Expression, purification, and characterization of two N,N-dimethyltransferases, tylM1 and desVI, involved in the biosynthesis of mycaminose and desosamine

Glycosyl Exchange of Unactivated Glycosidic Bonds: Suppressing or Embracing Side Reactivity in Catalytic Glycosylations

While developing boron-catalyzed glycosylations using glycosyl fluoride donors and trialkylsilyl ether acceptors, competing pathways involving productive glycosylation or glycosyl exchange were observed. Experimental and computational mechanistic studies suggest a novel mode of reactivity where a dioxolenium ion is a key intermediate that promotes both pathways through addition to either a silyl ether or to the acetal of an existing glycosidic linkage. Modifications in catalyst structure enable either pathway to be favored, and with this understanding, improved multicomponent iterative couplings and glycosyl exchange processes were demonstrated.

Carboxyl Methyltransferase Catalysed Formation of Mono- and Dimethyl Esters under Aqueous Conditions: Application in Cascade Biocatalysis

Carboxyl methyltransferase (CMT) enzymes catalyse the biomethylation of carboxylic acids under aqueous conditions and have potential for use in synthetic enzyme cascades. Herein we report that the enzyme FtpM from Aspergillus fumigatus can methylate a broad range of aromatic mono- and dicarboxylic acids in good to excellent conversions. The enzyme shows high regioselectivity on its natural substrate fumaryl-l-tyrosine, trans, trans-muconic acid and a number of the dicarboxylic acids tested. Dicarboxylic acids are generally better substrates than monocarboxylic acids, although some substituents are able to compensate for the absence of a second acid group. For dicarboxylic acids, the second methylation shows strong pH dependency with an optimum at pH 5.5-6. Potential for application in industrial biotechnology was demonstrated in a cascade for the production of a bioplastics precursor (FDME) from bioderived 5-hydroxymethylfurfural (HMF).

Cloning and identification of the Frigocyclinone biosynthetic gene cluster from Streptomyces griseus strain NTK 97

Frigocyclinone is a novel antibiotic with antibacterial and anticancer activities. It is produced by both Antarctica-derived Streptomyces griseus NTK 97 and marine sponge-associated Streptomyces sp. M7_15. Here, we first report the biosynthetic gene cluster of frigocyclinone in the S. griseus NTK 97. The frigocyclinone gene cluster spans a DNA region of 33-kb which consists of 30 open reading frames (ORFs), encoding minimal type II polyketide synthase, aromatase and cyclase, redox tailoring enzymes, sugar biosynthesis-related enzymes, C-glycosyltransferase, a resistance protein, and three regulatory proteins. Based on the bioinformatic analysis, a biosynthetic pathway for frigocyclinone was proposed. Second, to verify the cloned gene cluster, CRISPR-Cpf1 mediated gene disruption was conducted. Mutant with the disruption of beta-ketoacyl synthase encoding gene frig20 fully loses the ability of producing frigocyclinone, while inactivating the glycosyltransferase gene frig1 leads to the production of key intermediate of anti-MRSA anthraquinone tetrangomycin.

Structural and Functional Characterization of Sulfonium Carbon-Oxygen Hydrogen Bonding in the Deoxyamino Sugar Methyltransferase TylM1

The N-methyltransferase TylM1 from Streptomyces fradiae catalyzes the final step in the biosynthesis of the deoxyamino sugar mycaminose, a substituent of the antibiotic tylosin. The high-resolution crystal structure of TylM1 bound to the methyl donor S-adenosylmethionine (AdoMet) illustrates a network of carbon-oxygen (CH···O) hydrogen bonds between the substrate's sulfonium cation and residues within the active site. These interactions include hydrogen bonds between the methyl and methylene groups of the AdoMet sulfonium cation and the hydroxyl groups of Tyr14 and Ser120 in the enzyme. To examine the functions of these interactions, we generated Tyr14 to phenylalanine (Y14F) and Ser120 to alanine (S120A) mutations to selectively ablate the CH···O hydrogen bonding to AdoMet. The TylM1 S120A mutant exhibited a modest decrease in its catalytic efficiency relative to that of the wild type (WT) enzyme, whereas the Y14F mutation resulted in an approximately 30-fold decrease in catalytic efficiency. In contrast, site-specific substitution of Tyr14 by the noncanonical amino acid p-aminophenylalanine partially restored activity comparable to that of the WT enzyme. Correlatively, quantum mechanical calculations of the activation barrier energies of WT TylM1 and the Tyr14 mutants suggest that substitutions that abrogate hydrogen bonding with the AdoMet methyl group impair methyl transfer. Together, these results offer insights into roles of CH···O hydrogen bonding in modulating the catalytic efficiency of TylM1.

