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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
J Am Chem Soc. Author manuscript; available in PMC Aug 10, 2012.
Published in final edited form as:
PMCID: PMC3162038
NIHMSID: NIHMS312876

Pseudoglycosyltransferase Catalyzes Non-Glycosidic C-N Coupling in Validamycin A Biosynthesis

Abstract

Glycosyltransferases are ubiquitous in nature. They catalyze a glycosidic bond formation between sugar donors and sugar or non-sugar acceptors to produce oligo/polysaccharides, glycoproteins, glycolipids, glycosylated natural products, and other sugar-containing entities. However, a trehalose 6-phosphate synthase-like protein has been found to catalyze an unprecedented non-glycosidic C-N bond formation in the biosynthesis of the aminocyclitol antibiotic validamycin A. This dedicated ‘pseudoglycosyltransferase’ catalyzes a condensation between GDP-valienol and validamine 7-phosphate to give validoxylamine A 7′-phosphate with net retention of the ‘anomeric’ configuration of the donor cyclitol in the product. The enzyme operates in sequence with a phosphatase, which dephosphorylates validoxylamine A 7′-phosphate to validoxylamine A.

INTRODUCTION

Glycosyltransferases (EC 2.4.x.y) are amongst the most abundant enzymes in nature and play a central role in structural and physiological aspects of living organisms. They catalyze the transfer of a sugar moiety from an activated donor sugar onto sugar or non-sugar acceptors, resulting in sugar-containing products, such as oligo/polysaccharides, glycoproteins, glycolipids, glycosylated natural products, and many others. To date, there are more than 44,000 genes which encode proteins across the 92 known families of glycosyltransferases listed in the NCBI database, and this number is fast growing.1 However, only a fraction of them have actually been functionally characterized.

Whereas all glycosyltransferases characterized to date catalyze glycosylation reactions, some glycosyltransferase-like proteins have recently been implicated in non-glycosidic C-N bond formations in natural products biosyntheses. Examples include ValL/VldE, trehalose 6-phosphate synthase-like proteins involved in the biosynthesis of the crop protectant validamycin A (1) (Figure 1) in several Streptomyces hygroscopicus sp.,2, 3 and AcbS/GacS, glycosyltransferase-like enzymes proposed to be involved in the biosynthesis of the anti-diabetic drug acarbose (2) in Actinoplanes sp. SE50/110 and Streptomyces glaucescens.4, 5

Figure 1
Chemical structures of validamycin A, acarbose, and related natural products

Validamycin A (1) is a bacterial-derived natural product that consists of an aminocyclitol moiety, validoxylamine A, and glucose. The validoxylamine A core structure is built of an unsaturated cyclitol and a saturated cyclitol, which are connected through a shared single nitrogen bridge. Previous investigations using isotopically labeled precursors and biochemical experiments demonstrated that these cyclitol units are derived from sedoheptulose 7-phosphate via a set of cyclic intermediates,69 and further downstream the pathway requires a coupling reaction between the two cyclitols. However, the biochemical mechanism underlying this coupling reaction had been poorly understood, stimulating speculations around how the nitrogen bridge in this compound is formed.2, 3, 6 Here we describe the first biochemical evidence of an unprecedented non-glycosidic C-N coupling in validamycin biosynthesis catalyzed by a trehalose 6-phosphate synthase-like enzyme, VldE. This dedicated ‘pseudoglycosyltransferase’ recognizes GDP-valienol and validamine 7-phosphate as substrates and operates in sequence with a phosphatase (VldH), which dephosphorylates the VldE product validoxylamine A 7′-phosphate to validoxylamine A, a process that is similar to that of trehalose biosynthesis by trehalose 6-phosphate synthase and trehalose 6-phosphate phosphatase.

RESULTS

Validation of VldB as a cyclitol nucleotidyltransferase

Biosynthetic gene clusters of validamycin have been identified in a number of Streptomyces strains, e.g., S. hygroscopicus subsp. jinggangensis 5008 and S. hygroscopicus subsp. limoneus KCCM 11405 (IFO 12704).2, 3 Direct comparison of the former (the val cluster) and the latter (the vld cluster) has shown that both clusters contain almost identical sets of genes necessary for the biosynthesis of validamycin A (Figure 2A),10 suggesting that both producers may be closely related. As part of our investigation on validamycin biosynthesis in S. hygroscopicus 5008, we recently established the role of ValB, a cyclitol nucleotidyltransferase that catalyzes the conversion of valienol 1-phosphate to GDP-valienol.11 However, its homologue (VldB) from S. hygroscopicus subsp. limoneus KCCM 11405 was reported as a glucose 1-phosphate uridylyltransferase that catalyzes the formation of UDP-glucose.12 To validate the function of VldB as a cyclitol nucleotidyltransferase, the recombinant protein was tested using synthetic valienol 1-phosphate11 as substrate in the presence of all five naturally occuring NTPs (ATP, GTP, CTP, dTTP, and UTP). The results showed that VldB, in reactions containing 1 mM Mg2+, effectively catalyzes the conversion of valienol 1-phosphate to GDP-valienol and less effectively to ADP-valienol or other NDP derivatives (Figure 2C). However, interestingly, in reactions containing 10 mM Mg2+ the enzyme more effectively used ATP as nucleotidyl donor than GTP or other NTPs (Figure 2C). Although the reason for this nucleotidyl donor preference switch is unclear, the results confirm our hypothesis that VldB is a cyclitol nucleotidyltransferase that plays a critical role in supplying NDP-valienol, most likely GDP-valienol, in validamycin biosynthesis. Therefore, the formation of validoxylamine A may involve a coupling reaction between GDP-valienol and an amino-bearing cyclitol counterpart.

Figure 2
Biosynthetic gene clusters for validamycin A and characterization of VldB. A, Genetic organizations of the validamycin biosynthetic gene clusters from S. hygroscopicus 5008 and S. hygroscopicus subsp. limoneus; B, SDS-PAGE of recombinant VldB, VldE, VldH, ...

Syntheses of validamine (16) and validamine 7-phosphate (21)

To explore the above-proposed coupling reaction we synthesized two potential aminocyclitol substrates, validamine (16) and validamine 7-phosphate (21). The two compounds were prepared from validoxylamine A (8) as shown in Scheme 1. Thus, complete benzylation of validoxylamine A using BnBr and NaH furnished perbenzylated validoxylamine A (9), which was then treated with NBS to give a mixture of four products, tetrabenzylvalienone (10), tetrabenzylvalidone (11), tetrabenzylvalidamine (12) and tetrabenzylvalienamine (13). The benzylated validamine and valienamine products were chromatographically separated from their keto-analogues, and the mixture was subjected to Cbz-protection to give 1-Cbz-2,3,4,7-tetrabenzylvalienamine (14) and 1-Cbz-2,3,4,7-tetrabenzylvalidamine (15). The compounds were then chromatographically separated, and complete deprotection of 15 by hydrogenolysis afforded validamine (16) in a quantitative yield.

