Studies on the nonmevalonate pathway of terpene biosynthesis. The role of 2C-methyl-D-erythritol 2,4-cyclodiphosphate in plants.

2C-methyl-D-erythritol 2,4-cyclodiphosphate was recently shown to be formed from 2C-methyl-D-erythritol 4-phosphate by the consecutive action of IspD, IspE, and IspF proteins in the nonmevalonate pathway of terpenoid biosynthesis. To complement previous work with radiolabelled precursors, we have now demonstrated that [U-13C5]2C-methyl-D-erythritol 4-phosphate affords [U-13C5]2C-methyl-D-erythritol 2,4-cyclodiphosphate in isolated chromoplasts of Capsicum annuum and Narcissus pseudonarcissus. Moreover, chromoplasts are shown to efficiently convert 2C-methyl-D-erythritol 4-phosphate as well as 2C-methyl-D-erythritol 2,4-cyclodiphosphate into the carotene precursor phytoene. The bulk of the kinetic data collected in competition experiments with radiolabeled substrates is consistent with the notion that the cyclodiphosphate is an obligatory intermediate in the nonmevalonate pathway to terpenes. Studies with [2,2'-13C2]2C-methyl-D-erythritol 2,4-cyclodiphosphate afforded phytoene characterized by pairs of jointly transferred 13C atoms in the positions 17/1, 18/5, 19/9, and 20/13 and, at a lower abundance, in positions 16/1, 4/5, 8/9, and 12/13. A detailed scheme is presented for correlating the observed partial scrambling of label with the known lack of fidelity of the isopentenyl diphosphate/dimethylethyl diphosphate isomerase.

Biosynthesis of thiophenes in Tagetes patula.

The biosynthesis of 5-(3-buten-1-ynyl)-2,2'-bithiophene was studied in root cultures of Tagetes patula. Organ cultures were grown with [U-13C(6)]glucose or [1-13C]glucose. The bithiopene and amino acids from cell protein were isolated and analysed by quantitative NMR spectroscopy. Retrobiosynthetic analysis establish acetyl-CoA or a closely related compound (e.g. malonyl-CoA) as building blocks and their orientations in the bithiophene. The data confirm a previously suggested biosynthetic route via long-chain fatty acids and polyacetylenes. However, a polyketide-like biosynthesis via a carbocyclic intermediate cannot be excluded.

Homo-phytochelatins are synthesized in response to cadmium in azuki beans.

In a recent report, it was claimed that azuki beans (Vigna angularis) do not synthesize phytochelatins (PCs) upon exposure to cadmium, although glutathione (GSH), the substrate for PC synthesis, is present in this plant. This legume species thus would be the first exception in the plant kingdom that would fail to complex heavy metals by PCs. Here, we report that not GSH, but only homoglutathione can be detected in this plant and that homo-phytochelatins are formed when azuki beans are challenged with heavy metals such as cadmium. We also show that the 5,5'-dithiobis(2-nitrobenzoic acid)-oxidized GSH reductase recycling assay, used for GSH quantification in the recent study of heavy metal tolerance in azuki beans, reacts both with GSH and homoglutathione and therefore cannot be used when biological samples should be analyzed exclusively for GSH.

Tracer studies with 13C-labeled carbohydrates in cultured plant cells. Retrobiosynthetic analysis of chelidonic acid biosynthesis.

The biosynthesis of chelidonic acid was studied in cell suspension cultures of Leucojum aestivum. Cell cultures were supplied with [U-13C]glucose, [l-13C]glucose or [U-13Cs]ribose/ribulose in standard medium containing unlabeled glucose. 13C labeling patterns of amino acids obtained by hydrolysis of biomass were determined by NMR spectroscopy and compared to the labeling pattern of chelidonic acid. The data document the incorporation of a contiguous 4-carbon fragment derived from the pentose phosphate pool into chelidonic acid. This suggests a biosynthetic pathway involving the condensation of phosphoenolpyruvate with a pentose phosphate followed by dehydration, dehydrogenation, ring closure and decarboxylation conducive to the loss of C-5 of the pentose precursor.

Identification of phytochelatin-related peptides in maize seedlings exposed to cadmium and obtained enzymatically in vitro.