Structural, mechanistic and functional insight into gliotoxin bis-thiomethylation in Aspergillus fumigatus

Gliotoxin is an epipolythiodioxopiperazine (ETP) class toxin, contains a disulfide bridge that mediates its toxic effects via redox cycling and is produced by the opportunistic fungal pathogen Aspergillus fumigatus Self-resistance against gliotoxin is effected by the gliotoxin oxidase GliT, and attenuation of gliotoxin biosynthesis is catalysed by gliotoxin S-methyltransferase GtmA. Here we describe the X-ray crystal structures of GtmA-apo (1.66 Å), GtmA complexed to S-adenosylhomocysteine (1.33 Å) and GtmA complexed to S-adenosylmethionine (2.28 Å), providing mechanistic insights into this important biotransformation. We further reveal that simultaneous elimination of the ability of A. fumigatus to dissipate highly reactive dithiol gliotoxin, via deletion of GliT and GtmA, results in the most significant hypersensitivity to exogenous gliotoxin observed to date. Indeed, quantitative proteomic analysis of ΔgliT::ΔgtmA reveals an uncontrolled over-activation of the gli-cluster upon gliotoxin exposure. The data presented herein reveal, for the first time, the extreme risk associated with intracellular dithiol gliotoxin biosynthesis-in the absence of an efficient dismutation capacity. Significantly, a previously concealed protective role for GtmA and functionality of ETP bis-thiomethylation as an ancestral protection strategy against dithiol compounds is now evident.

Elucidation of the glycosylation steps during biosynthesis of antitumor macrolides PM100117 and PM100118 and engineering for novel derivatives

BACKGROUND

Antitumor compounds PM100117 and PM100118 are glycosylated polyketides derived from the marine actinobacteria Streptomyces caniferus GUA-06-05-006A. The organization and characterization of the PM100117/18 biosynthesis gene cluster has been recently reported.

RESULTS

Based on the preceding information and new genetic engineering data, we have outlined the pathway by which PM100117/18 are glycosylated. Furthermore, these genetic engineering experiments have allowed the generation of novel PM100117/18 analogues. Deletion of putative glycosyltranferase genes and additional genes presumably involved in late biosynthesis steps of the three 2,6-dideoxysugars appended to the PM100117/18 polyketide skeleton, resulted in the generation of a series of intermediates and novel derivatives.

CONCLUSIONS

Isolation and identification of the novel compounds constitutes an important contribution to our knowledge on PM100117/18 glycosylation, and set the basis for further characterization of specific enzymatic reactions, additional genetic engineering and combinatorial biosynthesis approaches.

An Iterative O-Methyltransferase Catalyzes 1,11-Dimethylation of Aspergillus fumigatus Fumaric Acid Amides

S-adenosyl-l-methionine (SAM)-dependent methyltransfer is a common biosynthetic strategy to modify natural products. We investigated the previously uncharacterized Aspergillus fumigatus methyltransferase FtpM, which is encoded next to the bimodular fumaric acid amide synthetase FtpA. Structure elucidation of two new A. fumigatus natural products, the 1,11-dimethyl esters of fumaryl-l-tyrosine and fumaryl-l-phenylalanine, together with ftpM gene disruption suggested that FtpM catalyzes iterative methylation. Final evidence that a single enzyme repeatedly acts on fumaric acid amides came from an in vitro biochemical investigation with recombinantly produced FtpM. Size-exclusion chromatography indicated that this methyltransferase is active as a dimer. As ftpA and ftpM homologues are found clustered in other fungi, we expect our work will help to identify and annotate natural product biosynthesis genes in various species.

