Shusaku Daikoku

Both Isoforms of Human UDP-glucose:glycoprotein Glucosyltransferase are Enzymatically Active

Being recognized as an important constituent of the glycoprotein folding cycle, uridine diphosphate-glucose:glycoprotein glucosyltransferase (UGGT) has been a subject of intense study. Up to now, it is two isoforms, UGGT1 and 2 have been identified, which share ∼ 50% amino acid identity. UGGT1 is a well-documented enzyme which functions as a folding sensor in the endoplasmic reticulum, by the virtue of its ability to transfer a glucose residue to non-glucosylated high-mannose-type glycans of immature glycoproteins exhibiting non-native conformation. On the other hand, direct evidence to support the glucosyltransferase activity of UGGT2 has been lacking, leaving it unclear as to whether it has any function in the glycoprotein folding process. This study aimed to reveal the property of human UGGT2 by using synthetic substrates such as fluorescently labeled glycans and N-glycosylated proteins. The analysis, for the first time, revealed the glucosyltransferase activity of UGGT2, whose specificity was shown to be quite similar to UGGT1, in terms of both glycan specificity and preferential recognition of proteins having non-native conformations. Finally, Sep15 was found to form the heterodimeric complex with both isoforms of UGGT and markedly enhanced its glucosyltransferase activity.

Analysis of a series of isomeric oligosaccharides by energy-resolved mass spectrometry: a challenge on homobranched trisaccharides

Glycans exist as part of glycoproteins and glycolipids, which are involved in a variety of biological functions. The analysis of glycan structures, particularly that of structural isomers, is fundamentally important since isomeric glycans often show distinct functions; however, a method for their structural elucidation has not yet been established. Anomeric configurations, linkage positions and branching are the major factors in glycans and their alteration results in a large diversity of glycan structures. The analysis of vicinally substituted oligosaccharides is extremely difficult because the product ions formed in tandem mass spectrometry (MS/MS) often have the same m/z values. In our endeavor to address the issue, we analyzed a series of homo-substituted trisaccharides consisting only of glucose by collision-induced dissociation (CID), especially energy-resolved mass spectrometry (ERMS). It was found that these structurally related glycans could be distinguished by taking advantage of differences in their activation energies in ERMS.

Profiling Aglycon-Recognizing Sites of UDP-glucose:glycoprotein Glucosyltransferase by Means of Squarate-Mediated Labeling

Because of its ability to selectively glucosylate misfolded glycoproteins, UDP-glucose:glycoprotein glucosyltransferase (UGGT) functions as a folding sensor in the glycoprotein quality control system in the endoplasmic reticulum (ER). The unique property of UGGT derives from its ability to transfer a glucose residue to N-glycan moieties of incompletely folded glycoproteins. We have previously discovered nonproteinic synthetic substrates of this enzyme, allowing us to conduct its high-sensitivity assay in a quantitative manner. In this study, we aimed to conduct site-selective affinity labeling of UGGT using a functionalized oligosaccharide probe to identify domain(s) responsible for recognition of the aglycon moiety of substrates. To this end, a probe 1 was designed to selectively label nucleophilic amino acid residues in the proximity of the canonical aglycon-recognizing site of human UGGT1 (HUGT1) via squaramide formation. As expected, probe 1 was able to label HUGT1 in the presence of UDP. Analysis by nano-LC-ESI/MS(n) identified a unique lysine residue (K1424) that was modified by 1. Kyte-Doolittle analysis as well as homology modeling revealed a cluster of hydrophobic amino acids that may be functional in the folding sensing mechanism of HUGT1.

Analysis of behavior of sodiated sugar hemiacetals under low-energy collision-induced dissociation conditions and application to investigating mutarotation and mechanism of a glycosidase

Analysis of anomericity is one of the most important issues in the structure elucidation of carbohydrates. Mass spectrometry (MS)-based methods are of particular interest and important to address the issue related to resolving anomericity of monosaccharide units in a glycan. However, direct analysis of hemiacetals has not been possible by MS because of the nonavailability of information regarding the gas-phase behavior of such ion species. We addressed this issue by using stage-discriminated energy-resolved mass spectrometry (ERMS) at the stages of MSn and MSn+1 and showed that such analysis can be made. This was achieved by proving that individual anomers can be identified and that the equilibrium of sodium adducted ion species of α- and β-anomers can be negated in the gas phase under collision-induced dissociation (CID) conditions. On the basis of these results, we could 1) observe the mutarotation of lactose and 2) speculate the hydrolysis mechanism of endo-glycosylceramidase by using mass spectrometry.

Energy-Resolved Structural Details Obtained from Gangliosides

Gangliosides, a family of glycosphingolipids (GSLs) that comprise sialic acid residue(s), are an important class of molecules that exist on the outer surface of the plasma membrane. To assess the functions of a particular series of gangliosides that play important roles in brain functions, their structures and localizations need to be investigated. We studied the structures of these gangliosides by collision-induced dissociation using quadrupole ion-trap mass spectrometry. The dissociation processes were investigated in detail based on energy-resolved mass spectrometry using sodiated molecules. The decision of utilization of the positive mode was based on the assumption that it was the generally applicable method for GSLs, including neutral ones. In this investigation, sialic acid residues were esterified to stabilize the linkages and to generate multiple fragment ions for successful structural investigations. A detailed analysis of a series of sodiated species of gangliosides based on energy-resolved mass spectrometry revealed that the GM1-equivelent fragments generated from the precursor ions under low energy CID conditions had the structural characteristics of their individual precursors. It was suggested that this information will be useful in determining the structures of their precursor gangliosides.

