A new motif for inhibitors of geranylgeranyl diphosphate synthase

The enzyme geranylgeranyl diphosphate synthase (GGDPS) is believed to receive the substrate farnesyl diphosphate through one lipophilic channel and release the product geranylgeranyl diphosphate through another. Bisphosphonates with two isoprenoid chains positioned on the α-carbon have proven to be effective inhibitors of this enzyme. Now a new motif has been prepared with one isoprenoid chain on the α-carbon, a second included as a phosphonate ester, and the potential for a third at the α-carbon. The pivaloyloxymethyl prodrugs of several compounds based on this motif have been prepared and the resulting compounds have been tested for their ability to disrupt protein geranylgeranylation and induce cytotoxicity in myeloma cells. The initial biological studies reveal activity consistent with GGDPS inhibition, and demonstrate a structure–function relationship which is dependent on the nature of the alkyl group at the α-carbon.     

Biotechnological approaches to enhance the biosynthesis of ginkgolides and bilobalide in Ginkgo biloba

Ginkgo biloba is one of the oldest living tree species and its extracts or powdered leaves are one of the best selling herbal preparations. The main bioactive constituents are flavonoids and the terpene trilactones, ginkgolides and bilobalide, which are responsible for their pharmacological activity. However, there are many difficulties for ginkgo leaves supply and the chemical synthesis is far from of being applicable for commercial-scale production. G. biloba cell cultures have arisen as a useful alternative source of pharmacologically active terpene trilactones. This review sheds light on the chemistry and biosynthesis of terpene trilactones with the aim of increasing the production of these high value compounds by biotechnological approaches. Different biotechnological strategies to improve ginkgolides and bilobalide production will be discussed, including screening and selection of in vitro ginkgo cultures, cell differentiation levels of these cultures, optimization of culture conditions, feeding and elicitation strategies. Special attention will be paid in developing new methodologies to enhance ginkgo cell biomass and provide high amounts of these bioactive terpene trilactones using large-scale cell cultures.     

Cloning and characterization of Ginkgo biloba levopimaradiene synthase which catalyzes the first committed step in ginkgolide biosynthesis

Levopimaradiene synthase, which catalyzes the initial cyclization step in ginkgolide biosynthesis, was cloned and functionally characterized. A Ginkgo biloba cDNA library was prepared from seedling roots and a probe was amplified using primers corresponding to conserved gymnosperm terpene synthase sequences. Colony hybridization and rapid amplification of cDNA ends yielded a full-length clone encoding a predicted protein (873 amino acids, 100,289 Da) similar to known gymnosperm diterpene synthases. The sequence includes a putative N-terminal plastid transit peptide and three aspartate-rich regions. The full-length protein expressed in Escherichia coli cyclized geranylgeranyl diphosphate to levopimaradiene, which was identical to a synthetic standard by GC/MS analysis. Removing 60 or 79 N-terminal residues increased levopimaradiene production, but a 128-residue N-terminal deletion lacked detectable activity. This is the first cloned ginkgolide biosynthetic gene and the first in vitro observation of an isolated ginkgolide biosynthetic enzyme.     

Interaction with the small subunit of geranyl diphosphate synthase modifies the chain length specificity of geranylgeranyl diphosphate synthase to produce geranyl diphosphate.

Geranyl diphosphate synthase belongs to a subgroup of prenyltransferases, including farnesyl diphosphate synthase and geranylgeranyl diphosphate synthase, that catalyzes the specific formation, from C(5) units, of the respective C(10), C(15), and C(20) precursors of monoterpenes, sesquiterpenes, and diterpenes. Unlike farnesyl diphosphate synthase and geranylgeranyl diphosphate synthase, which are homodimers, geranyl diphosphate synthase from Mentha is a heterotetramer in which the large subunit shares functional motifs and a high level of amino acid sequence identity (56-75%) with geranylgeranyl diphosphate synthases of plant origin. The small subunit, however, shares little sequence identity with other isoprenyl diphosphate synthases; yet it is absolutely required for geranyl diphosphate synthase catalysis. Coexpression in Escherichia coli of the Mentha geranyl diphosphate synthase small subunit with the phylogenetically distant geranylgeranyl diphosphate synthases from Taxus canadensis and Abies grandis yielded a functional hybrid heterodimer that generated geranyl diphosphate as product in each case. These results indicate that the geranyl diphosphate synthase small subunit is capable of modifying the chain length specificity of geranylgeranyl diphosphate synthase (but not, apparently, farnesyl diphosphate synthase) to favor the production of C(10) chains. Comparison of the kinetic behavior of the parent prenyltransferases with that of the hybrid enzyme revealed that the hybrid possesses characteristics of both geranyl diphosphate synthase and geranylgeranyl diphosphate synthase.     