Natural riboflavin analogs

Riboflavin analogs have a good potential to serve as basic structures for the development of novel anti-infectives. Riboflavin analogs have multiple cellular targets, since riboflavin (as a precursor to flavin cofactors) is active at more than one site in the cell. As a result, the frequency of developing resistance to antimicrobials based on riboflavin analogs is expected to be significantly lower. The only known natural riboflavin analog with antibiotic function is roseoflavin from the bacterium Streptomyces davawensis. This antibiotic negatively affects flavoenzymes and FMN riboswitches. Another roseoflavin producer, Streptomyces cinnabarinus, was recently identified. Possibly, flavin analogs with antibiotic activity are more widespread than anticipated. The same could be true for flavin analogs yet to be discovered, which could constitute tools for cellular chemistry, thus allowing a further extension of the catalytic spectrum of flavoenzymes.

Biocatalytic synthesis of pikromycin, methymycin, neomethymycin, novamethymycin, and ketomethymycin

A biocatalytic platform that employs the final two monomodular type I polyketide synthases of the pikromycin pathway in vitro followed by direct appendage of D-desosamine and final C-H oxidation(s) in vivo was developed and applied toward the synthesis of a suite of 12- and 14-membered ring macrolide natural products. This methodology delivered both compound classes in 13 steps (longest linear sequence) from commercially available (R)-Roche ester in >10% overall yields.

A novel N,N-8-amino-8-demethyl-D-riboflavin Dimethyltransferase (RosA) catalyzing the two terminal steps of roseoflavin biosynthesis in Streptomyces davawensis

Streptomyces davawensis synthesizes the antibiotic roseoflavin (RoF) (8-dimethylamino-8-demethyl-D-riboflavin). It was postulated that RoF is synthesized from riboflavin via 8-amino- (AF) and 8-methylamino-8-demethyl-D-riboflavin (MAF). In a cell-free extract of S. davawensis, an S-adenosyl methionine-dependent conversion of AF into MAF and RoF was observed. The corresponding N,N-8-amino-8-demethyl-d-riboflavin dimethyltransferase activity was enriched by column chromatography. The final most active fraction still contained at least five different proteins that were analyzed by enzymatic digestion and concomitant de novo sequencing by MS/MS. One of the sequences matched a hypothetical peptide fragment derived from an as yet uncharacterized open reading frame (sda77220) located in the middle of a (putative) gene cluster within the S. davawensis genome. Expression of ORF sda77220 in Escherichia coli revealed that the corresponding gene product had N,N-8-amino-8-demethyl-d-riboflavin dimethyltransferase activity. Inactivation of ORF sda77220 led to a S. davawensis strain that synthesized AF but not MAF or RoF. Accordingly, as the first identified gene of RoF biosynthesis, ORF sda77220 was named rosA. RosA (347 amino acids; 38 kDa) was purified from a recombinant E. coli strain (as a His(6)-tagged protein) and was biochemically characterized (apparent K(m) for AF = 57.7 ± 9.2 μm; apparent K(D) for AF = 10.0 μm; k(cat) = 0.37 ± 0.02 s(-1)). RosA is a unique enzyme and may be useful for a variety of applications.