An analytical strategy based on the sensitivity of electrospray tandem mass spectrometry following a simplified and reproducible sample preparation procedure was evaluated for the determination of Cd-induced phytochelatins (PC) and related peptides in four maize varieties. In addition to the three known families of PC (PC, desGly-PC and iso-PC(Glu)) that were observed, novel PC and desGly-PC homologues lacking the N-terminal gamma-linked Glu were isolated from maize root extracts for the first time. Additionally the complete sequence of iso-PC3(Glu) was determined by tandem mass spectrometry. Peptides obtained in vivo and in vitro as the result of the reaction of glutathione with the enzyme phytochelatin synthase were compared. Minor forms detected from in vitro reactions include compounds with intramolecular or intermolecular disulfide bonds resulting from the oxidation of SH groups, phytochelatin homologues lacking the N-terminal gamma-linked Glu, and new PC-related peptides with a Cys-Cys motif. Since peptides lacking a gammaGlu residue could be generated as artifacts in electrospray mass spectrometry, the application of capillary electrophoresis with online electrospray mass spectrometry allowed the separation and detection of such peptides as endogenous molecules present in planta and as products of in vitro reactions.

Biosynthesis of cannabinoids. Incorporation experiments with (13)C-labeled glucoses.

The biosynthesis of cannabinoids was studied in cut sprouts of Cannabis sativa by incorporation experiments using mixtures of unlabeled glucose and [1-(13)C]glucose or [U-(13)C(6)]glucose. (13)C-labeling patterns of cannabichromenic acid and tetrahydrocannabinolic acid were analyzed by quantitative NMR spectroscopy. (13)C enrichments and coupling patterns show that the C(10)-terpenoid moiety is biosynthesized entirely or predominantly (> 98%) via the recently discovered deoxyxylulose phosphate pathway. The phenolic moiety is generated by a polyketide-type reaction sequence. The data support geranyl diphosphate and the polyketide, olivetolic acid, as specific intermediates in the biosynthesis of cannabinoids.

Phosphorylation of 1-deoxy-D-xylulose by D-xylulokinase of Escherichia coli.

1-deoxy-D-xylulose 5-phosphate serves as a precursor for the biosynthesis of the vitamins thiamine and pyridoxal and for the formation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate via the nonmevalonate pathway of terpenoid biosynthesis. Earlier studies had shown that Escherichia coli incorporates unphosphorylated 1-deoxy-D-xylulose into the terpenoid side chain of ubiquinones with high efficacy. We show that D-xylulokinase of E. coli (EC 2.7.1.17) catalyzes the phosphorylation of 1-deoxy-D-xylulose at the hydroxy group of C-5 at a rate of 1.6 micromol.mg min-1. This reaction constitutes a potential salvage pathway for the generation of 1-deoxy-D-xylulose 5-phosphate from exogenous or endogenous 1-deoxy-D-xylulose as starting material for the biosynthesis of terpenoids, thiamine and pyridoxal.

Strong induction of phytochelatin synthesis by zinc in marine green alga, Dunaliella tertiolecta.

Synthesis of phytochelatins (PCs), heavy-metal-sequestering peptides, in the marine green alga, Dunaliella tertiolecta, was evaluated under various conditions of exposure to heavy metals. To investigate the effect of heavy metals on both PC synthesis and their upstream biosynthetic reactions, an ion-pair-HPLC system was developed in this study, by which PCs and their biosynthetic intermediates, cysteine (Cys), gamma-glutamylcysteine (gammaEC) and glutathione (GSH), could be determined simultaneously with high sensitivity. When the cells were exposed to Zn2+, the level of PCs was maximal at 200 microM and significantly higher than that obtained after exposure to 400 microM Cd2+, which is the strongest inducer of PC synthesis in higher plants in vivo and in vitro as well as in microalgae. The predominant PC subtype was PC4, followed by PC3 and PC5, whereas PC2, which is generally abundant in higher plants, has the lowest level among PC2 to PC5. These results suggest that the characteristics of PC synthase in D. tertiolecta including the requirement of heavy metals for its catalysis and substrate specificity towards GSH and PC(n) are considerably different from those in higher plants and other algae. While PC synthesis proceeded in the heavy-metal-treated cells, the level of GSH did not appreciably change. To maintain the same size of the GSH pool, GSH must be newly synthesized to balance the amount consumed for PC synthesis.

Characterization of proteins in latex of the opium poppy (Papaver somniferum) using two-dimensional gel electrophoresis and microsequencing.

