Reversible sugar transfer by glycosyltransferases as a tool for natural product (bio)synthesis

Biosynthesis and Transport of Nucleotide Sugars for Plant Hemicellulose

Hemicellulose is entangled with cellulose through hydrogen bonds and meanwhile acts as a bridge for the deposition of lignin monomer in the secondary wall. Therefore, hemicellulose plays a vital role in the utilization of cell wall biomass. Many advances in hemicellulose research have recently been made, and a large number of genes and their functions have been identified and verified. However, due to the diversity and complexity of hemicellulose, the biosynthesis and regulatory mechanisms are yet unknown. In this review, we summarized the types of plant hemicellulose, hemicellulose-specific nucleotide sugar substrates, key transporters, and biosynthesis pathways. This review will contribute to a better understanding of substrate-level regulation of hemicellulose synthesis.

Seamless stitching of biosynthetic gene cluster containing type I polyketide synthases using Red/ET mediated recombination for construction of stably co-existing plasmids

Type I polyketides are natural products with diverse functions that are important for medical and agricultural applications. Manipulation of large biosynthetic gene clusters containing type I polyketide synthases (PKS) for heterologous expression is difficult due to the existence of conservative sequences of PKS in multiple modules. Red/ET mediated recombination has permitted rapid manipulation of large fragments; however, it requires insertion of antibiotic selection marker in the cassette, raising the problem of interference of expression by leaving "scar" sequence. Here, we report a method for precise seamless stitching of large polyketide biosynthetic gene cluster using a 48.4kb fragment containing type I PKS involved in fostriecin biosynthesis as an example. rpsL counter-selection was used to assist seamless stitching of large fragments, where we have overcome both the size limitations and the restriction on endonuclease sites during the Red/ET recombination. The compatibility and stability of the co-existing vectors (p184 and pMT) which respectively accommodate 16kb and 32.4kb inserted fragments were demonstrated. The procedure described here is efficient for manipulation of large DNA fragments for heterologous expression.

A two-step O- to C-glycosidic bond rearrangement using complementary glycosyltransferase activities

An efficient 2'-O- to 3'-C-β-d-glucosidic bond rearrangement on the dihydrochalcone phloretin to convert phlorizin into nothofagin was achieved by combining complementary O-glycosyltransferase (OGT) and C-glycosyltransferase (CGT) activities in a one-pot transformation containing catalytic amounts of uridine 5'-diphosphate (UDP). Two separate enzymes or a single engineered dual-specific O/CGT were applied. Overall (quantitative) conversion occurred in two steps via intermediary UDP-glucose and phloretin.

A simple strategy for glycosyltransferase-catalyzed aminosugar nucleotide synthesis

A set of 2-chloro-4-nitrophenyl glucosamino-/xylosaminosides were synthesized and assessed as potential substrates in the context of glycosyltransferase-catalyzed formation of the corresponding UDP/TDP-α-D-glucosamino-/xylosaminosugars and in single-vessel model transglycosylation reactions. This study highlights a robust platform for aminosugar nucleotide synthesis and reveals OleD Loki to be a proficient catalyst for U/TDP-aminosugar synthesis and utilization

Broadening deoxysugar glycodiversity: natural and engineered transaldolases unlock a complementary substrate space

The majority of prokaryotic drugs are produced in glycosylated form, with the deoxygenation level in the sugar moiety having a profound influence on the drug's bioprofile. Chemical deoxygenation is challenging due to the need for tedious protective group manipulations. For a direct biocatalytic de novo generation of deoxysugars by carboligation, with regiocontrol over deoxygenation sites determined by the choice of enzyme and aldol components, we have investigated the substrate scope of the F178Y mutant of transaldolase B, TalB(F178Y), and fructose 6-phosphate aldolase, FSA, from E. coli against a panel of variously deoxygenated aldehydes and ketones as aldol acceptors and donors, respectively. Independent of substrate structure, both enzymes catalyze a stereospecific carboligation resulting in the D-threo configuration. In combination, these enzymes have allowed the preparation of a total of 22 out of 24 deoxygenated ketose-type products, many of which are inaccessible by available enzymes, from a [3×8] substrate matrix. Although aliphatic and hydroxylated aliphatic aldehydes were good substrates, D-lactaldehyde was found to be an inhibitor possibly as a consequence of inactive substrate binding to the catalytic Lys residue. A 1-hydroxy-2-alkanone moiety was identified as a common requirement for the donor substrate, whereas propanone and butanone were inactive. For reactions involving dihydroxypropanone, TalB(F178Y) proved to be the superior catalyst, whereas for reactions involving 1-hydroxybutanone, FSA is the only choice; for conversions using hydroxypropanone, both TalB(F178Y) and FSA are suitable. Structure-guided mutagenesis of Ser176 to Ala in the distant binding pocket of TalB(F178Y), in analogy with the FSA active site, further improved the acceptance of hydroxypropanone. Together, these catalysts are valuable new entries to an expanding toolbox of biocatalytic carboligation and complement each other well in their addressable constitutional space for the stereospecific preparation of deoxysugars.

