Breeding with rare defective alleles (BRDA): a natural Populus nigra HCT mutant with modified lignin as a case study

Next-generation (NG) sequencing in a natural population of Populus nigra revealed a mutant with a premature stop codon in the gene encoding hydroxycinnamoyl-CoA : shikimate hydroxycinnamoyl transferase1 (HCT1), an essential enzyme in lignin biosynthesis. The lignin composition of P. nigra trees homozygous for the defective allele was compared with that of heterozygous trees and trees without the defective allele. The lignin was characterized by phenolic profiling, lignin oligomer sequencing, thioacidolysis and NMR. In addition, HCT1 was heterologously expressed for activity assays and crosses were made to introduce the mutation in different genetic backgrounds. HCT1 converts p-coumaroyl-CoA into p-coumaroyl shikimate. The mutant allele, PnHCT1-Δ73, encodes a truncated protein, and trees homozygous for this recessive allele have a modified lignin composition characterized by a 17-fold increase in p-hydroxyphenyl units. Using the lignin pathway as proof of concept, we illustrated that the capture of rare defective alleles is a straightforward approach to initiate reverse genetics and accelerate tree breeding. The proposed breeding strategy, called 'breeding with rare defective alleles' (BRDA), should be widely applicable, independent of the target gene or the species.

Construction and assessment of reaction models of Class I EPSP synthase. Part II: investigation of the EPSP ketal

Although the proposed mechanisms are reasonable, there are still many questions about the 5-enolpyruvyl shikimate-3-phosphate (EPSP) synthase mechanism that are difficult to answer by experimental means alone. EPSP synthase is a key enzyme in the shikimic acid pathway, which is found only in plants and some micro-organisms and is also molecular target of glyphosate, active component of one of the top-selling herbicides. In the study of reaction mechanism of EPSP synthase, in addition to inorganic phosphate and EPSP products, after long time at equilibrium, it was shown that a side product is formed, the EPSP ketal. In this line, studies using density functional theory (DFT) techniques were performed to investigate the reaction mechanism of formation of EPSP and the corresponding ketal. Our findings indicate some key amino acid residues in the EPSP synthase mechanism and a possible route for the formation of the EPSP ketal.

Chemical constituents comparison between Rhizoma Smilacis Glabrae and Rhizoma Smilacis Chinae by HPLC-DAD-MS/MS

Rhizoma Smilacis Glabrae (RSG) and Rhizoma Smilacis Chinae (RSC) are two herbal materials that belong to the same genera and are both listed in the Chinese Pharmacopoeia. Chemical constituents in the two species were compared by HPLC-DAD-MS/MS. Many common constituents were found in both species, including shikimic acid, 5-O-caffeoylshikimic acid, trans-resveratrol, taxifolin, astilbin and its three stereoisomers, engeletin and isoengeletin. However, syringic acid was found only in RSG, while chlorogenic acid was found only in RSC.

Development of HPLC fingerprint for species differentiation and quality assessment of Rhizoma Smilacis Glabrae

Rhizoma Smilacis Glabrae (RSG) is a commonly used herbal material in functional food and Traditional Chinese Medicine. A HPLC chromatographic fingerprint was developed for its quality control and species differentiation. Nine peaks were found in the chromatogram of RSG and all these peaks were identified by diode array detection and electrospray ionization-MS/MS: 5-O-caffeoylshikimic acid, taxifolin, engeletin, isoengeletin, trans-resveratrol, astilbin and its three stereoisomers. Six of these constituents were consistently found in 18 batches of samples. The standard fingerprint of RSG was generated by mean simulation of all tested samples. Using the standard fingerprint, RSG could be easily differentiated from Rhizoma Smilacis Chinae and Rhizoma Heterosmilacis, the two species that can be confused with RSG.

Stereostructure reassignment and determination of the absolute configuration of pericosine D(o) by a synthetic approach

A combination of chemical synthesis and NMR methods was used to reassign the structure of pericosine D(o) (8), a cytotoxic marine natural product produced by the fungus Periconia byssoides OUPS-N133 that was originally derived from the sea hare Aplysia kurodai. Chemical synthesis was used to prepare pericoisne D(o) (8) from a known chlorohydrin that was in turn derived from (-)-quinic acid. The absolute configuration of natural pericosine D(o) (8) was determined to be methyl (3R,4S,5S,6S)-6-chloro-3,4,5-trihydroxy-1-cyclohexene-1-carboxylate. HPLC analyses using a chiral-phase column indicated that pericosine D(o) (8) exists in an enantiomerically pure form in nature.

