Synthesis and evaluation of 2,5-dihydrochorismate analogues as inhibitors of the chorismate-utilising enzymes.

A library of 2,5-dihydrochorismate analogues were designed as inhibitors of the chorismate-utilising enzymes including anthranilate synthase, isochorismate synthase, salicylate synthase and 4-amino-4-deoxychorismate synthase. The inhibitors were synthesised in seven or eight steps from shikimic acid, sourced from star anise. The compounds exhibited moderate but differential inhibition against the four chorismate-utilising enzymes.

Design and synthesis of aromatic inhibitors of anthranilate synthase.

Anthranilate synthase catalyses the conversion of chorismate to anthranilate, a key step in tryptophan biosynthesis. A series of 3-(1-carboxy-ethoxy) benzoic acids were synthesised as chorismate analogues, with varying functionality at C-4, the position of the departing hydroxyl group in chorismate. Most of the compounds were moderate inhibitors of anthranilate synthase, with inhibition constants between 20-30 microM. The exception was 3-(1-carboxy-ethoxy) benzoic acid, (C-4 = H), for which K(I)= 2.4 microM. These results suggest that a hydrogen bonding interaction with the active site general acid (Glu309) is less important than previously assumed for inhibition of the enzyme by these aromatic chorismate analogues.

Salicylate biosynthesis: overexpression, purification, and characterization of Irp9, a bifunctional salicylate synthase from Yersinia enterocolitica.

In some bacteria, salicylate is synthesized using the enzymes isochorismate synthase and isochorismate pyruvate lyase. In contrast, gene inactivation and complementation experiments with Yersinia enterocolitica suggest the synthesis of salicylate in the biosynthesis of the siderophore yersiniabactin involves a single protein, Irp9, which converts chorismate directly into salicylate. In the present study, Irp9 was for the first time heterologously expressed in Escherichia coli as a hexahistidine fusion protein, purified to near homogeneity, and characterized biochemically. The recombinant protein was found to be a dimer, each subunit of which has a molecular mass of 50 kDa. Enzyme assays, reverse-phase high-pressure liquid chromatography and 1H nuclear magnetic resonance (NMR) spectroscopic analyses confirmed that Irp9 is a salicylate synthase and converts chorismate to salicylate with a K(m) for chorismate of 4.2 microM and a k(cat) of 8 min(-1). The reaction was shown to proceed through the intermediate isochorismate, which was detected directly using 1H NMR spectroscopy.

Identification of 4-amino-4-deoxychorismate synthase as the molecular target for the antimicrobial action of (6s)-6-fluoroshikimate.

(6S)-6-Fluoroshikimate has antimicrobial activity. The molecular basis of this effect had not been identified, but there was speculation that (6S)-6-fluoroshikimate is first converted in vivo into 2-fluorochorismate, which then could inhibit 4-amino-4-deoxychorismate synthase (ADCS). 2-Fluorochorismate was prepared from E-fluorophosphoenolpyruvate and erythose-4-phosphate by the sequential reactions of DAHP synthase, dehydroquinate synthase, dehydroquinase, shikimate dehydrogenase, EPSP synthase, and chorismate synthase. Inhibition studies on ADCS showed that it was inhibited rapidly and irreversibly by 2-fluorochorismate. Electrospray mass spectrometry of the inactivated enzyme showed an additional mass of 198 +/- 10 Da. A novel peptide of 1087.6 Da was identified in the HPLC trace for the tryptic digest of 2-fluorochorismate-inactivated ADCS. Sequencing of this peptide by MS/MS showed that the peptide corresponded to residues 272-279 with a modification of 206.1 Da on Lys-274. This observation is particularly exciting in the context of a recent proposal for the catalytic mechanism of ADCS.

(S,S)-2,3-Dihydroxy-2,3-dihydrobenzoic acid: microbial access with engineered cells of Escherichia coli and application as starting material in natural-product synthesis.

