Deoxyxylulose phosphate pathway to terpenoids.

Recently, a mevalonate-independent pathway was discovered in bacteria and plants that leads to the formation of isopentenyl diphosphate and dimethylallyl diphosphate, the two basic precursors of isoprenoids. Although many details of the widely distributed pathway are unknown, some intermediates, mechanisms, enzymes and genes of this novel route have been identified. Information on this pathway could provide the basis for the development of new antibiotics, herbicides and antimalarials.

The non-mevalonate pathway of isoprenoids: genes, enzymes and intermediates.

Although the mevalonate pathway had been considered for a long time as the unique source of biosynthetic isoprenoids, an alternative pathway has recently been discovered. The first intermediate, 1-deoxy-D-xylulose 5-phosphate, is assembled by condensation of glyceraldehyde 3-phosphate and pyruvate. A skeletal rearrangement coupled with a reduction step affords the branched-chain polyol, 2C-methyl-D-erythritol 4-phosphate, which is subsequently converted into a cyclic 2,4-diphosphate by the consecutive action of three enzymes via nucleotide diphosphate intermediates. The genes specifying these enzymes have been cloned from bacteria, plants and protozoa. Their expression in recombinant bacterial hosts has opened the way to the identification of several novel pathway intermediates.

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.

Anti-malarial drug targets: screening for inhibitors of 2C-methyl-D-erythritol 4-phosphate synthase (IspC protein) in Mediterranean plants.

The recently discovered non-mevalonate pathway of isoprenoid biosynthesis serves as the unique source of terpenoids in numerous pathogenic eubacteria and in apicoplast-type protozoa, most notably Plasmodium, but is absent in mammalian cells. It is therefore an attractive target for anti-infective chemotherapy. The first committed step of the non-mevalonate pathway is catalyzed by 2C-methyl-D-erythritol 4-phosphate synthase (IspC). Using photometric and NMR spectroscopic assays, we screened extracts of Mediterranean plants for inhibitors of the enzyme. Strongest inhibitory activity was found in leaf extracts of Cercis siliquastrum.

Isoprenoid biosynthetic pathways as anti-infective drug targets.

IPP (isopentenyl diphosphate) and DMAPP (dimethylallyl diphosphate) serve as the universal precursors for the biosynthesis of isoprenoids. Besides the well-known mevalonate pathway, the existence of a second biosynthetic pathway conducive to IPP and DMAPP formation through 1-deoxy-D-xylulose 5-phosphate and 2C-methyl-D-erythritol 4-phosphate was discovered approx. 10 years ago in plants and certain eubacteria. It is now known that this pathway is widely distributed in the bacterial kingdom including major human pathogens, such as Mycobacterium tuberculosis and Helicobacter pylori. The pathway is also essential in the malaria vector Plasmodium falciparum. During the last few years, the genes, enzymes, intermediates and mechanisms of the biosynthetic route have been elucidated by a combination of comparative genomics, enzymology, advanced NMR technology and crystallography. The results provide the basis for the development of novel anti-infective drugs.

Tissue-specific regulation of cytochrome c oxidase efficiency by nucleotides.

Cytochrome c oxidase from bovine heart and liver was reconstituted in liposomes in the absence or presence of nucleotides. Intraliposomal ADP, and to a smaller extent intraliposomal ATP, increased the respiratory activity of the heart but not of the liver isozyme under uncoupled but not under coupled conditions, leading to increased respiratory control ratios. In a preceding publication [Anthony, G., Reimann, A., & Kadenbach, B. (1992) Proc. Natl. Acad. Sci. U.S.A. 90, 1652-1656], the stimulatory effect of intraliposomal ADP could be related to interaction with the matrix domain of subunit VIa-h (heart type). The data suggest a regulatory effect of matrix nucleotides in heart and skeletal muscle mitochondria on the efficiency of energy transduction in COX.

Biosynthesis of isoprenoids via the non-mevalonate pathway.

The mevalonate pathway for the biosynthesis of the universal terpenoid precursors, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), is known in considerable detail. Only recently, the existence of a second mevalonate-independent pathway for the biosynthesis of IPP and DMAPP was detected in plants and certain eubacteria. Experiments with 13C and/or 2H-labelled precursors were crucial in the elucidation of this novel route. The pathway is essential in plants, many eubacteria and apicomplexan parasites, but not in archaea and animals. The genes, enzymes and intermediates of this pathway were rapidly unravelled over the past few years. Detailed knowledge about the mechanisms of this novel route may benefit the development of novel antibiotics, antimalarials and herbicides.

Dihydroneopterin triphosphate epimerase of Escherichia coli: purification, genetic cloning, and expression.

The enzyme catalyzing the epimerization at position 2' of dihydroneopterin triphosphate was purified by a factor of about 10,000 from cell extract of Escherichia coli. The cognate gene was cloned, sequenced, expressed, and mapped to kb 2427 on the E. coli chromosome.

Enoate reductases of Clostridia. Cloning, sequencing, and expression.

The enr genes specifying enoate reductases of Clostridium tyrobutyricum and Clostridium thermoaceticum were cloned and sequenced. Sequence comparison shows that enoate reductases are similar to a family of flavoproteins comprising 2,4-dienoyl-coenzyme A reductase from Escherichia coli and old yellow enzyme from yeast. The C. thermoaceticum enr gene product was expressed in recombinant Escherichia coli cells growing under anaerobic conditions. The recombinant enzyme was purified and characterized.