Characterization of the TDP-D-ravidosamine biosynthetic pathway: one-pot enzymatic synthesis of TDP-D-ravidosamine from thymidine-5-phosphate and glucose-1-phosphate

Ravidomycin V and related compounds, e.g., FE35A-B, exhibit potent anticancer activities against various cancer cell lines in the presence of visible light. The amino sugar moieties (D-ravidosamine and its analogues, respectively) in these molecules contribute to the higher potencies of ravidomycin and analogues when compared to closely related compounds with neutral or branched sugars. Within the ravidomycin V biosynthetic gene cluster, five putative genes encoding NDP-D-ravidosamine biosynthetic enzymes were identified. Through the activities of the isolated enzymes in vitro, it is demonstrated that ravD, ravE, ravIM, ravAMT and ravNMT encode TDP-D-glucose synthase, TDP-4-keto-6-deoxy-D-glucose-4,6-dehydratase, TDP-4-keto-6-deoxy-D-glucose-3,4-ketoisomerase, TDP-3-keto-6-deoxy-D-galactose-3-aminotransferase, and TDP-3-amino-3,6-dideoxy-D-galactose-N,N-dimethyl-transferase, respectively. A protocol for a one-pot enzymatic synthesis of TDP-D-ravidosamine has been developed. The results presented here now set the stage to produce TDP-D-ravidosamine routinely for glycosylation studies.

Molecular architecture of TylM1 from Streptomyces fradiae: an N,N-dimethyltransferase involved in the production of dTDP-D-mycaminose

d-Mycaminose is an unusual dideoxy sugar found attached to the antibiotic tylosin, a commonly used veterinarian therapeutic. It is synthesized by the Gram-positive bacterium Streptomyces fradiae as a dTDP-linked sugar. The last step in its biosynthesis involves the dimethylation of the hexose C-3' amino group by an S-adenosylmethionine (SAM) dependent enzyme referred to as TylM1. Here we report two high-resolution X-ray structures of TylM1, one in which the enzyme contains bound SAM and dTDP-phenol and the second in which the protein is complexed with S-adenosylhomocysteine (SAH) and dTDP-3-amino-3,6-dideoxyglucose, its natural substrate. Combined, these two structures, solved to 1.35 and 1.79 Å resolution, respectively, show the orientations of SAM and the dTDP-linked sugar substrate within the active site region. Specifically, the C-3' amino group of the hexose is in the correct position for an in-line attack at the reactive methyl group of SAM. Both Tyr 14 and Arg 241 serve to anchor the dTDP-linked sugar to the protein. To test the role of His 123 in catalysis, two site-directed mutant proteins were constructed, H123A and H123N. Both mutant proteins retained catalytic activity, albeit with reduced rates. Specifically, the k(cat)/K(m) was reduced to 1.8% and 0.37% for the H123A and H123N mutant proteins, respectively. High-resolution X-ray models showed that the observed perturbations in the kinetic constants were not due to major changes in their three-dimensional folds. Most likely the proton on the C-3' amino group is transferred to one of the water molecules lining the active site pocket as catalysis proceeds.

Biosynthesis of spinosyn in Saccharopolyspora spinosa: synthesis of permethylated rhamnose and characterization of the functions of SpnH, SpnI, and SpnK

Spinosyn A is a polyketide-derived macrolide produced by Saccharopolyspora spinosa and is an active ingredient in several commercial insecticides. It is glycosylated by a tri-O-methylated rhamnose at C-9 and a forosamine at C-17. Previous studies indicated that the rhamnose methyltransferases are encoded by the spnH, spnI, and spnK genes. To verify the functions of these methyltransferases and to study how they are coordinated to achieve the desired level of methylation of rhamnose, we studied the catalytic properties of the spnH, spnI, and spnK gene products and validated their roles in the permethylation process of spinosyn A. Our data reported herein firmly established that SpnH, SpnI, and SpnK are the respective rhamnose 4'-, 2'-, and 3'-O-methyltransferase. Investigation of the order of the methylation events revealed that only one route catalyzed by SpnI, SpnK, and SpnH in sequence is productive for the permethylation of the rhamnose moiety. Moreover, the completion of rhamnose permethylation is likely achieved by the proper control of the expression levels of the methyltransferase genes involved. These results set the stage for future exploitation of the spinosyn biosynthetic pathway to produce targeted spinosyn derivatives and, perhaps, new analogues.