Cyclohexadiene-trans-5,6-diols such as (S,S)-2,3-dihydroxy-2,3-dihydrobenzoic acid (2,3-trans-CHD) have been shown to be of importance as chiral starting materials for the syntheses of bioactive substances, especially for the syntheses of carbasugars. By using methods of metabolic-pathway engineering, the Escherichia coli genes entB and entC, which encode isochorismatase and isochorismate synthase, were cloned and over-expressed in E. coli strains with a deficiency of entA, which encodes 2,3-dihydroxybenzoate synthase. A 30-fold increase in the corresponding EntB/EntC enzyme activities affects the accumulation of 2,3-trans-CHD in the cultivation medium. Although the strains did not contain deletions in chorismate-utilising pathways towards aromatic amino acids, neither chorismate nor any other metabolic intermediates were found as by-products. Fermentation of these strains in a 30 L pH-controlled stirred tank reactor showed that 2,3-trans-CHD could be obtained in concentrations of up to 4.6 g L(-1). This demonstrates that post-chorismate metabolites are accessible on a preparative scale by using techniques of metabolic-pathway engineering. Isolation and separation from fermentation salts could be performed economically in one step through anion-exchange chromatography or, alternatively, by reactive extraction. Starting from 2,3-trans-CHD as an example, we established short syntheses towards new carbasugar derivatives.

Investigation of the enzymatic mechanism of the yeast chorismate mutase by docking a transition state analog.

The structure of the complex of the chorismate mutase from the yeast Saccharomyces cerevisiae with a transition state analog is constructed using a suite of docking tools. The construction finds the best location for the active site in the enzyme, and the best orientation of the analog compound in the active site. The resulting complex shows extensive salt links and hydrogen bonds between the enzyme and the compound, including those mediated by water molecules. A network of polar interactions between amino acid residues is found to solidify the active site of the enzyme. The enzymatic mechanism suggested for a bacterial chorismate mutase, that the active site is by design capable of selecting an active conformer of the substrate, and of stabilizing the transition state, is apparently intact in the yeast enzyme. No direct evidence is found to support an alternative mechanism which involves specific catalytic groups, although the possibility is not eliminated. This finding reinforces the notion of a function being evolutionarily conserved via a common mechanism, rather than via sequential or structural homology.

Mechanisms of catalysis and allosteric regulation of yeast chorismate mutase from crystal structures.

BACKGROUND: Chorismate mutase (CM) catalyzes the Claisen rearrangement of chorismate to prephenate, notably the only known enzymatically catalyzed pericyclic reaction in primary metabolism. Structures of the enzyme in complex with an endo-oxabicyclic transition state analogue inhibitor, previously determined for Bacillus subtilis and Escherichia coli CM, provide structural insight into the enzyme mechanism. In contrast to these bacterial CMs, yeast CM is allosterically regulated in two ways: activation by tryptophan and inhibition by tyrosine. Yeast CM exists in two allosteric states, R (active) and t (inactive). RESULTS: We have determined crystal structures of wild-type yeast CM cocrystallized with tryptophan and an endo-oxabicyclic transition state analogue inhibitor, of wild-type yeast CM co-crystallized with tyrosine and the endo-oxabicyclic transition state analogue inhibitor and of the Thr226-->Ser mutant of yeast CM in complex with tryptophan. Binding of the transition state analogue inhibitor to CM keeps the enzyme in a 'super R' state, even if the inhibitory effector tyrosine is bound to the regulatory site. CONCLUSIONS: The endo-oxabicyclic inhibitor binds to yeast CM in a similar way as it does to the distantly related CM from E. coli. The inhibitor-binding mode supports a mechanism by which polar sidechains of the enzyme bind the substrate in the pseudo-diaxial conformation, which is required for catalytic turnover. A lysine and a protonated glutamate sidechain have a critical role in the stabilization of the transition state of the pericyclic reaction. The allosteric transition from T-->R state is accompanied by a 15 degrees rotation of one of the two subunits relative to the other (where 0 degrees rotation defines the T state). This rotation causes conformational changes at the dimer interface which are transmitted to the active site. An allosteric pathway is proposed to include residues Phe28, Asp24 and Glu23, which move toward the activesite cavity in the T state. In the presence of the transition-state analogue a super R state is formed, which is characterised by a 22 degrees rotation of one subunit relative to the other.