Scheme 1
Synthesis of validamine (16) and validamine 7-phosphate (21)

For the synthesis of validamine 7-phosphate (21), 15 was regioselectively debenzylated and acetylated using ZnCl2, Ac2O and AcOH, followed by a deacetylation step to furnish a free alcohol product, 1-Cbz-2,3,4-tribenzylvalidamine (19). The alcohol 19 was then treated with dibenzyl diisopropyl phosphoramidite in the presence of 1H-tetrazole to form the phosphite intermediate, which was readily oxidized by m-chloroperoxybenzoic acid (m-CPBA) to afford a phosphate product (20). Finally, debenzylation was achieved by hydrogenolysis with Pd/C to give the desired product validamine 7-phosphate (21).

Characterization of the glycosyltransferase-like protein VldE

As validoxylamine A is structurally similar to trehalose, the trehalose 6-phosphate synthase-like protein VldE was suspected to catalyze this coupling reaction. Trehalose 6-phosphate synthases (EC 2.4.1.15) catalyze the synthesis of trehalose 6-phosphate from UDP-glucose and glucose 6-phosphate with net retention of the anomeric configuration of the donor sugar in the product (Scheme S1). The notion that its homologue is involved in validoxylamine formation is intriguing, but raises fundamental questions as to how a glycosyltransferase-like enzyme catalyzes a condensation of two non-sugar molecules. To gain insight into the catalytic function of VldE and its involvement in the coupling reaction, we cloned and heterologously expressed the gene in E. coli and characterized its biochemical activity. As the potential donor substrate GDP-valienol is relatively unstable, the compound was prepared in-situ using VldB, valienol 1-phosphate, and GTP. In addition to these components, recombinant VldE and the synthetic amino-bearing cyclitol, either validamine or validamine 7-phosphate, were included in the reaction mixture (Scheme 2). The results showed that only the incubation containing GDP-valienol and validamine 7-phosphate gave a product (m/z 416 [M+H]+), which is consistent with the molecular mass of the expected product validoxylamine A 7′-phosphate (Figures 3G, S2 and S3). MS/MS analysis of the product gave fragment ions consistent with those observed in authentic validoxylamine A, except that most of the fragment ions of validoxylamine A 7′-phosphate are 80 amu higher than those of validoxylamine A, indicating the presence of a phosphate moiety in the former compound (Figure 3L). HR-ESTOFMS analysis of the product revealed a molecular formula of C14H26NO11P, consistent with that of validoxylamine A 7′-phosphate. Reactions containing validamine did not give any corresponding product, suggesting that VldE does not recognize an unphosphorylated substrate (Figure S3).

Figure 3
HPLC and ESI-MS analyses of VldB, VldE, and VldH reaction products. A, HPLC traces of reaction mixture containing VldB, valienol 1-phosphate, and GTP. GDP-valienol (cyan block arrow) eluted in two tautomeric forms; B, HPLC trace of reaction mixture containing ...
Scheme 2
Biosynthetic pathway to validamycin A and characterization of VldB, VldE, and VldH.

Synthesis of validoxylamine A 7′-phosphate (26) and confirmation of the VldE product

To further confirm the product of VldE, we synthesized validoxylamine A 7′-phosphate (26) and compared the VldE product with the synthetic compound. The synthesis was carried out using validoxylamine A (8), which was prepared from acid hydrolysis of validamycin A (1), as starting material. Validoxylamine A 7′-phosphate (26) was previously synthesized by Davies and co-workers in 10 steps starting from 8.13 However, using a modified procedure, we were able to obtain 26 from 8 in 5 steps (Scheme 3). Thus, 8 was treated with benzaldehyde dimethyl acetal in the presence of p-TsOH to selectively protect the 4′- and 7′-hydroxy groups. The product was then perbenzylated with benzyl bromide and NaH to give 28. Compound 28 was treated with DIBAL-H for selective acetal opening to give 29. The free alcohol functionality was then phosphorylated as described for the synthesis of validamine 7-phosphate (21) and the product was debenzylated using BBr3 at −40 °C to give validoxylamine A 7′-phosphate (26). This compound was used as reference for the assay of VldE catalysis. HPLC, MS, and MS/MS analyses of the VldE product gave chromatographic and spectral data identical to those of the synthetic compound, confirming its identity as validoxylamine A 7′-phosphate (26).

Scheme 3
Synthesis of validoxylamine A 7′-phosphate (26).

Kinetic properties of VldE

The kinetic properties of VldE were determined using a pyruvate kinase/lactate dehydrogenase (PK/LDH) coupled colorimetric assay (Scheme 2).14 Reactions were initiated by addition of a saturating amount of either GDP-valienol (25) or validamine 7-phosphate (21), and the oxidation of NADH to NAD+, which is stoichiometrically equivalent to the production of GDP, was measured by the decrease in A340. Compound 25 was obtained from valienol 1-phosphate (24) and GTP catalyzed by VldB and the concentration was determined using a coupled colorimetric assay containing inorganic pyrophosphatase and purine nucleoside phosphorylase (PNP) [EnzChek Pyrophosphate Assay Kit15 (Molecular Probes)].11 For the kinetic studies, a protein concentration of 1–3 μM, a pH of 7.5 and a reaction time of 1 to 3 min, which showed linear decrease of absorbance, were chosen. The apparent kinetic values, obtained from Hanes-Woolf plots, were Km 308.0 ± 34.5 μM and kcat 7.0 ± 0.4 min−1 for 21 and Km 59.7 ± 11.9 μM and kcat 7.0 ± 0.7 min−1 for 25 (Figure 4).

Figure 4
Kinetic studies of VldE. A, Michaelis-Menten curve for validamine 7-phosphate; B, Michaelis-Menten curve for GDP-valienol.

VldE is a dedicated pseudoglycosyltransferase

Whereas VldE is genetically similar to trehalose 6-phosphate synthases, it does not have any trehalose 6-phosphate synthase activity. Incubation of the enzyme with UDP- or GDP-glucose and glucose 6-phosphate did not give any product (Figure S5). The same result was observed when only one of the cyclitol derivatives was replaced by its glucose analogue (Figure S6). These results suggest that VldE is a dedicated pseudoglycosyltransferase that only recognizes GDP-valienol and validamine 7-phosphate as substrates, and not a trehalose 6-phosphate synthase with relaxed substrate specificity.

The E. coli trehalose 6-phosphate synthase OtsA does not recognize GDP-valienol and validamine 7-phosphate as substrates

To investigate if a trehalose 6-phosphate synthase can alternatively catalyze the formation of validoxylamine A 7′-phosphate, we cloned and expressed in E. coli the trehalose 6-phosphate synthase gene otsA and characterized its catalytic function. While the recombinant OtsA can catalyze the formation of trehalose 6-phosphate, the enzyme was not able to catalyze the formation of validoxylamine A 7′-phosphate (Figure S7). Replacements of GDP-valienol with UDP-/GDP-glucose or validamine 7-phosphate with glucose 6-phosphate did not give any hybrid products (Figure S8), indicating that OtsA does not recognize any activated cyclitols as substrate.

VldE is not a validoxylamine A glycosyltransferase

Interestingly, VldE has previously been reported as a glycosyltransferase that catalyzes the glucosylation of validoxylamine A to give validamycin A.16 However, a wealth of evidence is available to conclude that another glycosyltransferase, ValG (VldK), is responsible for this conversion.2, 17 Nevertheless, we explored the reported glycosyltransferase activity of VldE by incubating the enzyme with validoxylamine A and UDP-glucose and analyzing the product by ESIMS. No validamycin A could be identified in this reaction (Figure S9). The results confirmed the distinct role of VldE as a dedicated validoxylamine A 7′-phosphate synthase and not a validoxylamine A glycosyltransferase.