TDP-L-megosamine biosynthesis pathway elucidation and megalomicin a production in Escherichia coli

In vivo reconstitution of the TDP-l-megosamine pathway from the megalomicin gene cluster of Micromonospora megalomicea was accomplished by the heterologous expression of its biosynthetic genes in Escherichia coli. Mass spectrometric analysis of the TDP-sugar intermediates produced from operons containing different sets of genes showed that the production of TDP-l-megosamine from TDP-4-keto-6-deoxy-d-glucose requires only five biosynthetic steps, catalyzed by MegBVI, MegDII, MegDIII, MegDIV, and MegDV. Bioconversion studies demonstrated that the sugar transferase MegDI, along with the helper protein MegDVI, catalyzes the transfer of l-megosamine to either erythromycin C or erythromycin D, suggesting two possible routes for the production of megalomicin A. Analysis in vivo of the hydroxylation step by MegK indicated that erythromycin C is the intermediate of megalomicin A biosynthesis.

Cloning and characterization of the ravidomycin and chrysomycin biosynthetic gene clusters

The gene clusters responsible for the biosynthesis of two antitumor antibiotics, ravidomycin and chrysomycin, have been cloned from Streptomyces ravidus and Streptomyces albaduncus, respectively. Sequencing of the 33.28 kb DNA region of the cosmid cosRav32 and the 34.65 kb DNA region of cosChry1-1 and cosChryF2 revealed 36 and 35 open reading frames (ORFs), respectively, harboring tandem sets of type II polyketide synthase (PKS) genes, D-ravidosamine and D-virenose biosynthetic genes, post-PKS tailoring genes, regulatory genes, and genes of unknown function. The isolated ravidomycin gene cluster was confirmed to be involved in ravidomycin biosynthesis through the production of a new analogue of ravidomycin along with anticipated pathway intermediates and biosynthetic shunt products upon heterologous expression of the cosmid, cosRav32, in Streptomyces lividans TK24. The identity of the cluster was further verified through cross complementation of gilvocarcin V (GV) mutants. Similarly, the chrysomycin gene cluster was demonstrated to be indirectly involved in chrysomycin biosynthesis through cross-complementation of gilvocarcin mutants deficient in the oxygenases GilOII, GilOIII, and GilOIV with the respective chrysomycin monooxygenase homologues. The ravidomycin glycosyltransferase (RavGT) appears to be able to transfer both amino- and neutral sugars, exemplified through the structurally distinct 6-membered D-ravidosamine and 5-membered D-fucofuranose, to the coumarin-based polyketide derived backbone. These results expand the library of biosynthetic genes involved in the biosyntheses of gilvocarcin class compounds that can be used to generate novel analogues through combinatorial biosynthesis.

Characterization and mechanistic studies of DesII: a radical S-adenosyl-L-methionine enzyme involved in the biosynthesis of TDP-D-desosamine

D-desosamine (1) is a 3-(N,N-dimethylamino)-3,4,6-trideoxyhexose found in a number of macrolide antibiotics including methymycin (2), neomethymycin (3), pikromycin (4), and narbomycin (5) produced by Streptomyces venezuelae . It plays an essential role in conferring biological activities to its parent aglycones. Previous genetic and biochemical studies of the biosynthesis of desosamine in S. venezuelae showed that the conversion of TDP-4-amino-4,6-dideoxy-D-glucose (8) to TDP-3-keto-4,6-dideoxy-D-glucose (9) is catalyzed by DesII, which is a member of the radical S-adenosyl-L-methionine (SAM) enzyme superfamily. Here, we report the purification and reconstitution of His(6)-tagged DesII, characterization of its [4Fe-4S] cluster using UV-vis and EPR spectroscopies, and the capability of flavodoxin, flavodoxin reductase, and NADPH to reduce the [4Fe-4S](2+) cluster. Also included are a steady-state kinetic analysis of DesII-catalyzed reaction and an investigation of the substrate flexibility of DesII. Studies of deuterium incorporation into SAM using TDP-[3-(2)H]-4-amino-4,6-dideoxy-D-glucose as the substrate provides strong evidence for direct hydrogen atom transfer to a 5'-deoxyadenosyl radical in the catalytic cycle. The fact that hydrogen atom abstraction occurs at C-3 also sheds light on the mechanism of this intriguing deamination reaction.

