthesis in Alcaligenes eutrophus H16. I. Properties and regulation of 3-deoxy-d-arabino heptulosonate 7-phosphate synthase

Properties and regulation of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHP-synthase), EC4.1.2.15, from Alcaligenes eutrophus H16 were investigated. DAHP synthase was unstable during manipulations such as dialysis, dilution, ammonium sulfate fractionation, chromatography on DEAE-cellulose or Sephadex G-200. For kinetic measurements Sephadex G-25 treated crude extracts were used. The enzyme was not affected by thiol reagents, EDTA or divalent metal ions. The activation energy, deltaH, amounted to 16100 cal/mole. Between pH 7.2 and pH 8.2 there was little change of enzyme activity. The Km-values for the two substrates were found to be 0.043 mM phosphoenolpyruvate and 0.055 mM erythrose-4-phosphate. DAHP-synthase was inhibited by 0.5 mM phenylalanine for 60% and by 0.5 mM tyrosine for 20%. In the presence of both amino acids cumulative inhibition occurred amounting to about 70%. No other amino acid exerted inhibitory effects. A repression of DAHP-synthase by the aromatic amino acids was not observed. Some other strains of hydrogen bacteria were included in this study. The DAHP synthase from strain 12/60/X and Corynebacterium autotrophicum 7C was unregulated. The enzyme from strain 33/X was subject to retro-tyrosine inhibition and from strain 3/2, H1 and H20 were subject to cumulative inhibition.