A diterpene synthase from the clary sage Salvia sclarea catalyzes the cyclization of geranylgeranyl diphosphate to (8R)-hydroxy-copalyl diphosphate

The bicyclic diterpene (−)-sclareol is accumulated in glandular trichomes in Salvia sclarea (Schmiderer et al., 2008), and is a major terpenoid component of this plant species. It is used as the starting material for Ambrox synthesis, a synthetic ambergris analog used in the flavor and fragrance industry. In order to investigate the formation of sclareol, cDNA prepared from secretory cells of glandular trichomes from S. sclarea inflorescence were randomly sequenced. A putative copalyl diphosphate synthase encoding EST, SscTPS1, was functionally expressed in Escherichia coli. Whereas reaction of geranylgeranyl diphosphate with the putative copalyl diphosphate synthase followed by hydrolysis with alkaline phosphatase yielded a diastereomeric mixture of (13R)- and (13S)-manoyl oxide, HCl hydrolysis yielded (−)-sclareol (1) and 13-epi-sclareol as products. The product of the reaction of SscTPS1 with geranylgeranyl diphosphate was subjected to analysis by LC-negative ion ESI-MS/MS without prior hydrolysis. EPI scans were consistent with copalyl diphosphate to which 18 mass units had added (m/z 467 [M+H]⁻). The enzymatic reaction was also carried out in the presence of 60% H₂¹⁸O. LC-negative ion ESI-MS/MS analysis established an additional reaction product consistent with the incorporation of ¹⁸O. Incubation in the presence of 60% ²H₂O resulted in the incorporation of one deuterium atom. These results suggest water capture of the carbocation intermediate, which is known to occur in reactions catalyzed by monoterpene synthases, but has been described only several times for diterpene synthases.     

Isolation of ginkgolides A, B, C, J and bilobalide from G. biloba extracts

Ginkgolides A, B, C and J, together with bilobalide, are unique terpenoid components of the Ginkgo biloba tree. Due to similar chemical properties, their separation is quite tedious. We have developed an efficient and rapid protocol for separation of individual ginkgolides and bilobalide from G. biloba extracts. The procedure takes advantage of enhanced susceptibility of ginkgolides B and C to benzylation and the ease of separation of these products from ginkgolides A and J which do not react. The protocol is applicable to the previously reported enriched extracts prepared from G. biloba leaves. A single chromatographic step prior to benzylation provides bilobalide and mixture of ginkgolides A, B, C, and J. After benzylation, the individual ginkgolides are separated by chromatography.     

Relationship between intron 4b splicing of the rat geranylgeranyl diphosphate synthase gene and the active enzyme expression level

Geranylgeranyl diphosphate synthase (GGPS) is a branch point enzyme in the mevalonate pathway that catalyzes the synthesis of geranylgeranyl diphosphate used for the geranylgeranylation of Rho, Rac and Rab proteins. The current study showed the production of multiple forms of GGPS mRNA from a single GGPS gene in rat. The mRNAs resulted from combinations of multiple alternative introns and two poly(A) sites in the 3'-translated and 3'-untranslated regions. These are classified into 1a-type and 1b-type mRNAs, based on the splicing of intron 4b resulting in the difference in deduced amino acid sequence between the C-terminal regions. The 1a-type and 1b-type proteins expressed in both Escherichia coli and HeLa cells were active and inactive, respectively. In the case of HeLa cells, the latter protein expression level was about 10% relative to the former one. This was also observed for Cos-7 and 293 cells. When fusions of beta-galactosidase with C-terminal regions differing between the 1a-type and 1b-type proteins were expressed in HeLa cells, the expressed fusion proteins were both found to be active but the latter fusion protein expression level was considerably low compared with the former one. The expression level of 1a-type mRNA was higher than that of 1b-type mRNA in brain, liver, heart, and thymus, but the two expression levels were the same in testis and ovary. During testis development the total GGPS mRNA expression level increased, accompanied by an increase in 1b-type mRNA, the expression level of 1a-type mRNA encoding active GGPS remaining kept unchanged. These results indicate that the expression level of rat active GGPS is at least regulated through the splicing of intron 4b of its gene.     