The opium poppy (Papaver somniferum) belongs to the group of latex-containing plants. Latex is the milky-like fluid within laticifer cells. In this study, poppy latex was analyzed with respect to ultrastructure, alkaloid, and protein content. The main goal of this project was the examination of the proteins by two-dimensional gel electrophoresis. In a proteomics approach, we investigated two main fractions of the latex, namely the cytosolic serum and the sedimented fraction containing the alkaloid-accumulating vesicles. Of the serum, representing the protein-rich part of the latex, 75 spots were analyzed by internal peptide microsequencing, followed by a database searching. For 69 proteins a function could be assigned due to homology to known proteins, whereas six spots could not be identified. Furthermore, codeinone reductase, a representative of the specific enzyme system in morphine biosynthesis, could be detected within the cytosolic serum fraction. In the vesicle-containing pellet, 23 protein spots were analyzed. An attempt was also made to separate the vesicle pellet by density centrifugation, followed by investigation of the alkaloid content, ultrastructure, and protein pattern. This study describes the first database of soluble proteins present in the latex of P. somniferum

Biosynthesis of terpenoids: 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase from tomato.

The putative catalytic domain (residues 81-401) of a predicted tomato protein with similarity to 4-diphosphocytidyl-2-C-methyl-d-erythritol kinase of Escherichia coli was expressed in a recombinant E. coli strain. The protein was purified to homogeneity and was shown to catalyze the phosphorylation of the position 2 hydroxy group of 4-diphosphocytidyl-2-C-methyl-d-erythritol at a rate of 33 micromol small middle dotmg(-1) small middle dotmin(-1). The structure of the reaction product, 4-diphosphocytidyl-2-C-methyl-d-erythritol 2-phosphate, was established by NMR spectroscopy. Divalent metal ions, preferably Mg(2+), are required for activity. Neither the tomato enzyme nor the E. coli ortholog catalyzes the phosphorylation of isopentenyl monophosphate.

Biosynthesis of terpenoids: 4-diphosphocytidyl-2C-methyl-D-erythritol synthase of Arabidopsis thaliana.

A hypothetical gene with similarity to the ispD gene of Escherichia coli was cloned from Arabidopsis thaliana cDNA. The ORF of 909 bp specifies a protein of 302 amino acid residues. The cognate chromosomal gene consists of 2,071 bp and comprises 11 introns with a size range of 78-202 bp. A fragment comprising amino acid residues 76-302 was expressed in a recombinant E. coli strain. The protein was purified to homogeneity and was shown to catalyze the formation of 4-diphosphocytidyl-2C-methyl-d-erythritol from 2C-methyl-d-erythritol 4-phosphate with a specific activity of 67 micromol small middle dotmin(-1) mg(-1). The Michaelis constants for 4-diphosphocytidyl-2C-methyl-d-erythritol and CTP were 500 microM and 114 microM, respectively.

Synthesis of 3,3',5-trihydroxybiphenyl-2-carboxylic acid, a component of the bitterest natural product amarogentin and its coenzyme A and N-acetyl cysteamine thiol esters.

3,3',5-Trihydroxybiphenyl-2-carboxylic acid (6), an ester component of the bitter-tasting natural products amarogentin and amaroswerin, was synthesized in six steps in 13.6% overall yield. Its N-acetyl cysteamine thiol ester (9) and its coenzyme A thiol ester (8), a likely biosynthetic precursor of the amarums, were also prepared.

Biosynthesis of terpenoids: YgbB protein converts 4-diphosphocytidyl-2C-methyl-D-erythritol 2-phosphate to 2C-methyl-D-erythritol 2,4-cyclodiphosphate.

In many microorganisms, the putative orthologs of the Escherichia coli ygbB gene are tightly linked or fused to putative orthologs of ygbP, which has been shown earlier to be involved in terpenoid biosynthesis. The ygbB gene of E. coli was expressed in a recombinant E. coli strain and was shown to direct the synthesis of a soluble, 17-kDa polypeptide. The recombinant protein was found to convert 4-diphosphocytidyl-2C-methyl-D-erythritol 2-phosphate into 2C-methyl-D-erythritol 2,4-cyclodiphosphate and CMP. The structure of the reaction product was established by NMR spectroscopy using (13)C-labeled substrate samples. The enzyme-catalyzed reaction requires Mn(2+) or Mg(2+) but no other cofactors. Radioactivity from [2-(14)C]2C-methyl-D-erythritol 2,4-cyclodiphosphate was diverted efficiently to carotenoids by isolated chromoplasts from Capsicum annuum and, thus, was established as an intermediate in the deoxyxylulose phosphate pathway of isoprenoid biosynthesis. YgbB protein also was found to convert 4-diphosphocytidyl-2C-methyl-D-erythritol into 2C-methyl-D-erythritol 3,4-cyclophosphate. This compound does not serve as substrate for the formation of carotenoids by isolated chromoplasts and is assumed to be an in vitro product without metabolic relevance.