The role of the phosphatase VldH

The production of validoxylamine A 7′-phosphate by VldE suggests that a dephosphorylation reaction is necessary downstream in the pathway to provide a phosphate-free substrate for the glycosyltransferase ValG (VldK).2, 17 The only candidate phosphatase gene within the cluster is vldH, which encodes a protein that has high identity with proteins from the haloacid dehalogenase-like superfamily, which includes L-2-haloacid dehalogenase, epoxide hydrolase, phosphoserine phosphatase, phosphomannomutase, phosphoglycolate phosphatase, P-type ATPase, and many others.18 Interestingly, the G+C content of the vldH gene is only 60%, relatively low compared to that of other genes in the cluster (68–72%).

To investigate the function of VldH as a phosphatase, the recombinant protein was produced (Figure 2B) and co-incubated with VldB and VldE in the presence of valienol 1-phosphate, GTP, and validamine 7-phosphate (Scheme 2). The results showed that addition of VldH to the mixture resulted in a phosphate-free product, validoxylamine A (m/z 336 [M+H]+) (Figures 3D and 3I). In addition, MS/MS analysis of the product gave fragment ions identical to those of authentic validoxylamine A (Figures 3N and 3O), providing compelling evidence for the critical roles of VldB, VldE, and VldH in the formation of validoxylamine A. However, in contrast to VldE, VldH appears to have broader substrate permissiveness, as it could also convert trehalose 6-phosphate to trehalose (Figure S10). On the other hand, the trehalose 6-phosphate phosphatase OtsB, which was recombinantly produced in E. coli (Figure 2B), does not dephosphorylate validoxylamine A 7′-phosphate (Figure S4), suggesting that the role of VldH in validamycin A biosynthesis cannot be assumed by OtsB.

DISCUSSION

The biochemical analysis of VldB, VldE, and VldH and the establishment of their roles in validoxylamine A biosynthesis have opened up a new dimension in our understanding of pseudoglycoside biosynthesis. Particularly, the involvement of VldE and VldH, which resemble OtsA and OtsB, in validoxylamine A formation is evolutionarily and mechanistically intriguing. VldE might have evolved from an ancestral trehalose 6-phosphate synthase to the extent that it no longer recognizes activated sugars as substrates, but still retains some of the catalytic properties, including the net retention of the anomeric configuration of the product.

In addition, the use of the aminocyclitol unit as pseudosugar acceptor by VldE to form N-pseudoglycosyl linkage is a significant departure from the OtsA catalyzed reaction. In comparison to the O-glycosyltransferases such as OtsA, glycogen synthase, cellulose synthase, and others, N-glycosyltransferase enzymes are less abundant, but play significant roles, in biological systems. Among the known N-glycosyltransferases are the eukaryotic oligosaccharyltransferase (OST), which catalyzes asparagine-linked glycosylation of proteins, and UGT72B1, a bifunctional N- and O-glycosyltransferase that is involved in xenobiotic metabolism in plants.19, 20 Other N-glycosyltransferases that are involved in natural products biosynthesis include the indolocarbazole N-glycosyltransferases RebG and StaG and the ansamitocin N-glycosyltransferase Asm25.2123 However, most, if not all, of these enzymes catalyze the formation of glycosidic bonds with inversion of configuration at the anomeric center of the sugar donors.

Whereas inverting glycosyltransferases generally operate through a direct displacement, SN2-like mechanism, retaining glycosyltransferases are believed to function via either a two-step double-displacement mechanism or an internal return (SNi)-like mechanism, which is also known as a DN*ANss ion pair mechanism.24 Structural studies of OtsA suggest that its activity is likely to occur via the latter mechanism.13 As VldE shares a high number of conserved active site residues with OtsA, they may have similar catalytic mechanisms. However, the question remains how a glycosyltransferase-like enzyme catalyzes a condensation of two non-glycosyl units with retention of stereochemistry. Whereas the detailed mechanism underlying this unique capability is yet to be determined, we believe that the olefinic moiety of GDP-valienol plays a critical role in facilitating the coupling reaction. This is somewhat analogous to the involvement of the allylic moiety in farnesyl diphosphate (FPP) synthase-catalyzed reactions (Figure 5).25, 26 In both VldE and FPP synthase, and also in glycosyltransferases, a nucleophilic substitution reaction takes place at a carbon center with a diphosphate or nucleotidyl diphosphate acting as leaving group. Therefore, it may be proposed that at the enzyme active site the π-electrons of the unsaturated cyclitol resonate to form stabilized short-lived ion pair intermediates similar to the oxocarbenium-ion like transition states and the diphosphate group acts as a base to deprotonate the ammonium (−NH3+) group of validamine 7-phosphate. The primary amine then attacks the ‘pseudo-anomeric’ carbon, leading to a product with retained configuration.

Figure 5
Proposed catalytic mechanism for VldE and its similarity to that of farnesyl diphosphate (FPP) synthase and trehalose 6-phosphate synthase (OtsA). A, Catalytic mechanism of FPP synthases; B, Proposed SNi-like mechanism for OtsA; C, Proposed catalytic ...

Adoption of the SNi-like mechanism in VldE catalysis, however, may have a consequence, as the ion pair intermediate could collapse to give back starting material, which may affect the rate constant of the enzyme.24 In fact, we observed a rather low turnover rate of VldE as evidenced from the MS data and its kinetic values (kcat/Km 0.023 min−1•μM−1 for validamine 7-phosphate and kcat/Km 0.12 min−1•μM−1 for GDP-valienol). However, this observation might be circumstantial, therefore, a more detailed study is necessary to determine the catalytic mechanism of this unusual enzyme. Nevertheless, the results of the present study suggest that certain glycosyltransferase-like enzymes are capable of catalyzing coupling reactions between non-glycosidic allylic cation-forming donors and amino-bearing acceptors. Some, if not all, of them are similar to members of certain groups of glycosyltransferases, e.g., VldE to trehalose 6-phosphate synthases (GT20 family) and AcbS/GacS to glycogen synthases (GT5 family),4, 5 suggesting that this poorly recognized class of enzymes may have evolved from or shared ancestral traits with known glycosyltransferases. Similar glycosyltransferase-like enzymes may be involved in the biosynthesis of other natural products, e.g., the adiposins (3), salbostatin (4), the trestatins (5), the amylostatins (6), and the pyralomicins (7) (Figure 1).27 Therefore, the results have not only laid a foundation for understanding and discovering new classes of pseudoglycosyltransferases, but may also facilitate the development of new tools for generating carbohydrate mimetics and redesigning glycoconjugates and glycosylated natural products.