Mechanism of gallic acid biosynthesis in bacteria (Escherichia coli) and walnut (Juglans regia)

Gallic acid (GA), a key intermediate in the synthesis of plant hydrolysable tannins, is also a primary anti-inflammatory, cardio-protective agent found in wine, tea, and cocoa. In this publication, we reveal the identity of a gene and encoded protein essential for GA synthesis. Although it has long been recognized that plants, bacteria, and fungi synthesize and accumulate GA, the pathway leading to its synthesis was largely unknown. Here we provide evidence that shikimate dehydrogenase (SDH), a shikimate pathway enzyme essential for aromatic amino acid synthesis, is also required for GA production. Escherichia coli (E. coli) aroE mutants lacking a functional SDH can be complemented with the plant enzyme such that they grew on media lacking aromatic amino acids and produced GA in vitro. Transgenic Nicotiana tabacum lines expressing a Juglans regia SDH exhibited a 500% increase in GA accumulation. The J. regia and E. coli SDH was purified via overexpression in E. coli and used to measure substrate and cofactor kinetics, following reduction of NADP(+) to NADPH. Reversed-phase liquid chromatography coupled to electrospray mass spectrometry (RP-LC/ESI-MS) was used to quantify and validate GA production through dehydrogenation of 3-dehydroshikimate (3-DHS) by purified E. coli and J. regia SDH when shikimic acid (SA) or 3-DHS were used as substrates and NADP(+) as cofactor. Finally, we show that purified E. coli and J. regia SDH produced GA in vitro.

Red clover coumarate 3'-hydroxylase (CYP98A44) is capable of hydroxylating p-coumaroyl-shikimate but not p-coumaroyl-malate: implications for the biosynthesis of phaselic acid

Red clover (Trifolium pratense) leaves accumulate several mumol of phaselic acid [2-O-caffeoyl-L-malate] per gram fresh weight. Post-harvest oxidation of such o-diphenols to o-quinones by endogenous polyphenol oxidases (PPO) prevents breakdown of forage protein during storage. Forages like alfalfa (Medicago sativa) lack both foliar PPO activity and o-diphenols. Consequently, breakdown of their protein upon harvest and storage results in economic losses and release of excess nitrogen into the environment. Understanding how red clover synthesizes o-diphenols such as phaselic acid will help in the development of forages utilizing this natural system of protein protection. We have proposed biosynthetic pathways in red clover for phaselic acid that involve a specific hydroxycinnamoyl-CoA:malate hydroxycinnamoyl transferase. It is unclear whether the transfer reaction to malate to form phaselic acid involves caffeic acid or p-coumaric acid and subsequent hydroxylation of the resulting p-coumaroyl-malate. The latter would require a coumarate 3'-hydroxylase (C3'H) capable of hydroxylating p-coumaroyl-malate, an activity not previously described. Here, a cytochrome P450 C3'H (CYP98A44) was identified and its gene cloned from red clover. CYP98A44 shares 96 and 79% amino acid identity with Medicago truncatula and Arabidopsis thaliana C3'H proteins that are capable of hydroxylating p-coumaroyl-shikimate and have been implicated in monolignol biosynthesis. CYP98A44 mRNA is expressed in stems and flowers and to a lesser extent in leaves. Immune serum raised against CYP98A44 recognizes a membrane-associated protein in red clover stems and leaves and cross-reacts with C3'H proteins from other species. CYP98A44 expressed in Saccharomyces cerevisiae is capable of hydroxylating p-coumaroyl-shikimate, but not p-coumaroyl-malate. This finding indicates that in red clover, phaselic acid is likely formed by transfer of a caffeoyl moiety to malic acid, although the existence of a second C3'H capable of hydroxylating p-coumaroyl-malate cannot be definitively ruled out.