Indoxyl-UDPG-glucosyltransferase from Baphicacanthus cusia.

The enzyme catalyzing the transfer of glucose from uridine diphosphate glucose to indoxyl yielding the indoxyl glucoside indican was isolated from Baphicacanthus cusia Bremek (Acanthaceae). The indoxyl-uridine diphosphate glucose (UDPG)-glucosyltransferase was purified to homogeneity in six chromatographic steps. The decisive step for the recovery of a homogeneous enzyme was the application of immobilized metal affinity chromatography yielding an 863-fold purified enzyme. From a total of 60 substances tested, in addition to the natural substrate 3-OH-indole (indoxyl), only 4-OH-, 5-OH-, 6-OH-, and 7-OH-indole were accepted as substrates by the glucosyltransferase. However, the latter substrates were metabolized to varying extent. The optimum pH of the enzyme was 8.5, the optimum temperature was 30 degrees C and the isoelectric point was pH 6.5. The M(r) of the enzyme was determined to be 60 +/- 2 x 10(3). Indoxyl as substrate yielded a K(m) of 1.2 mM, while a K(m) of 1.7 mM was found for UDPG.

Biosynthesis of terpenoids: YchB protein of Escherichia coli phosphorylates the 2-hydroxy group of 4-diphosphocytidyl-2C-methyl-D-erythritol.

A comparative analysis of all published complete genomes indicated that the putative orthologs of the unannotated ychB gene of Escherichia coli follow the distribution of the dxs, dxr, and ygbP genes, which have been shown to specify enzymes of the deoxyxylulose phosphate pathway of terpenoid biosynthesis, thus suggesting that the hypothetical YchB protein also is involved in that pathway. To test this hypothesis, the E. coli ychB gene was expressed in a homologous host. The recombinant protein was purified to homogeneity and was shown to phosphorylate 4-diphosphocytidyl-2C-methyl-D-erythritol in an ATP-dependent reaction. The reaction product was identified as 4-diphosphocytidyl-2C-methyl-D-erythritol 2-phosphate by NMR experiments with various (13)C-labeled substrate samples. A (14)C-labeled specimen of this compound was converted efficiently into carotenoids by isolated chromoplasts of Capsicum annuum. The sequence of E. coli YchB protein is similar to that of the protein predicted by the tomato cDNA pTOM41 (30% identity), which had been implicated in the conversion of chloroplasts to chromoplasts.

Biosynthesis of terpenoids: 1-deoxy-D-xylulose-5-phosphate reductoisomerase from Escherichia coli is a class B dehydrogenase.

1-Deoxy-D-xylulose-5-phosphate is converted into 2-C-methyl-D-erythritol-4-phosphate by the catalytic action of 1-deoxy-D-xylulose-5-phosphate reductoisomerase (Dxr protein) using NADPH as cofactor. The stereochemical features of this reaction were investigated in in vitro experiments with the recombinant Dxr protein of Escherichia coli using (4R)- or (4S)-[4-(2)H(1)]NADPH as coenzyme. The enzymatically formed 2-C-methyl-D-erythritol-4-phosphate was isolated and converted into 1,2:3,4-di-O-isopropylidene-2-C-methyl-D-erythritol; NMR spectroscopic investigation of this derivative indicated that only (4S)-[4-(2)H(1)]NADPH affords 2-C-methyl-D-erythritol-4-phosphate labelled exclusively in the H(Re) position of C-1. Stereospecific transfer of H(Si) from C-4 of the cofactor identifies the Dxr protein of E. coli as a class B dehydrogenase.

Cytidine 5'-triphosphate-dependent biosynthesis of isoprenoids: YgbP protein of Escherichia coli catalyzes the formation of 4-diphosphocytidyl-2-C-methylerythritol.