Molecular cloning and sequence analysis of a novel chalcone synthase cDNA from Ginkgo biloba.

A chalcone synthase (CHS) gene was cloned from Ginkgo biloba for the first time and it was also the first cloned gene involved in flavonoids metabolic pathway in G. biloba. The full-length cDNA of G. biloba CHS (designated as Gbchs) was 1608bp with poly(A) tailing and it contained a 1173bp open reading frame (ORF) encoding a 391 amino acid protein. Gbchs was found to have extensive homology with those of other plant chs genes via multiple alignments. The active sites of the CoA binding, coumaroyl pocket and cyclization pocket in CHS protein of Medicago sativa were also found in GbCHS. Molecular modeling of GbCHS indicated that the three-dimensional structure of GbCHS strongly resembled that of M. sativa (MsCHS2), implying GbCHS may have similar functions with MsCHS2. Phylogenetic tree analysis revealed that GbCHS had closer relationship with CHSs from gymnosperm plants than from other plants. Gbchs is a useful tool to study the regulation of flavonoids metabolism in G. biloba.     

Structures,mechanisms and inhibitors of undecaprenyl diphosphate synthase: A cis-prenyltransferase for bacterial peptidoglycan biosynthesis

Isoprenoids are an intensive group of compounds made from isopentenyl diphosphate (IPP), catalyzed by prenyltransferases such as farnesyl diphosphate (FPP) cyclases, squalene synthase, protein farnesyltransferases and geranylgeranyltransferases, aromatic prenyltransferases as well as a group of prenyltransferases (cis- and trans-types) catalyzing consecutive condensation reactions of FPP with specific numbers of IPP to generate linear products with designate chain lengths. These prenyltransferases play significant biological functions and some of them are drug targets. In this review, structures, mechanisms, and inhibitors of a cis-prenyltransferase, undecaprenyl diphosphate synthase (UPPS) that mediates bacterial peptidoglycan biosynthesis, are summarized for comparison with the most related trans-prenyltransferases and other prenyltransferases.     

Molecular cloning and characterization of a glutathione S-transferase gene from Ginkgo biloba.

Glutathione S-transferases (GSTs) play an important role in the response of plants to changing environmental conditions. Here, we report the cloning of the GST gene for GST from Ginkgo biloba, a native medicinal plant species in China, by rapid amplification of cDNA ends (RACE). The full-length cDNA (designated as GbGST) was 1008 bp and contained a 684 bp open reading frame (ORF) encoding a polypeptide of 228 amino acids. The genomic sequence of GbGST was also obtained. Semi-quantitative RT-PCR analysis revealed that GbGST expressed in all tested tissues of G. biloba, including leaf, root and stem and the expression of GbGST could be induced by UV, MJ and drought treatments, suggesting that GbGST was potentially involved in plant's stress tolerance. To our knowledge, this is the first GST cDNA cloned from Ginkgoaceae. Based on comparative analyses of amino acid sequence, phylogeny, predicted three-dimensional structure together with the gene structure, the GbGST should be classified into the tau class.     

Effect of Ginkgo biloba extract on rat hepatic microsomal CYP1A activity: role of ginkgolides, bilobalide, and flavonols