A novel RPMXR motif among class II 5-enolpyruvylshikimate-3-phosphate synthases is required for enzymatic activity and glyphosate resistance

The shikimate pathway enzyme 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase is an attractive target for drugs and herbicides. Here we identified a novel RPMXR motif that is strictly conserved among class II EPSP synthases. Site-directed mutational analysis of this motif showed that substitutions of the four strictly conserved amino acid residues, Arg127, Pro128, Met129, and Arg131, resulted in complete loss of enzymatic activity, whereas changes in the non-conserved Asn130 residue strongly influenced glyphosate resistance (all numbering according to Pseudomonas stutzeri A1501 EPSP synthase). These experimental results, combined with 3D structure modeling of the location and interaction of the RPMXR motif with phosphoenolpyruvate (PEP) and shikimate-3-phosphate (S3P), demonstrate that the novel motif is required for enzymatic activity and glyphosate resistance of class II EPSP synthases.

Prenylated C6-C3 compounds from the fruits of Illicium simonsii

Two new prenylated C(6)-C(3) compounds, 4-epi-illicinone E-12-shikimate (1) and 3-hydroxyillifunone B (2), together with five known prenylated C(6)-C(3) compounds (3-7), were isolated from the fruits of Illicium simonsii. Their structures were elucidated on the basis of extensive spectroscopic methods, including 1D and 2D NMR, CD spectra, and ESI-MS analysis.

Evolution of rosmarinic acid biosynthesis

Rosmarinic acid and chlorogenic acid are caffeic acid esters widely found in the plant kingdom and presumably accumulated as defense compounds. In a survey, more than 240 plant species have been screened for the presence of rosmarinic and chlorogenic acids. Several rosmarinic acid-containing species have been detected. The rosmarinic acid accumulation in species of the Marantaceae has not been known before. Rosmarinic acid is found in hornworts, in the fern family Blechnaceae and in species of several orders of mono- and dicotyledonous angiosperms. The biosyntheses of caffeoylshikimate, chlorogenic acid and rosmarinic acid use 4-coumaroyl-CoA from the general phenylpropanoid pathway as hydroxycinnamoyl donor. The hydroxycinnamoyl acceptor substrate comes from the shikimate pathway: shikimic acid, quinic acid and hydroxyphenyllactic acid derived from l-tyrosine. Similar steps are involved in the biosyntheses of rosmarinic, chlorogenic and caffeoylshikimic acids: the transfer of the 4-coumaroyl moiety to an acceptor molecule by a hydroxycinnamoyltransferase from the BAHD acyltransferase family and the meta-hydroxylation of the 4-coumaroyl moiety in the ester by a cytochrome P450 monooxygenase from the CYP98A family. The hydroxycinnamoyltransferases as well as the meta-hydroxylases show high sequence similarities and thus seem to be closely related. The hydroxycinnamoyltransferase and CYP98A14 from Coleus blumei (Lamiaceae) are nevertheless specific for substrates involved in RA biosynthesis showing an evolutionary diversification in phenolic ester metabolism. Our current view is that only a few enzymes had to be "invented" for rosmarinic acid biosynthesis probably on the basis of genes needed for the formation of chlorogenic and caffeoylshikimic acid while further biosynthetic steps might have been recruited from phenylpropanoid metabolism, tocopherol/plastoquinone biosynthesis and photorespiration.

Engineering and characterization of the isolated C-terminal domain of 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase

5-Enolpyruvylshikimate-3-phosphate (EPSP) synthase catalyzes the formation of EPSP and inorganic phosphate from shikimate-3-phosphate (S3P) and phosphoenolpyruvate (PEP) in the biosynthesis of aromatic amino acids. To delineate the domain-specific function, we successfully isolated the discontinuous C-terminal domain (residues 1-21, linkers, 240-427) of EPSP synthase (427 residues) by site-directed mutagenesis. The engineered C-terminal domains containing no linker (CTD), or with gly-gly (CTD(GG)) and gly-ser-ser-gly (CTD(GSSG)) linkers were purified and characterized as having distinct native-like secondary and tertiary structures. However, isothermal titration calorimetry (ITC), 15N-HSQC, and 31P-NMR revealed that neither its substrate nor inhibitor binds the isolated domain. The isolated domain maintained structural integrity, but did not function as the half of the full-length protein.