2-C-methylerythritol 4-phosphate has been established recently as an intermediate of the deoxyxylulose phosphate pathway used for biosynthesis of terpenoids in plants and in many microorganisms. We show that an enzyme isolated from cell extract of Escherichia coli converts 2-C-methylerythritol 4-phosphate into 4-diphosphocytidyl-2-C-methylerythritol by reaction with CTP. The enzyme is specified by the hitherto unannotated ORF ygbP of E. coli. The cognate protein was obtained in pure form from a recombinant hyperexpression strain of E. coli harboring a plasmid with the ygbP gene under the control of a T5 promoter and lac operator. By using the recombinant enzyme, 4-diphosphocytidyl-[2-(14)C]2-C-methylerythritol was prepared from [2-(14)C]2-C-methylerythritol 4-phosphate. The radiolabeled 4-diphosphocytidyl-2-C-methylerythritol was shown to be efficiently incorporated into carotenoids by isolated chromoplasts of Capsicum annuum. The E. coli ygbP gene appears to be part of a small operon also comprising the unannotated ygbB gene. Genes with similarity to ygbP and ygbB are present in the genomes of many microorganisms, and their occurrence appears to be correlated with that of the deoxyxylulose pathway of terpenoid biosynthesis. Moreover, several microorganisms have genes specifying putative fusion proteins with ygbP and ygbB domains, suggesting that both the YgbP protein and the YgbB protein are involved in the deoxyxylulose pathway. A gene from Arabidopsis thaliana with similarity to ygbP carries a putative plastid import sequence, which is well in line with the assumed localization of the deoxyxylulose pathway in the plastid compartment of plants.

Biosynthesis of Erythrina alkaloids in Erythrina crista-galli.

A precursor application system was developed to allow the study of Erythrina alkaloid formation in Erythrina crista-galli. Fruit wall tissue of this species was recognized as the major site of alkaloid biosynthesis. The application of radioactively and 13C-labelled potential precursors showed that the hitherto assumed precursor (S)-norprotosinomenine was not incorporated into the Erythrina alkaloids. In contrast, (S)-coclaurine as well as (S)-norreticuline were metabolized to erythraline and erythrinine, respectively, suggesting that a coclaurine-norreticuline pathway is operative in Erythrina alkaloid formation. Feeding of [1-13C]-labelled (S)-norreticuline with subsequent NMR spectroscopy demonstrated that the resulting erythraline was exclusively labelled at position C-10. Therefore, the participation of a symmetrical intermediate of the diphenoquinone type in Erythrina alkaloid biosynthesis can be excluded.

Biosynthesis of bitter acids in hops. A (13)C-NMR and (2)H-NMR study on the building blocks of humulone.

The biosynthesis of humulone, an antibacterial bitter acid from hops, was studied by isotope-incorporation experiments using (13)C-labelled glucose or (2)H(2)O. (13)C enrichments, (2)H enrichments and (13)C(13)C coupling patterns identify isovaleryl-CoA, malonyl-CoA and dimethylallyl pyrophosphate as precursors for humulone. Dimethylallyl pyrophosphate, which serves as a building block for the bitter acid, is generated via the deoxyxylulose pathway of terpenoid biosynthesis. The data confirm that a symmetrical intermediate is involved in humulone formation.

Purification and characterization of acetyl coenzyme A: 10-hydroxytaxane O-acetyltransferase from cell suspension cultures of Taxus chinensis.

An O-acetyltransferase that catalyzes the regiospecific acetylation of a range of taxanes possessing an unsubstituted 10-hydroxyl group was detected and purified to apparent electrophoretic homogeneity from a cytosolic fraction of Taxus chinensis cell cultures. The purification involved negative calcium phosphate adsorption, sephadex desalting, DEAE, AcA44 chromatography, HighQ, CHT II, HiTrap Blue, Phenylsepharose and Mimetic Green purification steps. The purified acetyltransferase was found to be a monomeric protein of 71 +/- 1.5 kDa that is highly regio- and stereospecific towards the 10 beta-hydroxyl group of the taxane molecule and is also active towards 10-desacetylbaccatine III. The acetyltransferase reaction had a pH optimum of 9.0 with halfmaximal activities at pH 6.8 and 10.8, respectively. The temperature optimum was at 35 degrees C and the isoelectric point at 5.6. The apparent K(m) values for 10-desacetyltaxuyunnanine C and acetyl CoA were 23 and 61 microM, respectively. The turnover rate for the enzyme using both substrates was 0.2 mol mol-1 of enzyme. The kinetic optimum was determined to be Kcat/K(m) = 8.7 s-1 L M-1.