Experimental Section

General

All chemical reactions were performed under an argon or nitrogen atmosphere employing oven-dried glassware. Analytical thin-layer chromatography (TLC) was performed using silica plates (60 Å) with a fluorescent indicator (254 nm), which were visualized with a UV lamp and ceric ammonium molybdate (CAM) solution. Chromatographic purification of products was performed on silica gel (60 Å, 72–230 mesh). Optical rotations were measured on a Jasco P1010 polarimeter (100 mm cells were used) at the sodium D line. Proton NMR spectra were recorded on Bruker 300 or 400 MHz spectrometers. Proton chemical shifts are reported in ppm (δ) relative to the residual solvent signals as the internal standard (CDCl3: δH 7.26; D2O: δH 4.79). Multiplicities in the 1H NMR spectra are described as follows: s = singlet, bs = broad singlet, d = doublet, bd = broad doublet, t = triplet, bt = broad triplet, q = quartet, m = multiplet; coupling constants are reported in Hz. Carbon NMR spectra were recorded on a Bruker 300 (75 MHz) spectrometer with complete proton decoupling. Carbon chemical shifts are reported in ppm (δ) relative to the residual solvent signal as the internal standard (CDCl3: δC 77.16), or with sodium 2,2-dimethylsilapentane-5-sulphonate (DSS) (δ 0.0) as an external standard. Phosphorus NMR spectra were recorded on a Bruker 400 (122 MHz) spectrometer with complete proton decoupling. Phosphorus chemical shifts are reported in ppm (δ) relative to an 85% H3PO4 (δ 0.0) external standard. Low-resolution electrospray ionization (ESI) mass spectra were recorded on a ThermoFinnigan liquid chromatograph-ion trap mass spectrometer, and high-resolution electrospray mass spectra were recorded on a Waters/Micromass LCT spectrometer. Size exclusion chromatography was done on Sephadex LH-20 (Pharmacia).

Chemical Synthesis of Validamine (16) and Validamine 7-phosphate (21)

Benzyl ((1S,2S,3S,4R,5R)-2,3,4-tris(benzyloxy)-5-((benzyloxy)methyl)cyclohexyl)-carbamate (15)

To a solution of (1S,4R,5S,6S)-4,5,6-tris(benzyloxy)-3-((benzyloxy)methyl)-N-((1S,2S,3S,4R,5R)-2,3,4-tris(benzyloxy)-5-((benzyloxy)methyl)-cyclohexyl)cyclohex-2-enamine (9)28 (1.77 g, 1.676 mmol) in CH3CN/H2O (4:1; 40 mL) was added NBS (447 mg, 2.51 mmol), and the reaction mixture was stirred for 15 h at rt. The reaction mixture was diluted with water (150 mL) and EtOAc (150 mL), and the aqueous layer extracted with EtOAc (150 mL). The combined organic layer was washed with brine (150 mL), dried over MgSO4, filtered and concentrated under vacuum to give crude product. Column chromatography (silica gel, Hex-EtOAc = 3:1 and then EtOAc-MeOH = 4:1) of the product yielded a mixture of amine products (626 mg).

Benzyl chloroformate (439 mg, 2.57 mmol; 50% solution in toluene) in CHCl3 (8 mL) was slowly added to a solution of the above amine products (626 mg) and NaHCO3 (491 mg, 5.85 mmol) in MeOH (16 mL). The reaction mixture was stirred for 15 h at rt and diluted with EtOAc (150 mL) and water (150 mL). The organic layer was washed with brine (150 mL), dried over MgSO4, filtered and concentrated under vacuum. Column chromatography (silica gel, Hex-EtOAc = 5:1 and then toluene-EtOAc = 10:1) yielded 15 (8.7 mg, 17 % from 9); [α]20D= +47.1° (c 0.72, CHCl3); 1H NMR (300 MHz, CDCl3): δ 7.25–7.42 (m, 25H), 5.17 (d, J = 1.5, 2H), 5.12 (m, 1H), 4.98 (d, J = 10.5 Hz, 1H), 4.92 (d, J = 10.8 Hz, 1H), 4.82 (d, J = 10.8 Hz, 1H), 4.75 (d, J = 11.4 Hz, 1H), 4.60 (d, J = 11.7 Hz, 1H), 4.59 (d, J = 10.8 Hz, 1H), 4.48 (s, 2H), 3.55–3.66 (m, 3H), 3.48 (dd, J = 9.0, 2.4 Hz, 1H), 2.30 (bd, J = 14.7 Hz, 1H), 1.95 (m, 1H), 1.62 (m, 1H); 13C NMR (75 MHz, CDCl3): δ 156.5, 138.94, 138.85, 138.6, 138.0, 136.6, 128.6, 128.5, 128.4, 128.19, 128.15, 128.08, 127.97, 127.96, 127.8, 127.6, 129.6, 83.8, 80.9, 80.3, 75.8, 75.3, 73.2, 71.7, 69.8, 66.8, 48.1, 37.5, 28.7. HRMS (ESI) m/z 672.3338 (calcd for C43H46NO6 [M + H]+: 672.3325).

Validamine (6)

A mixture of 15 (15 mg, 22.33 μmol) and 10% Pd/C (15.0 mg) in MeOH (1.8 mL) and EtOAc (1.2 mL) containing 12 M HCl (30 μL) was hydrogenated under atmospheric pressure for 20 h. The reaction mixture was filtered through Celite and a cellulose syringe filter, washed with MeOH, and concentrated. The crude product was further purified with silica gel column chromatography (CHCl3-MeOH-25% NH4OH = 2:3:1) to give the pure product (3.7 mg, 95%).

To compare its physicochemical properties with those reported in the literature,29, 30 the product (1.0 mg, 5.6 μmol) was acetylated using Ac2O (0.12 mL) in the presence of DMAP (1.4 mg, 11.3 μmol) in pyridine (0.8 mL) and the product was purified by silica gel column chromatography (CHCl3-MeOH = 15:1) to give pentaacetylvalidamine (17) (1.1 mg, 52%). The physicochemical data for the product are in agreement with those reported in the literature.29, 30 [α]19D= +59.1° (c 0.19, CHCl3) (lit.30 [α]20D= +60.2° (c 0.6, CHCl3)); 1H NMR (300 MHz, CDCl3): δ 5.55 (m, 1H), 5.21 (dd, J = 9.9, 10.2 Hz, 1H), 4.94–5.02 (m, 2H), 4.53 (m, 1H), 4.13 (dd, J = 4.5, 11.4 Hz, 1H), 3.90 (dd, J = 3.6, 11.1 Hz, 1H), 2.14–2.24 (m, 2H), 2.06 (s, 3H), 2.05 (s, 3H), 2.03 (s, 3H), 2.02 (s, 3H) (x2), 1.60–1.71 (m, 1H); 13C NMR (75 MHz, CDCl3): δ 170.9, 170.53, 170.47, 170.0, 169.7, 71.7, 71.5, 63.3, 47.0, 35.3, 23.7, 20.88, 20.82, 20.76 (x 3).