First Total Synthesis of Antitumor Natural Product (+)- and (−)-Pericosine A: Determination of Absolute Stereo Structure†

The first total synthesis of (+)- and (-)-pericosine A has been achieved, enabling the revision and determination of the absolute configuration of this antitumor natural product as methyl (3S,4S,5S,6S)-6-chloro-3,4,5-trihydroxy-1-cyclohexene-1-carboxylate. Every step of this total synthesis proceeded well with excellent stereoselectivity. Structures of the intermediates in crucial steps were confirmed by detailed 2D NMR analysis.

Directed Evolution of 2-Keto-3-deoxy-6-phosphogalactonate Aldolase To Replace 3-Deoxy-d-arabino-heptulosonic Acid 7-Phosphate Synthase

Directed evolution of 2-keto-3-deoxy-6-phosphogalactonate (KDPGal) aldolase for microbial synthesis of shikimate pathway products provides an alternate strategy to circumvent the competition for phosphoenolpyruvate between 3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) synthase and the phosphoenolpyruvate:carbohydrate phosphotransferase system in Escherichia coli. E. coli KDPGal aldolase was evolved using a combination of error-prone polymerase chain reaction, DNA shuffling, and multiple-site-directed mutagenesis to afford KDPGal aldolase variant NR8.276-2, which exhibits a 60-fold improvement in the ratio kcat/KM relative to that of wild-type E. coli KDPGal aldolase in catalyzing the addition of pyruvate to d-erythrose 4-phosphate to form DAHP. On the basis of its nucleotide sequence, NR8.276-2 contains seven amino acid changes from the wild-type E. coli KDPGal aldolase. Amplified expression of NR8.276-2 in the DAHP synthase and shikimate dehydrogenase-deficient E. coli strain NR7 under fed-batch fermentor-controlled cultivation conditions resulted in synthesis of 13 g/L 3-dehydroshikimic acid in 6.5% molar yield from glucose. Increased coexpression of the irreversible downstream enzyme 3-dehydroquinate synthase increased production of 3-dehydroshikimic acid to 19 g/L in 9.7% molar yield from glucose. Coamplification with transketolase, which increases d-erythrose 4-phosphate availability, afforded 16 g/L 3-dehydroshikimic acid in 8.5% molar yield.

Nanomolar Competitive Inhibitors ofMycobacterium tuberculosis andStreptomyces coelicolor Type II Dehydroquinase

Isomeric nitrophenyl and heterocyclic analogues of the known inhibitor (1S,3R,4R)-1,3,4-trihydroxy-5-cyclohexene-1-carboxylic acid have been synthesized and tested as inhibitors of M. tuberculosis and S. coelicolor type II dehydroquinase, the third enzyme of the shikimic acid pathway. The target compounds were synthesized by a combination of Suzuki and Sonogashira cross-coupling and copper(I)-catalyzed 2,3-dipolar cycloaddition reactions from a common vinyl triflate intermediate. These studies showed that a para-nitrophenyl derivative is almost 20-fold more potent as a competitive inhibitor against the S. coelicolor enzyme than that of M. tuberculosis. The opposite results were obtained with the meta isomer. Five of the bicyclic analogues reported herein proved to be potent competitive inhibitors of S. coelicolor dehydroquinase, with inhibition constants in the low nanomolar range (4-30 nM). These derivatives are also competitive inhibitors of the M. tuberculosis enzyme, but with lower affinities. The most potent inhibitor against the S. coelicolor enzyme, a 6-benzothiophenyl derivative, has a K(i) value of 4 nM-over 2000-fold more potent than the best previously known inhibitor, (1R,4R,5R)-1,5-dihydroxy-4-(2-nitrophenyl)cyclohex-2-en-1-carboxylic acid (8 microM), making it the most potent known inhibitor against any dehydroquinase. The binding modes of the analogues in the active site of the S. coelicolor enzyme (GOLD 3.0.1), suggest a key pi-stacking interaction between the aromatic rings and Tyr 28, a residue that has been identified as essential for enzyme activity.

