Synthesis of UDP-GalNAc analogues as probes for the study of polypeptide-α-GalNAc-transferases. Part 2

Demonstration of an oligosaccharide-diphosphodolichol diphosphatase activity whose subcellular localization is different than those of dolichyl-phosphate-dependent enzymes of the dolichol cycle

Oligosaccharyl phosphates (OSPs) are hydrolyzed from oligosaccharide-diphosphodolichol (DLO) during protein N-glycosylation by an uncharacterized process. An OSP-generating activity has been reported in vitro, and here we asked if its biochemical characteristics are compatible with a role in endoplasmic reticulum (ER)-situated DLO regulation. We demonstrate a Co(2+)-dependent DLO diphosphatase (DLODP) activity that splits DLO into dolichyl phosphate and OSP. DLODP has a pH optimum of 5.5 and is inhibited by vanadate but not by NaF. Polyprenyl diphosphates inhibit [(3)H]OSP release from [(3)H]DLO, the length of their alkyl chains correlating positively with inhibition potency. The diphosphodiester GlcNAc2-PP-solanesol is hydrolyzed to yield GlcNAc2-P and inhibits [(3)H]OSP release from [(3)H]DLO more effectively than the diphosphomonoester solanesyl diphosphate. During subcellular fractionation of liver homogenates, DLODP codistributes with microsomal markers, and density gradient centrifugation revealed that the distribution of DLODP is closer to that of Golgi apparatus-situated UDP-galactose glycoprotein galactosyltransferase than those of dolichyl-P-dependent glycosyltransferases required for DLO biosynthesis in the ER. Therefore, a DLODP activity showing selectivity toward lipophilic diphosphodiesters such as DLO, and possessing properties distinct from other lipid phosphatases, is identified. Separate subcellular locations for DLODP action and DLO biosynthesis may be required to prevent uncontrolled DLO destruction.

Recent advances in synthesis of bacterial rare sugar building blocks and their applications

This Highlight describes recent advances in the synthesis of the bacterial deoxy amino hexopyranoside building blocks and their application in constructing various biologically important bacterial O-glycans.

De novo synthesis of D- and L-fucosamine containing disaccharides

The availability of rare monosaccharides that cannot be isolated from natural sources is currently limiting the access to the synthesis and the biological evaluation of complex bacterial cell-surface glycans. Here, we report the synthesis of D- and L-fucosamine building blocks by a de novo approach from L- and D-Garner aldehydes. These differentially protected monosaccharide building blocks were utilized to prepare disaccharides present on the surface of Pseudomonas aeruginosa bacteria.

Evaluation of analogues of GalNAc as substrates for enzymes of the mammalian GalNAc salvage pathway

Changes in glycosylation are correlated to disease and associated with differentiation processes. Experimental tools are needed to investigate the physiological implications of these changes either by labeling of the modified glycans or by blocking their biosynthesis. N-Acetylgalactosamine (GalNAc) is a monosaccharide widely encountered in glycolipids, proteoglycans, and glycoproteins; once taken up by cells it can be converted through a salvage pathway to UDP-GalNAc, which is further used by glycosyltransferases to build glycans. In order to find new reporter molecules able to integrate into cellular glycans, synthetic analogues of GalNAc were prepared and tested as substrates of both enzymes acting sequentially in the GalNAc salvage pathway, galactokinase 2 (GK2) and uridylpyrophosphorylase AGX1. Detailed in vitro assays identified the GalNAc analogues that can be transformed into sugar nucleotides and revealed several bottlenecks in the pathway: a modification on C6 is not tolerated by GK2; AGX1 can use all products of GK2 although with various efficiencies; and all analogues transformed into UDP-GalNAc analogues except those with alterations on C4 are substrates for the polypeptide GalNAc transferase T1. Besides, all analogues that could be incorporated in vitro into O-glycans were also integrated into cellular O-glycans as attested by their detection on the cell surface of CHO-ldlD cells. Altogether our results show that GalNAc analogues can help to better define structural requirements of the donor substrates for the enzymes involved in GalNAc metabolism, and those that are incorporated into cells will prove valuable for the development of novel diagnostic and therapeutic tools.

Enzymatic synthesis of UDP-GlcNAc/UDP-GalNAc analogs using N-acetylglucosamine 1-phosphate uridyltransferase (GlmU)

Reports the generation of a library composed of UDP-GlcNAc/UDP-GalNAc and investigates the substrate specificity of Escherichia coli GlcNAc-1-P uridyltransferase GlmU.

Synthesis of uridine 5'-diphosphoiduronic acid: a potential substrate for the chemoenzymatic synthesis of heparin

An improved understanding of the biological activities of heparin requires structurally defined heparin oligosaccharides. The chemoenzymatic synthesis of heparin oligosaccharides relies on glycosyltransferases that use UDP-sugar nucleotides as donors. Uridine 5'-diphosphoiduronic acid (UDP-IdoA) and uridine 5'-diphosphohexenuronic acid (UDP-HexUA) have been synthesized as potential analogues of uridine 5'-diphosphoglucuronic acid (UDP-GlcA) for enzymatic incorporation into heparin oligosaccharides. Non-natural UDP-IdoA and UDP-HexUA were tested as substrates for various glucuronosyltransferases to better understand enzyme specificity.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.