((1R,2R,3S,4S,5S)-2,3,4-tris(benzyloxy)-5-(((benzyloxy)carbonyl)amino)-cyclohexyl)methyl acetate (18)

To a solution of 15 (135 mg, 0.20 mmol) in Ac2O/AcOH (2:1; 2 mL) was added ZnCl2 (328 mg, 2.4 mmol), and the reaction mixture was stirred for 5 h at rt. The reaction mixture was diluted with water (100 mL) and EtOAc (100 mL), and the organic layer washed with sat. aq. Na2CO3 (3 × 50 mL). The organic layer was dried over MgSO4, filtered and concentrated under reduced pressure to give a crude product. Column chromatography (silica gel, Hex-EtOAc = 3:1) of the product yielded 18 (82 mg, 66%); [α]20D= +29.3° (c 1.19, CHCl3); 1H NMR (300 MHz, CDCl3): δ 7.27–7.38 (m, 20H), 5.14 (s, 2H), 5.10 (m, 1H), 4.95 (d, J = 10.8 Hz, 1H), 4.90 (d, J = 10.8 Hz, 1H), 4.78 (d, J = 10.5 Hz, 1H), 4.71 (d, J = 11.4 Hz, 1H), 4.56 (d, J = 11.4 Hz, 2H), 4.23–4.28 (m, 2H), 4.13 (dd, J = 11.1, 2.7 Hz, 1H), 3.57–3.68 (m, 2H), 3.38 (dd, J = 8.1, 10.5 Hz, 1H), 2.26 (bd, J = 14.1 Hz, 1H), 2.03 (m, 1H), 2.00 (s, 3H), 1.37 (bt, J = 14.7 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 170.9, 156.5, 138.71, 138.33, 137.83, 136.5, 128.7, 128.54, 128.49, 128.18, 128.15, 128.09, 128.0, 127.95, 127.87, 127.7, 83.6, 80.8, 79.9, 75.8, 75.2, 71.7, 64.4, 66.9, 47.8, 36.2, 28.4, 20.9. HRMS (ESI) m/z 624.2982 (calcd for C38H42NO7 [M + H]+: 624.2961).

Benzyl ((1S,2S,3S,4R,5R)-2,3,4-tris(benzyloxy)-5-(hydroxymethyl)-cyclohexyl)carbamate (19)

To a solution of 18 (82 mg, 0.13 mmol) in MeOH (6.0 mL) was added 30% NaOMe in MeOH (2.4 ml) via syringe at 0 °C, and the reaction mixture was stirred for 4 h at rt. The reaction was quenched by addition of water (3.0 mL) and sat. aq. NH4Cl (5.0 mL), and diluted with water (50 mL) and EtOAc (50 mL). The organic layer was washed with brine (50 mL), dried over MgSO4, filtered and concentrated under reduced pressure to give a crude product. Column chromatography of the product (silica gel, Hex-EtOAc = 1:1) yielded 19 (68 mg, 90%); [α]20D= +59.3° (c 1.44, CHCl3); 1H NMR (300 MHz, CDCl3): δ 7.31–7.37 (m, 20H), 5.08–5.18 (m, 3H), 4.96 (d, J = 10.8 Hz, 1H), 4.94 (d, J = 11.1 Hz, 1H), 4.78 (d, J = 10.8 Hz, 1H), 4.68 (dd, J = 8.7, 11.1 Hz, 2H), 4.55 (d, J = 11.4 Hz, 1H), 4.23 (bs, 1H), 3.55–3.72 (m, 4H), 3.41 (bt, J = 8.7 Hz, 1H), 2.17 (bd, J = 14.4 Hz, 1H), 1.85 (m, 1H), 1.31 (td, J = 13.7, 2.7 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 156.5, 138.8, 138.4, 137.9, 136.5, 128.7, 128.55, 128.50, 128.3, 128.2, 128.11, 128.05, 128.02, 127.9, 127.7, 83.8, 81.6, 81.0, 75.8, 75.1, 71.7, 66.9, 64.3, 47.9, 38.6, 28.2. HRMS (ESI) m/z 582.2853 (calcd for C36H40NO6 [M + H]+: 582.2856).

Benzyl ((1S,2S,3S,4R,5R)-2,3,4-tris(benzyloxy)-5-(((bis(benzyloxy)phosphoryl)oxy)methyl)cyclohexyl)carbamate (20)

To a solution of 19 (42 mg, 72.7 μmol) and 1H-tetrazole (12.6 mg, 0.18 mmol) in CH2Cl2 (2 mL) was added dibenzyl N,N-diisopropylphosphoramidite (50 mg, 0.144 mmol) in CH2Cl2 (1 mL) at rt under Ar, and the reaction mixture was stirred for 1 h at rt. The reaction mixture was cooled to 0 °C and m-CPBA (35.6 mg (70%), 0.144 mmol) was added. After 10 min stirring at 0 °C the reaction mixture was diluted with EtOAc (25 mL) and washed with NaHCO3 (2 × 25 mL) and brine (3 × 25 mL). The organic layer was dried over Na2SO4, filtered and concentrated under reduced pressure. Column chromatography of the product (silica gel, Hex-EtOAc = 3:2) yielded 20 (41 mg, 68%); [α]19D= +27.1° (c 0.72, CHCl3); 1H NMR (300 MHz, CDCl3): δ 7.19–7.37 (m, 30H), 5.10 (s, 2H), 5.02 (d, J = 9.0 Hz, 5H), 4.90 (d, J = 10.8 Hz, 1H), 4.82 (d, J = 10.8 Hz, 1H), 4.73 (d, J = 10.8 Hz, 1H), 4.67 (d, J = 11.7 Hz, 1H), 4.51–4.56 (m, 2H), 4.25–4.31 (m, 1H), 4.13 (bs, 1H), 3.95–3.99 (m, 1H), 3.45–3.58 (m, 2H), 3.29 (bt, J = 8.4 Hz, 1H), 2.15 (d, J = 14.4 Hz, 1H), 1.86 (m, 1H), 1.27 (bt, J = 13.8, 1H); 13C NMR (75 MHz, CDCl3): δ 156.5, 138.8, 138.5, 137.9, 136.5, 136.03 (d, Jc-p = 1.8 Hz), 135.95 (d, Jc-p = 2.0 Hz), 128.75, 128.71, 128.59, 128.53, 128.49, 128.3, 128.22, 128.12, 128.08, 128.02, 127.9, 127.8, 127.7, 83.5, 80.5, 79.3, 75.8, 75.4, 71.7, 69.53 (d, Jc-p = 1.9 Hz), 69.46 (d, Jc-p = 1.9 Hz), 67.5 (d, Jc-p = 5.85 Hz), 66.9, 47.9, 37.4 (d, Jc-p = 8.76 Hz), 27.8. HRMS (ESI) m/z 842.3435 (calcd for C50H52NO9 [M + H]+: 842.3458).

Validamine 7-phosphate (21)

A mixture of 20 (10 mg, 11.9 μmol) and 10% Pd/C (5.0 mg) in MeOH (0.6 mL) and EtOAc (0.4 mL) containing 12 M HCl (10 μL) was hydrogenated under atmospheric pressure for 20 h. The reaction mixture was filtered through Celite and a cellulose syringe filter, washed with MeOH and concentrated. The crude product was further purified with Sephadex LH-20 to give the pure product 21 (2.1 mg, 70%); [α]23D= +46.7° (c 0.13, H2O); 1H NMR (300 MHz, D2O): δ 4.0–4.02 (m, 2H), 3.79–3.86 (m, 2H), 3.57 (t, J = 9.3 Hz, 1H), 3.46 (t, J = 9.0 Hz, 1H), 1.82–1.90 (m, 2H); 13C NMR (75 MHz, D2O; DSS): δ 76.3, 74.2, 72.7, 67.5 (d, Jc-p = 4.5 Hz), 54.0, 39.8 (d, Jc-p = 7.5 Hz), 28.6; 31P NMR (122 MHz, D2O; H3PO4): δ 1.70. HRMS (ESI) m/z 256.0576 (calcd for C7H15NO7P [M – H]: 256.0586).