Enolpyruvyl Activation by Enolpyruvylshikimate-3-phosphate Synthase

Enolpyruvylshikimate-3-phosphate synthase (AroA, also called EPSP synthase) is a carboxyvinyl transferase involved in aromatic amino acid biosynthesis, forming EPSP from shikimate 3-phosphate and phosphoenolpyruvate. Upon extended incubation, EPSP ketal, a side product, forms by intramolecular nucleophilic addition of O4 to C2' of the enolpyruvyl group. The catalytic significance of this reaction was unclear, as it was initially proposed to arise from nonenzymatic breakdown of tetrahedral intermediate that had dissociated from AroA. This study shows that EPSP ketal formed in AroA's active site, not nonenzymatically, by demonstrating its formation in the presence of excess AroA. It formed both in the normal reaction and during AroA-catalyzed EPSP hydrolysis. In addition, nonenzymatic EPSP hydrolysis was studied to elucidate the catalytic imperative for enolpyruvyl reactions. Hydrolysis was acid-catalyzed, with a rate enhancement of >5 x 10(8)-fold. There was no detectable EPSP breakdown after 16 days at 90 degrees C in 1 M KOH, a solution that is 1000-fold more nucleophilic than neutral aqueous solutions. Thus, an unactivated enolpyruvyl group is not susceptible to nucleophilic attack. Enzymatic EPSP ketal formation therefore requires enolpyruvyl activation through protonation of C3' to form either a cationic intermediate or a highly cation-like transition state. Forming an EPSP cation requires the investment of considerable catalytic power by AroA. Such an intermediate is a potential target motif for inhibitor design.

Enzymatic preparation of metabolic intermediates, 3-dehydroquinate and 3-dehydroshikimate, in the shikimate pathway

A method for enzymatic preparation of 3-dehydroquinate and 3-dehydroshikimate in the shikimate pathway was established by controlling the enzyme activity of 3-dehydroquinate dehydratase. When quinate was incubated with the membrane fraction of acetic acid bacteria at pH 5.0, 3-dehydroquinate was formed as the predominant product. 3-Dehydroshikimate was the sole product when incubated at pH 8.0. Mutual separation of the metabolic intermediates was also exemplified.

Purification and properties of NADP-dependent shikimate dehydrogenase from Gluconobacter oxydans IFO 3244 and its application to enzymatic shikimate production

NADP-Dependent shikimate dehydrogenae (SKDH, EC 1.1.1.25) was purified from Gluconobacter oxydans IFO 3244. SKDH showed a single protein band on native-PAGE accompanying enzyme activity. It required NADP exclusively and catalyzed only the shuttle reaction between shikimate and 3-dehydroshikimate. The optimum pH for shikimate oxidation and 3-dehydroshikimate reduction was found at pH 10 and 7 respectively. SKDH proved to be a useful catalyst for shikimate production from 3-dehydroshikimate.

Mechanism of phosphoryl transfer catalyzed by shikimate kinase from Mycobacterium tuberculosis

The structural mechanism of the catalytic functioning of shikimate kinase from Mycobacterium tuberculosis was investigated on the basis of a series of high-resolution crystal structures corresponding to individual steps in the enzymatic reaction. The catalytic turnover of shikimate and ATP into the products shikimate-3-phosphate and ADP, followed by release of ADP, was studied in the crystalline environment. Based on a comparison of the structural states before initiation of the reaction and immediately after the catalytic step, we derived a structural model of the transition state that suggests that phosphoryl transfer proceeds with inversion by an in-line associative mechanism. The random sequential binding of shikimate and nucleotides is associated with domain movements. We identified a synergic mechanism by which binding of the first substrate may enhance the affinity for the second substrate.

Novel and Efficient Syntheses of (−)-Methyl 4-epi-Shikimate and 4,5-Epoxy-Quinic and -Shikimic Acid Derivatives as Key Precursors to Prepare New Analogues

We have developed simple methods that provide a rapid entry into the synthesis of a series of quinate and shikimate analogues, including (-)-methyl 4-epi-shikimate and the 4,5-epoxy analogues of the parent acids. Epoxy derivatives of quinic and shikimic acids were converted into methyl scyllo-quinate and (+)-methyl 3-epi-shikimate, respectively, by processes involving a regio- and stereoselective epoxide ring opening. The strategies described take place through short, high-yield reaction sequences.