Enzymatic synthesis of TDP-deoxysugars

Many biologically active bacterial natural products contain highly modified deoxysugar residues that are often critical for the activity of the parent compounds. Most of these deoxysugars are secondary metabolites that are biosynthesized in the form of nucleotide diphosphate (NDP) sugars prior to their transfer to natural product aglycones by glycosyltransferases. Over the past decade, many biosynthetic pathways that lead to the formation of these unusual sugars have been unraveled, and the mechanisms of many key enzymatic transformations involved in these pathways have been elucidated. However, obtaining workable quantities of NDP-deoxysugars for in vitro studies is often a difficult task. This limitation has hindered an in-depth investigation of the substrate specificity of deoxysugar biosynthetic enzymes, many of which are promiscuous with respect to their NDP-sugar substrates and are, thus, potentially useful catalysts for natural product glycoengineering. Presented in this review are procedures for the enzymatic synthesis and purification of a variety of NDP-deoxysugars, including some early intermediates in NDP-deoxysugar biosynthetic pathways, and highly modified NDP-deoxysugars that are late intermediates in their respective biosynthetic pathways. The procedures described herein could be used as general guidelines for the development of specific protocols for the synthesis of other NDP-deoxysugars.

Functional analysis of MycCI and MycG, cytochrome P450 enzymes involved in biosynthesis of mycinamicin macrolide antibiotics

Macrolides are a class of valuable antibiotics that include a macrolactone ring, at least one appended sugar unit, and, in most cases, additional hydroxyl or epoxide groups installed by cytochrome P450 enzymes. These functional groups contribute to structural diversification and serve to improve the bioactivity profiles of natural products. Here, we have characterized in vitro two P450 enzymes from the mycinamicin biosynthetic pathway of Micromonospora griseorubida. First, MycCI was characterized as the C21 methyl hydroxylase of mycinamicin VIII, the earliest macrolide form in the postpolyketide synthase tailoring pathway. Moreover, we established that optimal activity of MycCI depends on the native ferredoxin MycCII. Second, MycG P450 catalyzes consecutive hydroxylation and epoxidation reactions with mycinamicin IV as initial substrate. These reactions require prior dimethylation of 6-deoxyallose to mycinose for effective conversion by the dual function MycG enzyme.

Identifying the minimal enzymes required for anhydrotetracycline biosynthesis

The cyclohexenone ring A of tetracyclines exhibits unique structural features not observed among other aromatic polyketides. These substitutions include the C2 primary amide, C4 dimethylamine, and the C12a tertiary alcohol. Here we report the identification and reconstitution of the minimum set of enzymes required for the biosynthesis of anhydrotetracycline (ATC, 5), the first intermediate in the tetracycline biosynthetic pathway that contains the fully functionalized ring A. Using a combination of in vivo and in vitro approaches, we confirmed OxyL, OxyQ, and OxyT to be the only enzymes required to convert 6-methylpretetramid 1 into 5. OxyL is a NADPH-dependent dioxygenase that introduces two oxygen atoms into 1 to yield the unstable intermediate 4-keto-ATC 2. The aminotransferase OxyQ catalyzes the reductive amination of C4-keto of 2, yielding 4-amino-ATC 3. Furthermore, the N, N-dimethyltransferase OxyT catalyzes the formation of 5 from 3 in a (S)-adenosylmethionine (SAM)-dependent manner. Finally, a "non-natural" anhydrotetracycline derivative was generated, demonstrating that our heterologous host/vector pair can be a useful platform toward the engineered biosynthesis of tetracycline analogues.