Escherichia coli chorismate synthase catalyzes the conversion of (6S)-6-fluoro-5-enolpyruvylshikimate-3-phosphate to 6-fluorochorismate. Implications for the enzyme mechanism and the antimicrobial action of (6S)-6-fluoroshikimate.

Chorismate synthase catalyzes the conversion of 5-enolpyruvylshikimate-3-phosphate to chorismate. It is the seventh enzyme of the shikimate pathway, which is responsible for the biosynthesis of aromatic metabolites from glucose. The chorismate synthase reaction involves a 1,4-elimination with unusual anti-stereochemistry and requires a reduced flavin cofactor. The substrate analogue (6S)-6-fluoro-5-enolpyruvylshikimate-3-phosphate is a competitive inhibitor of Neurospora crassa chorismate synthase (Balasubramanian, S., Davies, G. M., Coggins, J. R., and Abell, C. (1991) J. Am. Chem. Soc. 113, 8945-8946). We have shown that this analogue is converted to 6-fluorochorismate by Escherichia coli chorismate synthase at a rate 2 orders of magnitude slower than the normal substrate. The decreased rate of reaction is consistent with the destabilization of an allylic cationic intermediate. The formation of chorismate and 6-fluorochorismate involves a common protein-bound flavin intermediate although the fluoro substituent does influence the spectral characteristics of this intermediate. The fluoro substituent also decreased the rate of decay of the flavin intermediate by 280 times. These results are consistent with the antimicrobial activity of (6S)-6-fluoroshikimate not being mediated by the inhibition of chorismate synthase but by the inhibition of 4-aminobenzoic acid synthesis as previously proposed (Davies, G. M., Barrett-Bee, K. J., Jude, D. A., Lehan, M., Nichols, W. W., Pinder, P. E., Thain, J. L., Watkins, W. J., and Wilson, R. G. (1994) Antimicrobial Agents and Chemotherapy 38, 403-406).

The monofunctional chorismate mutase from Bacillus subtilis. Structure determination of chorismate mutase and its complexes with a transition state analog and prephenate, and implications for the mechanism of the enzymatic reaction.

Structures have been determined for chorismate mutase from Bacillus subtilis and of complexes of this enzyme with product and an endo-oxabicyclic transition state analog using multiple isomorphous replacement plus partial structure phase combination and non-crystallographic averaging. In addition to 522 water molecules, the model includes 1380 of the 1524 amino acid residues of the four trimers (each containing 3 x 127 amino acid residues) in the asymmetric unit. Refinement to 1.9 A resolution yields 0.194 for R and r.m.s. deviations from ideal values of 0.014 A for bond lengths and 2.92 degrees for bond angles. The trimer resembles a beta-barrel structure in which a core beta-sheet is surrounded by helices. The structures of the two complexes locate the active sites which are at the interfaces of adjacent pairs of monomers in the trimer. These structures have been refined at 2.2 A to a crystallographic R value of 0.18 and show r.m.s. deviations from ideal values of 0.013 A for bond lengths and 2.84 degrees or 3.05 degrees for bond angles, respectively. The final models have 1398 amino acid residues, nine prephenate molecules and 503 water molecules in the product complex, and 1403 amino acid residues, 12 inhibitor molecules and 530 water molecules in the transition state complex. The active sites of all three of these structures are very similar and provide a structural basis for the biochemical studies that indicate a pericyclic mechanism for conversion of chorismate to prephenate. The absence of reactive catalytic residues on the enzyme, the selective binding of the single reactive conformation of chorismate, the stabilization of the polar transition state, and the possible role of the C-terminal region in "capping" the active site are factors which relate these structures to the million-fold rate enhancement of this reaction.