Chemical Synthesis of Validoxylamine A 7′-phosphate (26)

(4aR,6S,7S,8S,8aR)-7,8-bis(benzyloxy)-2-phenyl-N-((1S,4R,5S,6S)-4,5,6-tris(benzyloxy)-3-((benzyloxy)methyl)cyclohex-2-en-1-yl)hexahydro-4H-benzo[d][1,3]dioxin-6-amine (28)

To a solution of validoxylamine A (8) (320 mg, 0.95 mmol) in 5 mL dry DMF, benzaldehyde dimethyl acetal (189 mg, 1.24 mmol) and p-TsOH (181 mg, 0.95 mmol) were added and the mixture was stirred at 60 °C for 3 h. The reaction was quenched with diethylamine, and the pH was adjusted to 8. To the same pot, NaH (275 mg, 11.5 mmol, 12 eq.) was added at 0 °C and stirred for 30 min. The mixture was brought to rt and benzyl bromide (1.4 mL, 11.5 mmol, 12 eq.) was added and the mixture was stirred for 3 h. The reaction was quenched with sat. aq. NH4Cl and extracted with EtOAc (3 × 5 mL). The EtOAc fractions were combined and washed with brine (3 × 5 mL), dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure. The residue was subjected to column chromatography (silica gel, hexane-EtOAc = 7:1) to afford 28 (420 mg, 2 steps, 46 %). 1H NMR (300 MHz, CDCl3): 7.13–7.54 (m), 5.92 (brs, 1H), 5.56 (s, 1H, HCPh), 4.89 (d, J = 11 Hz, 1H), 4.34–4.80 (m, 16H), 4.21 (d, J = 12 Hz, 1H), 3.81–4.14 (m, 6H), 3.62 (m, 1H), 3.51 (bt, J = 9 Hz), 3.31 (m, 3H), 2.54 (m, 1H), 1.88 (m, 1H), 1.63 (bd, J = 14 Hz, 1H), 1.54 (S, 1H), 0.823 (t, J = 14 Hz); 13C NMR (300 MHz, CDCl3); 139,1, 138.8, 138.4, 135.25, 127.3 – 128.1, 126.9, 126.2, 101.1, 84.6, 82.8, 82.3, 81.16, 81.16, 80.2, 76.6, 75.4, 74.6, 74.6, 73.3, 72.7, 72.3, 71.4, 71.3, 52.9, 51.4, 31.8, 28.7, 26.7. Electrospray MS: m/z 964.40 [M+H]+. HRMS (ESI) m/z 964.4743 (calcd for C63H66NO8 [M + H]+: 964.4788).

((1R,2R,3S,4S,5S)-2,3,4-tris(benzyloxy)-5-(((1S,4R,5S,6S)-4,5,6-tris(benzyloxy)-3-((benzyloxy)methyl)cyclohex-2-en-1-yl)amino)cyclohexyl)methanol (29)

To a solution of 28 (200 mg, 0.21 mmol) in CH2Cl2 (4 mL) DIBAL-H (1.0 M in hexane, 311 μL, 0.311 mmol) was added at 0 °C. After stirring at rt for 3 h, more DIBAL-H (150 μL) was added. The reaction mixture was stirred for 3.5 h and quenched by adding few drops of saturated solution of potassium sodium tartrate at 0 °C. The mixture was brought to rt with continuous addition of potassium sodium tartrate (5 mL). The mixture was diluted with EtOAc and vigorously stirred for 2.5 h. The aqueous phase was extracted three times with EtOAc. The organic fraction was washed with brine, dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure. The residue was subjected to column chromatography (silica gel, hexane-EtOAc = 3:1) to afford 29 (150 mg, 74.9 %) as a colorless oil. 1H NMR (300 MHz, CDCl3): 7.25 (m), 5.87 (brs, 1H), 4.90 (bt, J = 11 Hz, 2H), 4.58–4.87 (m, 13H), 4.22 (d, J = 12 Hz, 1H), 3.84–4.09 (m, 4H), 3.60 (m, 1H), 3.51 (m, 1H), 3.34 (m, 3H), 2.27 (m, 1H) 1.74 (brd, J = 14 Hz), 1.52 (s), 1.03 (t, J = 12 Hz); 13C NMR (300 MHz, CDCl3): 139.0, 138.9, 138.7, 135.6, 128.0–128.9, 127.9, 127.8, 102.2, 100.6, 83.9, 83.8, 83.5, 78.2, 78.1, 75.9, 75.2, 74.1, 73.8, 72.9, 72.3, 65.3, 51.4, 51.2, 37.7, 28.1, 21.2. Electrospray MS: m/z 966.47 [M+H]+. HRMS (ESI) m/z 966.4984 (calcd for C63H68NO8 [M + H]+: 966.4945).

Dibenzyl (((1R,2R,3S,4S,5S)-2,3,4-tris(benzyloxy)-5-(((1S,4R,5S,6S)-4,5,6-tris(benzyloxy)-3-((benzyloxy)methyl)cyclohex-2-en-1-yl)amino)cyclohexyl)methyl) phosphate (30)

A solution of 29 (70 mg, 0.0725 mmol) in CH2Cl2 (2 mL) was added to a mixture of dibenzyl N,N-diisopropylphosphoramidite (37.5 mg, 0.108 mmol) and 1H–tetrazole (15.2 mg, 0.21 mmol) in CH2Cl2 (2 mL), which had been stirred for 40 min, at rt and the reaction mixture was further stirred for 3 h. The reaction mixture was cooled to 0 °C and m-CPBA (12.5 mg, 0.0725 mmol) was added. After the cooling bath was removed, the reaction mixture was stirred for an additional 30 min at rt. The solvent was evapored in vacuo, and the residue was re-dissolved in EtOAc (5 mL). The EtOAc solution was washed with 10 % aqueous Na2SO3 (5 mL), saturated aqueous NaHCO3 (5 mL) and brine (5 mL), dried over anhydrous MgSO4, filtered and concentrated in a rotary evaporator. The product was purified using column chromatography (silica gel, toluene–EtOAc = 3: 1) to afford 30 (65 mg, 73%). 1H NMR (300 MHz, CDCl3): 7.25 (m), 5.92 (br s, 1H), 4.96 (d, J = 9 Hz, 4H), 4.85 (t, J = 10 Hz, 2H), 4.38 – 4.75 (m, 14H), 4.22 (d, J = 12 Hz, 2H), 4.02 (br d, J = 4 Hz, 1H), 3.88 (m, 3H), 3.71 (m, 1H), 3.55 (m, 1H), 3.38 (dd, J = 9 Hz, J = 4 Hz, 1H), 3.26 (m, 3H), 2.28 (m, 1H), 1.67 (m, 1H), 1.12 (t, J = 15 Hz); 13C NMR (300 MHz, CDCl3) 138.6–139.4, 126.4–128.9, 83.6, 83.2, 80.3, 77.9, 77.6, 77.4, 76.8, 75.7, 75.3, 74.2, 73.6, 72.6, 72.5, 71.1, 69.5, 69.4, 68.3, 51.5, 51.3, 36.7, 36.6, 27.9, 20.7. Electrospray MS: m/z 1226.53 [M+H]+. HRMS (ESI) m/z 1226.5573 (calcd for C77H81NO11P [M + H]+: 1226.5542).