In vitro characterization of the enzymes involved in TDP-D-forosamine biosynthesis in the spinosyn pathway of Saccharopolyspora spinosa

Forosamine (4-dimethylamino)-2,3,4,6-tetradeoxy-beta-D-threo-hexopyranose) is a highly deoxygenated sugar component of several important natural products, including the potent yet environmentally benign insecticide spinosyns. To study D-forosamine biosynthesis, the five genes (spnO, N, Q, R, and S) from the spinosyn gene cluster thought to be involved in the conversion of TDP-4-keto-6-deoxy-D-glucose to TDP-D-forosamine were cloned and heterologously expressed, and the corresponding proteins were purified and their activities examined in vitro. Previous work demonstrated that SpnQ functions as a pyridoxamine 5'-monophosphate (PMP)-dependent 3-dehydrase which, in the presence of the cellular reductase pairs ferredoxin/ferredoxin reductase or flavodoxin/flavodoxin reductase, catalyzes C-3 deoxygenation of TDP-4-keto-2,6-dideoxy-D-glucose. It was also established that SpnR functions as a transaminase which converts the SpnQ product, TDP-4-keto-2,3,6-trideoxy-D-glucose, to TDP-4-amino-2,3,4,6-tetradeoxy-D-glucose. The results presented here provide a full account of the characterization of SpnR and SpnQ and reveal that SpnO and SpnN functions as a 2,3-dehydrase and a 3-ketoreductase, respectively. These two enzymes act sequentially to catalyze C-2 deoxygenation of TDP-4-keto-6-deoxy-D-glucose to form the SpnQ substrate, TDP-4-keto-2,6-dideoxy-D-glucose. Evidence has also been obtained to show that SpnS functions as the 4-dimethyltransferase that converts the SpnR product to TDP-D-forosamine. Thus, the biochemical functions of the five enzymes involved in TDP-D-forosamine formation have now been fully elucidated. The steady-state kinetic parameters for the SpnQ-catalyzed reaction have been determined, and the substrate specificities of SpnQ and SpnR have been explored. The implications of this work for natural product glycodiversification and comparative mechanistic analysis of SpnQ and related NDP-sugar 3-dehydrases E1 and ColD are discussed.

Manipulating nature's sugar biosynthetic machineries for glycodiversification of macrolides: Recent advances and future prospects

Changing the sugar structures and glycosylation patterns of natural products is an effective means of altering the biological activity of clinically useful drugs. Several recent strategies have provided researchers with the opportunity to manipulate sugar structures and to change the sugar moieties attached to these natural products via a biosynthetic approach. In this review, we explore the utility of contemporary in vivo and in vitro methods to achieve natural product glycodiversification. This study will focus on recent progress from our laboratory in elucidating the biosynthesis of D-desosamine, a deoxysugar component of many macrolide antibiotics, and will highlight how we have engineered the D-desosamine biosynthetic pathway in Streptomyces venezuelae through targeted disruption and heterologous expression of the sugar biosynthetic genes to generate a variety of new glycoforms. The in vitro exploitation of the substrate flexibility of the endogenous D-desosamine glycosyltransferase (GT) to generate many non-natural glycoforms will also be discussed. These experiments are compared with recent work from other research groups on the same topics. Finally, the significance of these studies for the future prospects of natural product glycodiversification is discussed.

Deciphering the late steps in the biosynthesis of the anti-tumour indolocarbazole staurosporine: sugar donor substrate flexibility of the StaG glycosyltransferase

The indolocarbazole staurosporine is a potent inhibitor of a variety of protein kinases. It contains a sugar moiety attached through C-N linkages to both indole nitrogen atoms of the indolocarbazole core. Staurosporine biosynthesis was reconstituted in vivo in a heterologous host Streptomyces albus by using two different plasmids: the 'aglycone vector' expressing a set of genes involved in indolocarbazole biosynthesis together with staG (encoding a glycosyltransferase) and/or staN (coding for a P450 oxygenase), and the 'sugar vector' expressing a set of genes responsible for the biosynthesis of the sugar moiety. Attachment of the sugar to the two indole nitrogens of the indolocarbazole core was dependent on the combined action of StaG and StaN. When StaN was absent, the sugar was attached only to one of the nitrogen atoms, through an N-glycosidic linkage, as in the indolocarbazole rebeccamycin. The StaG glycosyltransferase showed flexibility with respect to the sugar donor. When the 'sugar vector' was substituted by constructs directing the biosynthesis of l-rhamnose, L-digitoxose, L-olivose and D-olivose, respectively, StaG and StaN were able to transfer and attach all of these sugars to the indolocarbazole aglycone.