Secondary metabolites from Senecio burtonii (Compositae)

A cacalolide derivative named 4alpha-[2'-hydroxymethylacryloxy]-1beta-hydroxy-14-(5-->6) abeo eremophilan-12,8-olide and a shikimic acid derivative named (3'E)-(1alpha)-3-hydroxymethyl-4beta,5alpha-dimethoxycyclohex-2-enyloctadec-3'-enoate along with three known compounds, octacosan-1-ol, 3beta-hydroxyolean-12-en-28-oic acid and 3beta-acetoxyolean-12-en-28-oic acid were isolated from Senecio burtonii. Their structures and relative configurations were established on the basis of spectroscopic analysis.

Plasmodium falciparum: interaction of shikimate analogues with antimalarial drugs

The shikimate pathway for aromatic biosynthesis presents a target for antimalarial drug development as this pathway is absent from animals. This study extends previous work on inhibitors of the shikimate pathway, by examining their interaction with the antimalarial drugs pyrimethamine and atovaquone. Combinations of atovaquone with several shikimate analogues exhibited synergistic effects. These findings highlight potential use of shikimate pathway inhibitors in combination therapy.

Benzene-free synthesis of catechol: interfacing microbial and chemical catalysis

The toxicity of aromatics frequently limits the yields of their microbial synthesis. For example, the 5% yield of catechol synthesized from glucose by Escherichia coli WN1/pWL1.290A under fermentor-controlled conditions reflects catechol's microbial toxicity. Use of in situ resin-based extraction to reduce catechol's concentration in culture medium and thereby its microbial toxicity during its synthesis from glucose by E. coli WN1/pWL1.290A led to a 7% yield of catechol. Interfacing microbial with chemical synthesis was then explored where glucose was microbially converted into a nontoxic intermediate followed by chemical conversion of this intermediate into catechol. Intermediates examined include 3-dehydroquinate, 3-dehydroshikimate, and protocatechuate. 3-Dehydroquinate and 3-dehydroshikimate synthesized, respectively, by E. coli QP1.1/pJY1.216A and E. coli KL3/pJY1.216A from glucose were extracted and then reacted in water heated at 290 degrees C to afford catechol in overall yields from glucose of 10% and 26%, respectively. The problematic extraction of these catechol precursors from culture medium was subsequently circumvented by high-yielding chemical dehydration of 3-dehydroquinate and 3-dehydroshikimate in culture medium followed by extraction of the resulting protocatechuate. After reaction of protocatechuate in water heated at 290 degrees C, the overall yields of catechol synthesized from glucose via chemical dehydration of 3-dehydroquinate and chemical dehydration of 3-dehydroshikimate were, respectively, 25% and 30%. Direct synthesis of protocatechuate from glucose using E. coli KL3/pWL2.46B followed by its extraction and chemical decarboxylation in water gave a 24% overall yield of catechol from glucose. In situ resin-based extraction of protocatechaute synthesized by E. coli KL3/pWL2.46B followed by chemical decarboxylation of this catechol percursor was then examined. This employment of both strategies for dealing with the microbial toxicity of aromatic products led to the highest overall yield with catechol synthesized in 43% overall yield from glucose.

A thermostable shikimate 5-dehydrogenase from the archaeon Archaeoglobus fulgidus

Shikimate 5-dehydrogenase (SKDH; EC 1.1.1.25) catalyzes the reversible reduction of 3-dehydroshikimate to shikimate and is a key enzyme in the aromatic amino acid biosynthesis pathway. The shikimate 5-dehydrogenase gene, aroE, from Archaeoglobus fulgidus was cloned and overexpressed in Escherichia coli. The recombinant enzyme purified as a homodimer and yielded a maximum specific activity of 732 U/mg at 87 degrees C (with NADP+ as coenzyme). Apparent Km values for shikimate, NADP+, and NAD+ were estimated at 0.17+/-0.03 mM, 0.19+/-0.01 mM, and 11.4+/-0.4 mM, respectively. The half-life of the A. fulgidus SKDH is 2 h at the assay temperature (87 degrees C) and 17 days at 60 degrees C. Addition of 1 M NaCl or KCl stabilized the enzyme's half-life to approximately 70 h at 87 degrees C and approximately 50 days at 60 degrees C. This work presents the first kinetic analysis of an archaeal SKDH.

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.