Validoxylamine A 7′-phosphate (26)

BBr3 (417 μL of 1.0 M solution in CH2Cl2, 0.417 mmol) was added to a solution of 30 (28.4 mg, 0.023 mmol) in CH2Cl2, and the reaction mixture was stirred for 60 min at −40 °C. The mixture was then placed in an ice bath and several drops of saturated aqueous NaHCO3 solution were slowly added until no more effervescence was observed. The mixture was then partitioned between H2O (1 mL) and CH2Cl2 (1 mL). The aqueous fraction was collected and dried in a speedvac rotator and the residue was dissolved in H2O (500 μL). The solution (500 μL) was subjected to Bio-gel P-2 Gel fine column chromatography (10 cm × 0.5 cm) and eluted with H2O. Fractions containing the product were pooled, dried, and purified on Sephadex LH-20 (75 cm × 1 cm) to afford 26 (4.5 mg, 46 %). 1H NMR (300 MHz, D2O): 5.98 (br s, 1H), 4.33 (s, 2H), 4.22 (br d, J = 4.5 Hz, 2H), 4.13 (m, 1H), 4.04 (br s, 2H), 3.90 (m, 3H), 3.69 (t, J = 8 Hz, 1H), 3.51 (t, J = 9 Hz, 1H), 2.31 (brd, J = 14 Hz), 2.03 (m, 1H), 1.86 (t, J = 14 Hz); 13C NMR (300 MHz, D2O): 114.9, 73.7, 71.9, 71.5, 70.7, 70.4, 66.5, 64.7, 61.2, 58.1, 56.9, 37.4, 24.5, 20.9. Electrospray MS: m/z 416.20 [M+H]+. HRMS (ESI) m/z 416.1320 (calcd for C14H27NO11P [M + H]+: 416.1322).

VldB, VldE and VldH expression plasmid construction

Genomic DNA of Streptomyces hygroscopicus subsp. limoneus was prepared by a general procedure.31 The vldB, vldE, and vldH genes were amplified by PCR from the genomic DNA using the following primer pairs: 5′-TGG GGC TCG AGG CAT ATG GAC GGA GTG CGT -3′ and 5′-TTC GAA CTC GAG CAG CGC CAC -3′ for vldB, 5′-AAG ATC TCG AGA CAT ATG ACC GGA TCT GAG -3′ and 5′-TCA GAA TTC TCA GAG GTC TGC -3′ for vldE, 5′-GGT GAC TCG AGA CAT ATG TAC AAG GTT GCA -3′ and 5′-TGA GAA TTC TCA GGA AGG ACC AAT ATG CGG -3′ for vldH (XhoI, NdeI, and EcoRI sites are underlined). The vldB PCR fragment was digested with NdeI and XhoI and ligated into pET20b (Novagen) to give pET20b-vldB. The vldE and vldH PCR fragments were digested with XhoI and EcoRI and ligated into pRSET-B (Invitrogen) to give pRSET-B-vldE and pRSET-B-vldH. The DNA sequences of vldB, vldE and vldH were confirmed by the Center for Genome Research and Biocomputing (CGRB) at Oregon State University.

OtsA and OtsB expression plasmid construction

The otsA and otsB genes from E. coli MG1655 were amplified by PCR from plasmid pTrc99A-otsAB (a gift from Dr. C-J. Kim) using the following primer pairs: 5′-AGG CTC GAG GCA TAT GAG TCG TTT AGT CGT -3′ and 5′-GAC GAA TTC TCA CGC AAG CTT TGG AAA GGT -3′ for otsA, 5′-GAC CTC GAG ACA TAT GAC AGA ACC GTT AAC -3′ and 5′-GCA GAA TTC TCA GAT ACT ACG ACT AAA CGA -3′ for otsB (XhoI and EcoRI sites are underlined). The otsA and otsB PCR fragments were digested with XhoI and EcoRI and ligated into pRSET-B (Invitrogen) to give pRSET-B-otsA and pRSET-B-otsB. DNA sequences of otsA and otsB were confirmed by CGRB.

VldB, VldE, and VldH production and purification

pET20-vldB, pRSETB-vldE and pRSETB-vldH were used to transform E. coli BL21(DE3) pLysS. Transformants were grown overnight at 37 °C on LB agar plate containing 100 μg/mL ampicillin and 25 μg/mL chloramphenicol. A single colony was inoculated into 3 mL of LB medium and cultured at 37 °C for 6 h and then 1 mL of seed culture was transferred into 100 mL of LB medium in a 500 mL flask and grown at 28 °C until OD600 reached 0.6. Then, the temperature was reduced to 18 °C and after 2 h adaptation 0.1 mM IPTG was added to induce the C-terminal hexahistidine-tagged VldB and the N-terminal hexahistidine-tagged VldE and VldH proteins. After further growth for 14 h, the cells were harvested by centrifugation (5,000 rpm, 10 min, 4 °C) and stored at −80 °C until used. Cell pellets from 50 mL of culture were washed with 1 mL of binding (B) buffer (40 mM HEPES (pH 8.0), 300 mM NaCl, 10% glycerol, and 10 mM imidazole) and centrifuged (6,000 rpm, 3 min, 4 °C). Then, 1 mL of B buffer was added and the mixtures were sonicated (8 watts, 15 sec, 4 times). After centrifugation (14,500 rpm, 20 min, 4 °C), 0.8 mL of the supernatants were each mixed with 0.2 mL of B buffer-equilibrated Ni-NTA resin (QIAGEN) and incubated for 1 h at 4 °C. After incubation, the mixtures were centrifuged (4,000 rpm, 3 min, 4 °C) and the supernatants were discarded. One mL of washing (W) buffer (40 mM HEPES pH 8.0, 300 mM NaCl, 10% glycerol, and 20 mM imidazole in the case of VldE and VldH or 100 mM imidazole in the case of VldB) was added and the mixture centrifuged (4,000 rpm, 3 min, 4 °C). This washing step was repeated three times. Subsequently, 0.5 mL of elution (E) buffer (40 mM HEPES pH 8.0, containing 300 mM NaCl, 10% glycerol, and 500 mM imidazole) was added to elute the desired proteins. This elution step was repeated twice. Eluted VldB, VldE, and VldH were dialyzed against 1 L of dialysis buffer (10 mM Tris-HCl pH 7.5, 0.1 mM DTT, 1 mM MgCl2) 3 times for 3 h each. Purified proteins were analyzed by SDS-PAGE and concentrated by ultrafiltration using Amicon YM-10 (Millipore). Protein concentration was determined using the Bradford assay (BIO-RAD) with BSA as standard.

Coupled VldB/VldE assay with ATP and GTP

To determine whether ATP or GTP is the natural nucleotidyl donor in the coupled VldB/VldE reaction, VldB (1 μM) and VldE (3 μM) were incubated with 4 mM valienol 1-phosphate, 4 mM validamine 7-phosphate, 6 mM of either ATP or GTP, and 2 mM MgCl2 in 60 mM Tris-HCl pH 7.5. Reaction mixtures were incubated at 30 °C for 3 h. Then, 2 volumes of MeOH were added and mixed. The mixture was centrifuged (14,500 rpm, 5 min, 4 °C) and the supernatant was subjected to MS analysis. The results revealed that GTP is used more effectively in the VldB/VldE coupled assay than ATP.

