Chemical synthesis and antigenic activity of a phosphatidylinositol mannoside epitope from Mycobacterium tuberculosis

Phosphatidylinositol mannosides (PIMs) have been investigated as lipidic antigens for a new subunit tuberculosis vaccine. A non-natural diacylated phosphatidylinositol mannoside (Ac2PIM2) was designed and synthesized by mimicking the natural PIM6 processing procedure in dentritic cells. This synthetic Ac2PIM2 was achieved from α-methyl d-glucopyranoside 1 in 17 steps in 2.5% overall yield. A key feature of the strategy was extending the use of the chiral myo-inositol building block A to the O-2 and O-6 positions of the inositol unit to allow for introducing the mannose building blocks B1 and B2, and to the O-1 position for the phosphoglycerol building block C. Building block A, being a flexible core unit, may facilitate future access to other higher-order PIM analogues. A preliminary antigenic study showed that the synthetic PIM epitope (Ac2PIM2) was significantly more active than natural Ac2PIM2, which indicated that the synthetic Ac2PIM2 can be strongly immunoactive and may be developed as a potential vaccine.

A microbial-enzymatic strategy for producing chondroitin sulfate glycosaminoglycans

Chondroitin sulfate has been widely used in both medical and clinical applications. Commercial chondroitin sulfate has been mainly acquired from animal tissue extraction. Here we report a new two-step biological strategy for producing chondroitin sulfate A and chondroitin sulfate C. First, the chondroitin biosynthesis pathway in a recombinant Bacillus subtilis strain using sucrose as carbon source was systematically optimized and the titer of chondroitin was significantly enhanced to 7.15 g/L. Then, specific sulfation transformation systems were successfully constructed and optimized by combining the purified aryl sulfotransferase IV (ASST IV), chondroitin 4-sulfotransferase (C4ST) and chondroitin 6-sulfotransferase (C6ST). Chondroitin sulfate A and C were enzymatically transformed from chondroitin at conversion rates of 98% and 96%, respectively. The present biological strategy has great potential to be scaled up for biosynthesis of chondroitin sulfate A and C from cheap carbon sources.

Divergent Synthesis of Chondroitin Sulfate Disaccharides and Identification of Sulfate Motifs that Inhibit Triple Negative Breast Cancer

Glycosaminoglycans (GAGs) regulate many important physiological processes. A pertinent issue to address is whether GAGs encode important functional information via introduction of position specific sulfate groups in the GAG structure. However, procurement of pure, homogenous GAG motifs to probe the “sulfation code” is a challenging task due to isolation difficulty and structural complexity. To this end, we devised a versatile synthetic strategy to obtain all the 16 theoretically possible sulfation patterns in the chondroitin sulfate (CS) repeating unit; these include rare but potentially important sulfated motifs which have not been isolated earlier. Biological evaluation indicated that CS sulfation patterns had differing effects for different breast cancer cell types, and the greatest inhibitory effect was observed for the most aggressive, triple negative breast cancer cell line MDA-MB-231.

Synthesis of a chondroitin sulfate disaccharide library and a GAG-binding protein interaction analysis

Chondroitin sulfate (CS), which belongs to the glycosaminoglycan (GAG) superfamily, is a linear sulfated polysaccharide involved in various biological processes. CS structure is very heterogeneous and contains various sulfation patterns owing to the multiple and random enzymatic modifications that occur during its biosynthesis. The resultant microdomain structure in the CS chain interacts with specific biomolecules to regulate biological functions. Therefore, an analysis of the structure-activity relationship of CS at the molecular level is necessary to clarify their biofunctions. In this study, we designed the common intermediate possessing an orthogonally removable protective group and systematically synthesized all 16 types of CS disaccharide structure generated by sulfation. In addition, we demonstrated the on-time analysis of the binding properties of GAG-binding proteins using 'Sugar Chip' immobilized CS disaccharide structures by surface plasmon resonance (SPR) imaging, indicating that our chip technology is effective for the evaluation of binding properties.

Syntheses of chondroitin sulfate tetrasaccharide structures containing 4,6-disulfate patterns and analysis of their interaction with glycosaminoglycan-binding protein

Chondroitin sulfate tetrasaccharide ligand conjugates, namely GlcA-GalNAc6S-GlcA-GalNAc4S6S (CS-C+E) 1, GlcA2S-GalNAc6S-GlcA2S-GalNAc4S6S (CS-D+T) 2, GlcA-GalNAc4S6S-GlcA-GalNAc4S (CS-E+A) 3, GlcA-GalNAc4S6S-GlcA-GalNAc6S (CS-E+C) 4, and GlcA-GalNAc4S6S-GlcA-GalNAc4S6S (CS-E+E) 5, were systematically synthesized using a disaccharide building block 6. Synthesized CS tetrasaccharide structures were immobilized onto gold-coated chips to prepare array-type sugar chips, and the binding properties of protein were evaluated by surface plasmon resonance imaging biosensor. CS-D+T, CS-E+A, CS-E+C, and CS-E+E showed greater affinity for basic fibroblast growth factor than did other tetrasaccharides (CS-C+D, C+E, D+D).