Characterization of glycosyltransferase DesVII and its auxiliary partner protein DesVIII in the methymycin/picromycin biosynthetic pathway

The in vitro characterization of the catalytic activity of DesVII, the glycosyltransferase involved in the biosynthesis of the macrolide antibiotics methymycin, neomethymycin, narbomycin, and pikromycin in Streptomyces venezuelae, is described. DesVII is unique among glycosyltransferases in that it requires an additional protein component, DesVIII, for activity. Characterization of the metabolites produced by a S. venezuelae mutant lacking the desVIII gene confirmed that desVIII is important for the biosynthesis of glycosylated macrolides but can be replaced by at least one of the homologous genes from other pathways. The addition of recombinant DesVIII protein significantly improves the glycosylation efficiency of DesVII in the in vitro assay. When affinity-tagged DesVII and DesVIII proteins were coproduced in Escherichia coli, they formed a tight (αβ)(3) complex that is at least 10(3)-fold more active than DesVII alone. The formation of the DesVII/DesVIII complex requires coexpression of both genes in vivo and cannot be fully achieved by mixing the individual protein components in vitro. The ability of the DesVII/DesVIII system to catalyze the reverse reaction with the formation of TDP-desosamine was also demonstrated in a transglycosylation experiment. Taken together, our data suggest that DesVIII assists the folding of DesVII during protein production and remains tightly bound during catalysis. This requirement must be taken into consideration in the design of combinatorial biosynthetic experiments with new glycosylated macrolides.

Functional characterization and substrate specificity of spinosyn rhamnosyltransferase by in vitro reconstitution of spinosyn biosynthetic enzymes

Spinosyn, a potent insecticide, is a novel tetracyclic polyketide decorated with d-forosamine and tri-O-methyl-L-rhamnose. Spinosyn rhamnosyltransferase (SpnG) is a key biocatalyst with unique sequence identity and controls the biosynthetic maturation of spinosyn. The rhamnose is critical for the spinosyn insecticidal activity and cell wall biosynthesis of the spinosyn producer, Saccharopolyspora spinosa. In this study, we have functionally expressed and characterized SpnG and the three enzymes, Gdh, Epi, and Kre, responsible for dTDP-L-rhamnose biosynthesis in S. spinosa by purified enzymes from Escherichia coli. Most notably, the substrate specificity of SpnG was thoroughly characterized by kinetic and inhibition experiments using various NDP sugar analogs made by an in situ combination of NDP-sugar-modifying enzymes. SpnG was found to exhibit striking substrate promiscuity, yielding corresponding glycosylated variants. Moreover, the critical residues presumably involved in catalytic mechanism of Gdh and SpnG were functionally evaluated by site-directed mutagenesis. The information gained from this study has provided important insight into molecular recognition and mechanism of the enzymes, especially SpnG. The results have made possible the structure-activity characterization of SpnG, as well as the use of SpnG or its engineered form to serve as a combinatorial tool to make spinosyn analogs with altered biological activities and potency.

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.

Enantioselective synthesis of 2,6-dideoxy carbasugars based on a desymmetrizing hydroformylation-carbonyl ene cyclization process

A practical one-pot process involving a desymmetrizing hydroformylation with the aid of a chiral catalyst-directing group (CDG*), followed by a carbonyl ene cyclization provides a straightforward access to both enantiomers of the resulting cyclohexanediol; further divergent, highly selective and protecting group-free transformations furnish the carbocyclic analogues of four important 2,6-dideoxysugars.

Natural-product sugar biosynthesis and enzymatic glycodiversification

Many biologically active small-molecule natural products produced by microorganisms derive their activities from sugar substituents. Changing the structures of these sugars can have a profound impact on the biological properties of the parent compounds. This realization has inspired attempts to derivatize the sugar moieties of these natural products through exploitation of the sugar biosynthetic machinery. This approach requires an understanding of the biosynthetic pathway of each target sugar and detailed mechanistic knowledge of the key enzymes. Scientists have begun to unravel the biosynthetic logic behind the assembly of many glycosylated natural products and have found that a core set of enzyme activities is mixed and matched to synthesize the diverse sugar structures observed in nature. Remarkably, many of these sugar biosynthetic enzymes and glycosyltransferases also exhibit relaxed substrate specificity. The promiscuity of these enzymes has prompted efforts to modify the sugar structures and alter the glycosylation patterns of natural products through metabolic pathway engineering and enzymatic glycodiversification. In applied biomedical research, these studies will enable the development of new glycosylation tools and generate novel glycoforms of secondary metabolites with useful biological activity.

SorF: a glycosyltransferase with promiscuous donor substrate specificity in vitro

Glycosylations are well-established steps in numerous biosynthetic pathways, and the attached sugar moieties often influence the specificity or pharmacology of the modified compounds. The sorangicins belong to the polyketide family of natural products, and exhibit antibiotic activity through inhibition of bacterial RNA polymerase. We have identified the sorangicin biosynthetic gene cluster in the producing myxobacterium Sorangium cellulosum So ce12. Within the cluster, sorF encodes a putative glycosyltransferase. To determine its function in sorangicin biosynthesis, SorF was heterologously expressed as a fusion protein in Escherichia coli. After purification by affinity chromatography, SorF was found to glucosylate sorangicin A in vitro, utilizing UDP-alpha-D-glucose as the natural donor substrate. Additionally, SorF showed high flexibility towards further UDP- and dTDP-sugars and was able to transfer several other sugar moieties-alpha-D-galactose, alpha-D-xylose, beta-L-rhamnose, and 6-deoxy-4-keto-alpha-D-glucose-onto the aglycon. SorF is therefore one of the rare glycosyltransferases able to transfer both D- and L-sugars, and could thus be used to generate novel sorangiosides.