Biosynthesis of terpenoids. 2C-Methyl-D-erythritol 2,4-cyclodiphosphate synthase (IspF) from Plasmodium falciparum.

The putative catalytic domain of an open reading frame from Plasmodium falciparum with similarity to the ispF gene of Escherichia coli specifying 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase was expressed in a recombinant E. coli strain. The recombinant protein was purified to homogeneity and was found to catalyze the formation of 2C-methyl-D-erythritol 2,4-cyclodiphosphate from 4-diphosphocytidyl-2C-methyl-D-erythritol 2-phosphate at a rate of 4.3 micromol x mg(-1) x min(-1). At lower rates, the recombinant protein catalyzes the formation of 2-phospho-2C-methyl-D-erythritol 3,4-cyclophosphate from 4-diphosphocytidyl-2C-methyl-D-erythritol 2-phosphate and the formation of 2C-methyl-D-erythritol 3,4-cyclophosphate from 4-diphosphocytidyl-2C-methyl-D-erythritol. Divalent metal ions such as magnesium or manganese are required for catalytic activity. The enzyme has a pH optimum at pH 7.0. Recombinant expression of the full-length open reading frame afforded insoluble protein that could not be folded in vitro. The enzyme is a potential target for antimalarial drugs directed at the nonmevalonate pathway of isoprenoid biosynthesis.

Biosynthesis of pteridines in Escherichia coli. Structural and mechanistic similarity of dihydroneopterin-triphosphate epimerase and dihydroneopterin aldolase.

An open reading frame located at 69.0 kilobases on the Escherichia coli chromosome was shown to code for dihydroneopterin aldolase, catalyzing the conversion of 7,8-dihydroneopterin to 6-hydroxymethyl-7,8-dihydropterin in the biosynthetic pathway of tetrahydrofolate. The gene was subsequently designated folB. The FolB protein shows 30% identity to the paralogous dihydroneopterin-triphosphate epimerase, which is specified by the folX gene located at 2427 kilobases on the E. coli chromosome. The folX and folB gene products were both expressed to high yield in recombinant E. coli strains, and the recombinant proteins were purified to homogeneity. Both enzymes form homo-octamers. Aldolase can use L-threo-dihydroneopterin and D-erythro-dihydroneopterin as substrates for the formation of 6-hydroxymethyldihydropterin, but it can also catalyze the epimerization of carbon 2' of dihydroneopterin and dihydromonapterin at appreciable velocity. Epimerase catalyzes the epimerization of carbon 2' in the triphosphates of dihydroneopterin and dihydromonapterin. However, the enzyme can also catalyze the cleavage of the position 6 side chain of several pteridine derivatives at a slow rate. Steady-state kinetic parameters are reported for the various enzyme-catalyzed reactions. We propose that the polarization of the 2'-hydroxy group of the substrate could serve as the initial reaction step for the aldolase as well as for the epimerase activity. A deletion mutant obtained by targeting the folX gene of E. coli has normal growth properties on complete medium as well as on minimal medium. Thus, the physiological role of the E. coli epimerase remains unknown. The open reading frame ygiG of Hemophilus influenzae specifies a protein with the catalytic properties of an aldolase. However, the genome of H. influenzae does not specify a dihydroneopterin-triphosphate epimerase.

Studies on the nonmevalonate pathway to terpenes: the role of the GcpE (IspG) protein.

Recombinant Escherichia coli cells engineered for the expression of the xylB gene in conjunction with genes of the nonmevalonate pathway were supplied with (13)C-labeled 1-deoxy-D-xylulose. Cell extracts were analyzed directly by NMR spectroscopy. (13)C-labeled 2C-methyl-D-erythritol 2,4-cyclodiphosphate was detected at high levels in cells expressing xylB, ispC, ispD, ispE, and ispF. The additional expression of the gcpE gene afforded 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate as an intermediate of the nonmevalonate pathway. Hypothetical mechanisms involving conserved cysteine residues are proposed for the enzymatic conversion of 2C-methyl-D-erythritol 2,4-cyclodiphosphate into 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate catalyzed by the GcpE protein.

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.

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.

Enzyme-assisted preparation of isotope-labeled 1-deoxy-d-xylulose 5-phosphate.

Recombinant 1-deoxy-D-xylulose 5-phosphate synthase of Bacillus subtilis was used for the preparation of isotope-labeled 1-deoxy-D-xylulose 5-phosphate using isotope-labeled glucose and/or isotope-labeled pyruvate as starting materials. The simple one-pot methods described afford almost every conceivable isotopomer of 1-deoxy-D-xylulose 5-phosphate carrying (13)C or (14)C from commercially available precursors with an overall yield around 50%.

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.

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: efficient multistep biotransformation procedures affording isotope-labeled 2C-methyl-D-erythritol 4-phosphate using recombinant 2C-methyl-D-erythritol 4-phosphate synthase.

This paper describes the recombinant expression of the ispC gene of Escherichia coli specifying 2C-methyl-D-erythritol 4-phosphate synthase in a modified form that can be purified efficiently by metal-chelating chromatography. The enzyme was used for the preparation of isotope-labeled 2C-methyl-D-erythritol 4-phosphate employing isotope-labeled glucose and pyruvate as starting materials. The simple one-pot methods described afford numerous isotopomers of 2C-methyl-D-erythritol 4-phosphate carrying (3)H, (13)C, or (14)C from commercially available precursors. The overall yield based on the respective isotope-labeled starting material is approximately 50%.

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.

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.