Chorismate aminations: partial purification of Escherichia coli PABA synthase and mechanistic comparison with anthranilate synthase.

Chorismate is converted by regiospecific amination/aromatization sequences to o-aminobenzoate and p-aminobenzoate (PABA) by anthranilate synthase (AS) and PABA synthase (PABS), respectively. We report here the first partial purification of the large subunit of Escherichia coli PABA synthase, previously reported to be quantitatively inactivated in purification attempts. The subunit encoded by the pabB gene was overexpressed from a T7 promoter and purified 9-fold to 25-30% homogeneity. The pabB subunit appears unusually sensitive to inactivation by glycerol so this cosolvent is contraindicated. The Km for chorismate is 42 microM in the ammonia-dependent conversion to PABA, and we estimate a turnover number of 2.6 min-1. A variety of chorismate analogues have been prepared and examined. Of these compounds, cycloheptadienyl analogue 11 has been found to be the most potent inhibitor of Serratia marcescens anthranilate synthase (Ki = 30 microM for an RS mixture) and of the E. coli pabB subunit of PABA synthase (Ki = 226 microM). Modifications in the substituents at C-3 [enolpyruyl ether, (R)- or (S)-lactyl ether, glycolyl ether] or C-4 (O-methyl) of chorismate lead to alternate substrates. The Vmax values for (R)- and (S)-lactyl ethers are down 10-20-fold for each enzyme, and V/K analyses show the (S)-lactyl chorismate analogue to be preferred by 12/1 over (R)-lactyl for anthranilate synthase while a 3/1 preference was observed for (R)-/(S)-lactyl analogues by PABA synthase. The glycolyl ether analogue of chorismate shows 15% Vmax vs. chorismate for anthranilate synthase but is actually a faster substrate (140%) than chorismate with PABA synthase, suggesting the elimination/aromatization step from an aminocyclohexadienyl species may be rate limiting with AS but not with PABS. Indeed, studies with (R)-lactyl analogue 14 and anthranilate synthase led to accumulation of an intermediate, isolable by high-performance liquid chromatography and characterized by NMR and UV-visible spectroscopy as 6-amino-5-[(1-carboxyethyl)oxy]-1,3-cyclohexadiene-1-carboxylic acid (17). This is the anticipated intermediate predicted by our previous work with conversion of synthetic trans-6-amino-5-[(1-carboxyethenyl)oxy]-1,3-cyclohexadiene-1-carbo xylic acid (2) to anthranilate by the enzyme. Compound 17 is quantitatively converted to anthranilate on reincubation with enzyme, but at a 1.3-10-fold lower Vmax than starting lactyl substrate 14 under the conditions investigated; the basis for this kinetic variation is not yet determined.

Chorismate mutase-prephenate dehydrogenase from Escherichia coli: positive cooperativity with substrates and inhibitors.

Investigations have been made at pH 6.0 of the effect of chorismate and adamantane derivatives on the mutase and dehydrogenase activities of hydroxyphenylpyruvate synthase from Escherichia coli. When used over a wide range of concentrations, chorismate 5,6-epoxide, chorismate 5,6-diol, adamantane-1,3-diacetate, adamantane-1-acetate, adamantane-1-carboxylate, and adamantane-1-phosphonate give rise to nonlinear plots of the reciprocal of the initial velocity of each reaction as a function of the inhibitor concentration. The inhibitors do not induce the enzyme to undergo polymerization and have only a small effect on the S20,w value of the enzyme as determined by using sucrose density gradient centrifugation. At low substrate concentration, low concentrations of adamantane-1-acetate cause activation of both the mutase and dehydrogenase activities while at higher concentrations this compound functions as an inhibitor. When chorismate and prephenate are varied over a wide range of concentrations, double-reciprocal plots of the data indicate that the reactions exhibit positive cooperativity. The addition of albumin eliminates the cooperative interactions associated with substrates but has little effect on those associated with inhibitors.