Solid‐Phase Synthesis of Sialyl Tn Antigen

Solid‐phase synthesis of sialyl Tn [α‐Neu5Ac‐(2→6)‐α‐GalNAc‐(1→O)‐Ser] antigen with Kenners acylsulfonamide linker is described. The acylsulfonamide bond was found to be stable under glycosylation reactions using dimethyl(methylthio)sulfonium triflate (DMTST) as a promoter and basic conditions used for the removal of protecting groups. The solid‐phase reaction was monitored by the inverse gated decoupling 13C NMR technique, which enabled quantitative analysis of the reaction progress. At the end of the synthesis, the sulfamyl group of the linker was activated by treatment with (trimethylsilyl)diazomethane to provide a N‐methyl‐N‐acylsulfonamide. The acyl group was displaced with hydroxide to give the corresponding precursors of sialyl Tn antigen and its anomeric isomers, which were deprotected to afford the target molecules.

Diastereomeric resolution directed towards chirality determination focussing on gas-phase energetics of coordinated sodium dissociation.

Defining chiral centres is addressed by introducing a pair of chiral auxiliary groups. Ions of diastereomeric pairs of molecules could be distinguished utilising energy-resolved mass spectrometry, and the applicability of the method to a series of compounds carrying amine, carboxylic acid, alcohol, and all the amino acids was verified. The method was further strengthened by distinguishing diastereomeric ions that did not undergo fragmentation. Mass spectrometric evaluation of the dissociation process of adducted sodium cations from the diastereomeric precursors agreed with the theoretical calculations, indicating the potential usefulness of the method for the determination of absolute configurations.

Multi-stage mass spectrometric information obtained by deconvolution of energy-resolved spectra acquired by triple-quadrupole mass spectrometry

Triple-quadrupole mass spectrometry (TQ-MS) provides the capability to carry out collision-induced dissociation (CID) and it offers advantages in quantification when connected with high-performance liquid chromatography through an electrospray ionization interface. However, although TQ-MS provides information on partial structures through the analysis of product ions obtained by CID experiments, the method only provides single-stage CID experiments, which limits the detailed structural information that can be obtained. Herein, a method of overcoming this limitation of TQ-MS is described. A spectrum obtained by energy-resolved mass spectrometry (ERMS) was used to deconvolute the fragmentation process, with a Galili-antigenic trisaccharide derivative being used as an example. A replot of the ERMS data showing the ratios of the product ions to the precursor ion resulted in a descriptive graph. Analysis of the sum of the ratios of individual product ions to the precursor ion at specific CID energies revealed that the members of a series of product ions were related to each other. The obtained relationships and the m/z values of the product ions provided information on the fragmentation process taking place during the dissociation, indicating that the ERMS spectrum obtained by TQ-MS contained equivalent information to that obtainable by multi-stage MS/MS (MS(n); n≥2). This method may allow users of triple-quadrupole mass spectrometers to obtain MS(n)-type information by performing a single ERMS experiment, which is even advantageous over quadrupole ion trap (QIT)-MS/MS because CID experiments on individual first-generation product ions are not required.

Non-enzymatic reaction of glycosyl oxazoline with peptides.

Recently, a number of chemoenzymatic strategies have been explored for achieving preparation of homogeneous glycopeptides and glycoproteins, especially by using endoglycanases and glycosyl oxazolines. However, concomitant occurrence of non-enzymatic reactions has been reported, but no further characterization of the byproducts was conducted. In this work, we made an attempt to identify the side product by using model substrates. Analysis of the product allowed us to propose that the oxazoline ring was attacked by the amino group of lysine, leading to the formation of disubstituted acetamidine.

Stereospecific generation and analysis of α- and β-hemiacetals of monosaccharides in gas phase

A series of Boc-protected 4-aminobutyl α- and β-glycosides of commonly found neutral monosaccharides were synthesized. The sodium adducted ions of these individual molecules were used in producing corresponding α- and β-anomers of hemiacetal species under collision-induced dissociation (CID) conditions. The Boc group was successfully removed under CID conditions producing 4-aminobutyl glycosides, which were then used as the precursors. An intramolecular attack of the aglyconic nitrogen atom onto C-1 position of aglycon assisted to leave hemiacetal ion species without affecting anomeric configurations. In this manner, stereospecific syntheses of sugar hemiacetals were first achieved in gas phase. The dissociation of sodium cation from a series of these hemiacetals was further studied according to energy-resolved mass spectrometry. In this study, it was found that all the sugar hemiacetals could be distinguished even if they have same m/z values. Furthermore, the order of affinity of Na(+) toward the hemiacetals was determined.