Characterization of glycosaminoglycan (GAG) sulfatases from the human gut symbiont Bacteroides thetaiotaomicron reveals the first GAG-specific bacterial endosulfatase

Despite the importance of the microbiota in human physiology, the molecular bases that govern the interactions between these commensal bacteria and their host remain poorly understood. We recently reported that sulfatases play a key role in the adaptation of a major human commensal bacterium, Bacteroides thetaiotaomicron, to its host (Benjdia, A., Martens, E. C., Gordon, J. I., and Berteau, O. (2011) J. Biol. Chem. 286, 25973-25982). We hypothesized that sulfatases are instrumental for this bacterium, and related Bacteroides species, to metabolize highly sulfated glycans (i.e. mucins and glycosaminoglycans (GAGs)) and to colonize the intestinal mucosal layer. Based on our previous study, we investigated 10 sulfatase genes induced in the presence of host glycans. Biochemical characterization of these potential sulfatases allowed the identification of GAG-specific sulfatases selective for the type of saccharide residue and the attachment position of the sulfate group. Although some GAG-specific bacterial sulfatase activities have been described in the literature, we report here for the first time the identity and the biochemical characterization of four GAG-specific sulfatases. Furthermore, contrary to the current paradigm, we discovered that B. thetaiotaomicron possesses an authentic GAG endosulfatase that is active at the polymer level. This type of sulfatase is the first one to be identified in a bacterium. Our study thus demonstrates that bacteria have evolved more sophisticated and diverse GAG sulfatases than anticipated and establishes how B. thetaiotaomicron, and other major human commensal bacteria, can metabolize and potentially tailor complex host glycans.

Synthesis of chondroitin/dermatan sulfate-like oligosaccharides and evaluation of their protein affinity by fluorescence polarization

Here, we present a novel approach for the chemical synthesis of chondroitin and dermatan sulfate oligosaccharides. A key point of this strategy is the preparation and use of an N-trifluoroacetyl galactosamine building block containing a 4,6-O-di-tert-butylsilylene group. Glycosylation reactions proceeded in good yields (74-91%) with our protecting group distribution. Using this approach, we have synthesized, for the first time, a chondroitin/dermatan sulfate-like tetrasaccharide that contains both types of uronic acids, D-glucuronic and L-iduronic acid. Moreover, we have employed a fluorescence polarization competition assay to evaluate the interactions between the synthesized oligosaccharides and FGF-2 (basic fibroblast growth factor). Our results show that this method, using standard instrumentation and minimal sample consumption, is a powerful tool for the rapid analysis of the glycosaminoglycan affinity for proteins in solution.

Chondroitin-4-O-sulfatase from Bacteroides thetaiotaomicron: exploration of the substrate specificity

Bacterial sulfatases can be good tools to increase the molecular diversity of glycosaminoglycan synthetic fragments. A chondroitin 4-O-sulfatase from the human commensal bacterium Bacteroides thetaiotaomicron has recently been identified and expressed. In order to use this enzyme for synthetic purposes, the minimal structure required for its activity has been determined. For that, four 4-O-sulfated monosaccharides and one 4-O-sulfated disaccharide have been synthesized and used as substrates with the sulfatase. The minimum structure was shown to be a disaccharide but in contrast to the natural substrate, which must have a 4,5-insaturation, the enzyme accepts as substrate, a disaccharide with a saturated glucuronic acid at the non-reducing end and even a glucopyranosyl moiety without the carboxylic acid functionality.

Synthetic and semi-synthetic chondroitin sulfate oligosaccharides, polysaccharides, and glycomimetics

Chondroitin sulfate (CS) is a sulfated polysaccharide involved in a myriad of biological processes. Due to the variable sulfation pattern of CS polymer chains, the need to study in detail structure-activity relationships regarding CS biomedical features has provoked much interest in obtaining synthetic CS species. This paper reviews two decades of synthetic and semi-synthetic CS oligosaccharides, polysaccharides, and glycomimetics obtained by chemical, chemoenzymatic, enzymatic, and microbiological-chemical strategies.

From polymer to size-defined oligomers: a highly divergent and stereocontrolled construction of chondroitin sulfate A, C, D, E, K, L, and M oligomers from a single precursor: part 2

An efficient, stereocontrolled, and highly divergent approach for the preparation of oligomers of chondroitin sulfate (CS) A, C, D, E, K, L, and M variants, starting from a single precursor easily obtained by semisynthesis from abundant natural polymer is reported for the first time. Common intermediates were designed that allowed the straightforward construction of O-sulfonated species either on the D-galactosamine unit (CS-A, -C, and -E) or on both D-glucuronic acid and D-galactosamine units (CS-D and CS-K, -L, and -M). This strategy represents a successful improvement and brings a definitive answer toward the synthesis of such complex molecules with numerous relevant biological functions.