Combined VldB, VldE and VldH assay

Typical reaction condition for activity assays were 60 mM Tris-HCl pH 7.5, 4 mM valienol 1-phosphate, 6 mM NTPs, 2 mM MgCl2, 1 μM VldB, 4 mM validamine 7-phosphate, 3 μM VldE and 3μM VldH. Reaction mixtures were incubated at 30 °C for 3 h. For enzyme inactivation, VldB, VldE, and VldH were heated at 100 °C for 5 min.

HPLC Analysis of VldB, VldE, and VldH products

For HPLC analysis, the reaction was quenched by adding 2 volumes of MeOH and centrifuged, and the supernatant was analyzed by HPLC (Shimadzu SPD-20A system) with a Gemini C18 column (4.6 × 150 mm, 5 μm, Phenomenex) using 0.5% MeOH in H2O containing 10 mM NH4HCO3 as eluent at a flow rate of 1 mL/min. Elution profiles were monitored at 210 nm.

High-resolution mass spectral analysis of VldB/VldE and VldB/VldE/VldH products

HR-ESTOFMS of the VldB/VldE product: m/z 416.1315 (calcd for C14H27NO11P [M + H]+: 416.1322). HR-ESTOFMS of the VldB/VldE/VldH product: m/z 336.1657 (calcd for C14H26NO8 [M + H]+: 336.1658).

Preparation of GDP-valienol for kinetic studies

A reaction mixture (100 μL) containing 60 mM Tris-HCl pH 7.5, 2 mM MgCl2, 5 mM valienol 1-phosphate, 7.5 mM GTP, and 3 μM VldB was incubated at 30 °C for 2 h and VldB was removed by ultrafiltration (YM-10, Amicon). GDP-valienol formed in the reaction mixture was measured by quantification of the generated inorganic diphosphate generated using the EnzChek pyrophosphate assay kit (E-6645, Invitrogen). Reaction mixtures lacking VldB were used as negative control (blank).

Km and kcat determination for VldE

Km and kcat values for VldE were determined by using the phosphopyruvate kinase (PK) and lactate dehydrogenase (LDH) coupled assay (Sigma-Aldrich). The reaction mixture (100 μL) contained 50 mM Tris-HCl pH 7.5, 5 mM MgCl2, 1.0–3.0 μM VldE, 1.5 mM phosphoenolpyruvate, 0.5 mM NADH, 37 U of PK, and 46 U of LDH, in addition to 0, 10, 20, 30, 40, 50, 60, 90, 120, 150, 300, 600 μM GDP-valienol and 1 mM validamine 7-phosphate for the determination of the value for GDP-valienol, and 0, 50, 100, 150, 200, 300, 500, 1000 μM validamine 7-phosphate and 300μM GDP-valienol for the determination of the value for validamine 7-phosphate. Oxidation of NADH to NAD+ was monitored in 96-well plates using a spectrophotometric microplate reader at 340 nm. Reaction mixtures lacking VldE were used as negative control (blank). The data were collected in duplicate (for GDP-valienol) and triplicate (for validamine 7-phosphate). Hanes-Woolf plot was used for Km and kcat value determination.

Assay of VldE for trehalose 6-phosphate synthase activity

To determine if VldE has trehalose 6-phosphate synthase activity, the enzyme (6 μM) was incubated with 1 mM donor substrate (either GDP-valienol, UDP-glucose, or GDP-glucose), 1 mM acceptor substrate (either validamine 7-phosphate or glucose 6-phosphate), 2 mM MgCl2, 16 μM VldH or OtsB in 50 mM Tris-HCl (pH 7.5). After a 3-hour incubation at 30 °C, 2 volumes of MeOH were added and mixed. The mixture was centrifuged (14,500 rpm, 5 min, 4 °C) and the supernatant was subjected to MS analysis. The results revealed that VldE does not recognize UDP-glucose, GDP-glucose, or glucose 6-phosphate as substrates.

OtsA and OtsB production and purification

The productions and purifications of N-terminal hexahistidine-tagged OtsA and OtsB were carried out by the same procedures as used for VldE and VldH.

Assay of OtsA for validoxylamine A 7′-phosphate synthase activity

To determine if OtsA has validoxylamine A 7′-phosphate synthase activity, the enzyme (6 μM) was incubated with 1 mM donor substrate (either GDP-valienol or UDP-glucose), 1 mM acceptor substrate (either validamine 7-phosphate or glucose 6-phosphate), 5 mM MgCl2, 16 μM OtsB or VldH in 50 mM Tris-HCl pH 7.5. After 3 hours incubation at 30 °C, 2 volumes of MeOH were added and mixed. The mixture was centrifuged (14,500 rpm, 5 min, 4 °C) and the supernatant was subjected to MS analysis. The results revealed that OtsA does not recognize GDP-valienol and validamine 7-phosphate as substrates.

Characterization of OtsB and VldH as trehalose 6-phosphate phosphatases

To determine if VldH has trehalose 6-phosphate phosphatase activity, the enzyme (8 μM) was incubated with 5 mM trehalose 6-phosphate, and 10 mM MgCl2 in 50 mM Tris-HCl pH 7.5. OtsB was used as positive control. After 1.5 hours incubation at 30 °C, 2 volumes of MeOH were added and mixed. The mixture was centrifuged (14,500 rpm, 5 min, 4 °C) and the supernatant was subjected to MS analysis. The results revealed that VldH was able to catalyze dephosphorylation of trehalose 6-phosphate to give trehalose.

Assay of VldE for validoxylamine A glycosyltransferase activity

To determine if VldE can catalyze the glycosylation of validoxylamine A to validamycin A, the enzyme (50 μM) was incubated with 1 mM validoxylamine A, 1 mM UDP-glucose, 10 mM MgCl2 and 10 mM NH4Cl in 50 mM Tris-HCl pH 8.0. After 3 hours incubation at 30 °C, 2 volumes of MeOH were added and mixed. The mixture was centrifuged (14,500 rpm, 5 min, 4 °C) and the supernatant was subjected to MS analysis. The results show that VldE is not a validoxylamine A glycosyltransferase.

Supplementary Material

1_si_001

Acknowledgments

This paper is dedicated to Professor (Emeritus) Isao Kitagawa on the occasion of his 80th birthday. The authors thank Chang-Joon Kim for providing plasmid pTrc99A-otsAB, Heinz G. Floss and Philip J. Proteau for their comments and assistance in the preparation of this manuscript. The project described was supported by grant R01AI061528 from the National Institute of Allergy and Infectious Diseases and by the Herman-Frasch Foundation. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Allergy and Infectious Diseases or the National Institutes of Health (NIH). This publication was made possible, in part, by the Mass Spectrometry Facilities ad Service Core of the Environmetal Health Sciences Center, Oregon State University, grant number P30 ES00210, National Institute of Environmental Health Sciences, NIH.

Footnotes

Supporting Information Available: Comparison of catalytic activities of trehalose 6-phosphate synthase and VldE (Scheme S1), complete HPLC analysis of VldB, VldE, and VldH (Figure S1), MS analysis of VldB, VldE, and VldH (Figures S2 – S10), amino acid alignment of trehalose 6-phosphate synthase and VldE (Figure S11), 1H-, 13C-, and 31P NMR data for synthetic compounds generated in this study (Figures S12 – S30). This material is available free of charge via the Internet at http://pubs.acs.org.

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