The present study investigated the in vitro effect of Ginkgo biloba extracts and some of the individual constituents (ginkgolides, bilobalide, and flavonols such as kaempferol, quercetin, isorhamnetin, and their glycosides) on CYP1A-mediated 7-ethoxyresorufin O-dealkylation in hepatic microsomes isolated from rats induced with beta-naphthoflavone. G. biloba extract competitively inhibited CYP1A activity, with an apparent Ki value of 1.6 +/- 0.4 microg/mL (mean +/- SE). At the concentrations present in the G. biloba extracts, ginkgolides A, B, C, and J and bilobalide did not affect CYP1A activity, whereas kaempferol (IC50 = 0.006 +/- 0.001 microg/mL, mean +/- SE), isorhamnetin (0.007 +/- 0.001 microg/mL), and quercetin (0.050 +/- 0.003 microg/mL) decreased this activity. The monoglycosides (1 and 10 microg/mL) and diglycosides (10 microg/mL) of kaempferol and quercetin but not those of isorhamnetin also inhibited CYP1A activity. The order of inhibitory potency was kaempferol approximately equal to isorhamnetin > quercetin, and for each of these flavonols the order of potency was aglycone > monoglycoside > diglycoside. In summary, G. biloba extract competitively inhibited rat hepatic microsomal CYP1A activity, but the effect was not due to ginkgolides A, B, C, or J, bilobalide, kaempferol, quercetin, isorhamnetin, or the respective flavonol monoglycosides or diglycosides.     

Molecular cloning and function analysis of an anthocyanidin synthase gene from Ginkgo biloba, and its expression in abiotic stress responses

Anthocyanidin synthase (ANS, leucoanthocyanidin oxygenase), a 2-oxoglutarate iron-dependent oxygenase, catalyzed the penultimate step in the biosynthesis of the anthocyanin class of flavonoids, from the colorless leucoanthocyanidins to the colored anthocyanidins. The full-length cDNA and genomic DNA sequences of ANS gene (designated as GbANS) were isolated from Ginkgo biloba for the first time. The full-length cDNA of GbANS contained a 1062-bp open reading frame (ORF) encoding a 354-amino-acid protein. The genomic DNA analysis showed that GbANS gene had three exons and two introns. The deduced GbANS protein showed high identities to other plant ANSs. The conserved amino acids (H-X-D) ligating ferrous iron and residues (R-X-S) participating in 2-oxoglutarate binding were found in GbANS at the similar positions like other ANSs. Southern blot analysis indicated that GbANS belonged to a multi-gene family. The expression analysis by real-time PCR showed that GbANS expressed in a tissue-specific manner in G. biloba. GbANS was also found to be up-regulated by all of the six tested abiotic stresses, UV-B, abscisic acid, sucrose, salicylic acid, cold and ethylene, consistent with the promoter region analysis of GbANS. The recombinant protein was successfully expressed in E. coli strain with pET28a vector. The in vitro enzyme activity assay by HPLC indicated that recombinant GbANS protein could catalyze the formation the cyanidin from leucocyanidin and conversion of dihydroquercetin to quercetin, suggesting GbANS is a bifunctional enzyme within the anthocyanidin and flavonol biosynthetic pathway.     

Cloning and characterization of 1-deoxy-D-xylulose 5-phosphate synthase from Streptomyces sp. Strain CL190, which uses both the mevalonate and nonmevalonate pathways for isopentenyl diphosphate biosynthesis

In addition to the ubiquitous mevalonate pathway, Streptomyces sp. strain CL190 utilizes the nonmevalonate pathway for isopentenyl diphosphate biosynthesis. The initial step of this nonmevalonate pathway is the formation of 1-deoxy-D-xylulose 5-phosphate (DXP) by condensation of pyruvate and glyceraldehyde 3-phosphate catalyzed by DXP synthase. The corresponding gene, dxs, was cloned from CL190 by using PCR with two oligonucleotide primers synthesized on the basis of two highly conserved regions among dxs homologs from six genera. The dxs gene of CL190 encodes 631 amino acid residues with a predicted molecular mass of 68 kDa. The recombinant enzyme overexpressed in Escherichia coli was purified as a soluble protein and characterized. The molecular mass of the enzyme was estimated to be 70 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 130 kDa by gel filtration chromatography, suggesting that the enzyme is most likely to be a dimer. The enzyme showed a pH optimum of 9.0, with a V(max) of 370 U per mg of protein and K(m)s of 65 microM for pyruvate and 120 microM for D-glyceraldehyde 3-phosphate. The purified enzyme catalyzed the formation of 1-deoxyxylulose by condensation of pyruvate and glyceraldehyde as well, with a K(m) value of 35 mM for D-glyceraldehyde. To compare the enzymatic properties of CL190 and E. coli DXP synthases, the latter enzyme was also overexpressed and purified. Although these two enzymes had different origins, they showed the same enzymatic properties.