Solid-phase synthesis of alpha-glucosamine sulfoforms with fragmentation analysis by tandem mass spectrometry

Sulfated epitopes of alpha-glucosamine (GlcN sulfoforms) were prepared by solid-phase synthesis as models of internal glucosamines within heparan sulfate. An orthogonally protected 2'-hydroxyethyl GlcN derivative was immobilized on a trityl resin support and subjected to regioselective deprotection and sulfonation conditions, which were optimized with the aid of on-resin infrared or Raman analysis. The sulfoforms were cleaved from the resin under mild Lewis acid conditions without affecting the O- or N-sulfate groups and purified by reversed-phase high-performance liquid chromatography (HPLC). The alpha-GlcN sulfoforms and their 4- O-benzyl ethers were examined by electrospray ionization tandem mass spectrometry (ESI-MS/MS), with product ion spectra produced by collision-induced dissociation (CID). ESI-MS/MS revealed significant differences in parent ion stabilities and fragmentation rates as a function of sulfate position. Ion fragmentation by CID resulted in characteristic mass losses with strong correlation to the positions of both free hydroxyl groups and sulfate ions. Most of these fragmentation patterns are consonant with elimination pathways, and suggest possible strategies for elucidating the structures of glucosamine-derived sulfoforms with identical m/ z ratios. In particular, fragmentation analysis can easily distinguish GlcN sulfoforms bearing the relatively rare 3- O-sulfate from isomers with the more common 6- O-sulfate.

Carbohydrate post-glycosylational modifications

Carbohydrate modification is a common phenomenon in nature. Many carbohydrate modifications such as some epimerization, O-acetylation, O-sulfation, O-methylation, N-deacetylation, and N-sulfation, take place after the formation of oligosaccharide or polysaccharide backbones. These modifications can be categorized as carbohydrate post-glycosylational modifications (PGMs). Carbohydrate PGMs further extend the complexity of the structures and the synthesis of carbohydrates and glycoconjugates. They also increase the capacity of the biological regulation that is achieved by finely tuning the structures of carbohydrates. Developing efficient methods to obtain structurally defined naturally occurring oligosaccharides, polysaccharides, and glycoconjugates with carbohydrate PGMs is essential for understanding the biological significance of carbohydrate PGMs. Combined with high-throughput screening methods, synthetic carbohydrates with PGMs are invaluable probes in structure-activity relationship studies. We illustrate here several classes of carbohydrates with PGMs and their applications. Recent progress in chemical, enzymatic, and chemoenzymatic syntheses of these carbohydrates and their derivatives are also presented.

Total synthesis of desferrisalmycin B

The first total synthesis of a naturally occurring siderophore antibiotic, desferrisalmycin B, is described, and the configuration of the unknown stereocenter is assigned. The synthesis features a synthetic strategy of constructing the novel amino-heptopyranoside component by stereoselective dihydroxylation followed by a Bose-modified Mitsunobu reaction. Through this convergent approach, other members of salmycins should also be synthetically accessible.

Combinatorial chemistry toward understanding the function(s) of carbohydrates and carbohydrate conjugates

Combinatorial chemistry has contributed significantly to understanding the structure-function relationships of biologically important molecules such as proteins and nucleic acids. However, carbohydrates and carbohydrate conjugates, which have been identified as key modulators of several biological functions have not enjoyed the same measure of success. The complexity and synthetic challenges of carbohydrate conjugates have resulted in a number of conceptual approaches to rapidly access sufficient quantities of these biomolecules. This article summarizes these combinatorial approaches and also highlights fully automated library synthesis of artificial glycopeptides with the goals of understanding their biological roles.

Design, synthesis and preliminary biological evaluation of a focused combinatorial library of stereodiverse carbohydrate-scaffold-based peptidomimetics

A focused combinatorial library of 126 mimetics of the RGD sequence based on sugar scaffolds have been rationally constructed using molecular modeling, with a particular emphasis on the stereodiversity of the library. A liquid phase, mix and divide synthesis was used, active compounds being identified by using orthogonal libraries and recursive deconvolution strategies.

Synthesis of natural-product-based compound libraries

Natural products cover a diversity space not yet available from synthetic libraries, with an unrivalled success rate as drug leads. The combinatorial synthesis of non-oligomeric natural-product-based libraries, however, is still limited to few examples because access to easily modified units strongly depends on the availability of a core structure either from a natural source, or through a suitable synthetic route. Only a few resourceful groups have managed the latter approach for more demanding multifunctional natural drug leads, such as epothilones.