Biosynthesis Pathways for Deoxysugars in Antibiotic-Producing Actinomycetes: Isolation, Characterization and Generation of Novel Glycosylated Derivatives

Many bioactive natural products synthesized by actinomycetes are glycosylated compounds in which the appended sugars contribute to specific interactions with their biological target. Most of these sugars are 6-deoxyhexoses, of which more than 70 different forms have been identified, and an increasing number of gene clusters involved in 6-deoxyhexoses biosynthesis are being characterized from antibiotic-producing actinomycetes. Novel glycosylated compounds have been generated by modifying natural deoxysugar biosynthesis pathways in the producer organisms, and/or the simultaneous expression in these strains of selected deoxysugar biosynthesis genes from other strains. Non-producing strains endowed with the capacity to synthesize novel deoxysugars through the expression of engineered deoxysugar biosynthesis clusters can also be used as alternative hosts. Transfer of these deoxysugars to a multiplicity of aglycones relies upon the existence of glycosyltransferases with an inherent degree of 'relaxed substrate specificity'. In this review, we analyze how the knowledge coming out from isolation and characterization of deoxysugar biosynthesis pathways from actinomycetes is being used to produce novel glycosylated derivatives of natural products.

Functional characterizations of novWUS involved in novobiocin biosynthesis from Streptomyces spheroides

NovW, novU, and novS gene products represent dTDP-4-keto-6-deoxy-D-glucose 3,5 epimarase, C-methyltransferase and dTDP-glucose-4-ketoreductase involved in noviose biosynthetic pathway, respectively. We have expressed three genes to elucidate the functions of NovW, NovU, and NovS in Escherichia coli. NovW and NovU catalyze the formation of dTDP-4-keto-6-deoxy-5-C-methyl-L-lyxo-hexose from dTDP-4-keto-6-deoxy-D-glucose. NovS reduces the product formed from the reaction of NovW with dTDP-4-keto-6-deoxy-D-glucose in the presence of NADH to result in dTDP-l-rhamnose. Furthermore, a pathway for the biosynthesis of noviose is proposed.

Engineering Biosynthetic Pathways for Deoxysugars: Branched-Chain Sugar Pathways and Derivatives from the Antitumor Tetracenomycin

Sugar biosynthesis cassette genes have been used to construct plasmids directing the biosynthesis of branched-chain deoxysugars: pFL942 (NDP-L-mycarose), pFL947 (NDP-4-deacetyl-L-chromose B), and pFL946/pFL954 (NDP-2,3,4-tridemethyl-L-nogalose). Expression of pFL942 and pFL947 in S. lividans 16F4, which harbors genes for elloramycinone biosynthesis and the flexible ElmGT glycosyltransferase of the elloramycin biosynthetic pathway, led to the formation of two compounds: 8-alpha-L-mycarosyl-elloramycinone and 8-demethyl-8-(4-deacetyl)-alpha-L-chromosyl-tetracenomycin C, respectively. Expression of pFL946 or pFL954 failed to produce detectable amounts of a novel glycosylated tetracenomycin derivative. Formation of these two compounds represents examples of the sugar cosubstrate flexibility of the ElmGT glycosyltransferase. The use of these cassette plasmids also provided insights into the substrate flexibility of deoxysugar biosynthesis enzymes as the C-methyltransferases EryBIII and MtmC, the epimerases OleL and EryBVII, and the 4-ketoreductases EryBIV and OleU.