Attachment of oligosaccharides to proteins is a major post-translational modification. Chemical syntheses of oligosaccharides have contributed to clarifying the functions of these oligosaccharides. However, syntheses of oligosaccharide-linked proteins are still challenging because of their inherent complicated structures, including diverse di- to tetra-antennary forms. We report a highly efficient strategy to access the representative two types of triantennary oligosaccharides through only 9- or 10-step chemical conversions from a biantennary oligosaccharide, which can be isolated in exceptionally homogeneous form from egg yolk. Four benzylidene acetals were successfully introduced to the terminal two galactosides and two core mannosides of the biantennary asialononasaccharide bearing 24 hydroxy groups, followed by protection of the remaining hydroxy groups with acetyl groups. Selective removal of one of the benzylidene acetals gave two types of suitably protected glycosyl acceptors. Glycosylation toward the individual acceptors with protected Gal-β-1,4-GlcN thioglycoside and subsequent deprotection steps successfully yielded two types of complex-type triantennary oligosaccharides.

The glycosylation of proteins contributes to the modulation of the structure and biological activity of glycoproteins. Asparagine-linked glycans (N-glycans) of glycoproteins naturally exhibit diverse antennary patterns, such as bi-, tri-, and tetra-antennary forms. However, there are no chemical or biological methods to obtain homogeneous glycoproteins via the intentional alteration of the antennary form of N-glycans. Thus, the functions of the individual antennary form of N-glycan at a molecular level remain unclear. Herein, we report the chemical synthesis of an erythropoietin (EPO) glycoform having a triantennary sialylglycan at position 83, as well as two biantennary sialylglycans at both positions 24 and 38. We demonstrated efficient liquid-phase condensation reactions to prepare a sialylglycopeptide having a triantennary N-glycan prepared by the addition of a Neu5Ac-α-2,6-Gal-β-1,4-GlcNAc element to the biantennary glycan under semisynthetic conditions. The molecular weight of the newly added antennary element was ∼3% of the EPO glycoform, and the introduced position was the most distant from the bioactive protein. However, in vivo assays using mice revealed that the additional antennary element at position 83 dramatically increased the hematopoietic activity compared to a commercially available native EPO. These unprecedented data clearly indicate that the antennary pattern of N-glycans inherently plays a critical role in the modulation of protein functions.

A "D-scan" of two small proteins, the disulfide-rich Ecballium elaterium trypsin inhibitor II (EETI-II) and a minimized Z domain of protein A (Z33), is reported. For each protein, the stereochemistry of one amino acid at a time was inverted to generate a series of diastereomers. In much the same way an alanine scan determines necessary residues for protein function, the D-scan elucidated the critical stereocenters of the 30-residue EETI-II and the 33-residue Z33. The folding properties and activity of each variant were investigated. A total of 24 out of 30 EETI-II D-scan analogues folded to give a three-disulfide product. Of the 24 variants that folded, half were high-affinity trypsin inhibitors, and three were as active as the wild type (WT). Of these 12 active variants, most were substantially less stable to reduction than WT EETI-II (WT first reduction potential -270.0 ± 1.5 mV, WT second reduction potential -307.2 ± 1.1 mV). Similarly, ten Z33 analogues retained high binding affinity to IgG (KD < 250 nM, WT: 24 ± 1 nM) and 12 additional analogues had reduced but appreciable IgG binding affinity (KD between 250 nM and 2.5 μM). As with EETI-II, most Z33 analogues were substantially less stable than the WT (ΔG(H2O, 263 K) = 2.4 ± 1.2 kcal/mol). Collectively, our findings show that the D-scan is powerful new strategy for studying how the stereochemistry of amino acids affects the structure and function of proteins.

Glycosylation is a major modification of secreted and cell surface proteins, and the resultant glycans show considerable heterogeneity in their structures. To understand the biological processes arising from each glycoform, the preparation of homogeneous glycoproteins is essential for extensive biological experiments. To establish a more robust and rapid synthetic route for the synthesis of homogeneous glycoproteins, we studied several key reactions based on amino thioacids. We found that diacyl disulfide coupling (DDC) formed with glycosyl asparagine thioacid and peptide thioacid yielded glycopeptides. This efficient coupling reaction enabled us to develop a new glycoprotein synthesis method, such as the bifunctional thioacid-mediated strategy, which can couple two peptides with the N-and C-termini of glycosyl asparagine thioacid. Previous glycoprotein synthesis methods required valuable glycosyl asparagine in the early stage and subsequent multiple glycoprotein synthesis routes, whereas the developed concept can generate glycoproteins within a few steps from peptide and glycosyl asparagine thioacid. Herein, we report the characterization of the DDC of amino thioacids and the efficient ability of glycosyl asparagine thioacid to be used for robust glycoprotein semisynthesis.

Biological systems, including ribosomes and enzymes, produce peptides with an extraordinary high speed and accuracy. On the other hand, a rational and regioselective α-peptide bond formation, without involving protecting groups, is difficult to achieve in chemical synthesis. In this study, α-amino thioacids were utilized for the generation of polypeptides without using any protecting groups. We found that an α-amino thioacid could oxidatively form a diaminoacyl-disulfide moiety and undergo a subsequent intramolecular S- to N-acyl transfer to form an α-peptide bond. Even the thioacid form of lysine, which has a free ε-amino group, generated a regioselective α-peptide bond. The oxidation of amino thioacids generated the oligomers of amino acids. Interestingly, this oligomerization reaction proceeded even in the presence of iron ore, a prebiotic element, thus suggesting a plausible prebiotic peptide bond forming reaction.

Prion diseases are transmissible, lethal neurodegenerative disorders caused by accumulation of the aggregated scrapie form of the prion protein (PrPSc) after conversion of the cellular prion protein (PrPC). The glycosylphosphatidylinositol (GPI) anchor of PrPC is involved in prion disease pathogenesis, and especially sialic acid in a GPI side chain reportedly affects PrPC conversion. Thus, it is important to define the location and structure of the GPI anchor in human PrPC. Moreover, the sialic acid linkage type in the GPI side chain has not been determined for any GPI-anchored protein. Here we report GPI glycan structures of human PrPC isolated from human brains and from brains of a knock-in mouse model in which the mouse prion protein (Prnp) gene was replaced with the human PRNP gene. LC–electrospray ionization–MS analysis of human PrPC from both biological sources indicated that Gly229 is the ω site in PrPC to which GPI is attached. Gly229 in human PrPC does not correspond to Ser231, the previously reported ω site of Syrian hamster PrPC. We found that ∼41% and 28% of GPI anchors in human PrPCs from human and knock-in mouse brains, respectively, have N-acetylneuraminic acid in the side chain. Using a sialic acid linkage-specific alkylamidation method to discriminate α2,3 linkage from α2,6 linkage, we found that N-acetylneuraminic acid in PrPC's GPI side chain is linked to galactose through an α2,3 linkage. In summary, we report the GPI glycan structure of human PrPC, including the ω-site amino acid for GPI attachment and the sialic acid linkage type.

Sequential mannose trimming of N-glycan, from M9 to M8B and then to oligosaccharides exposing the a1,6-linked mannosyl residue (M7A, M6 and M5), facilitates endoplasmic reticulum-associated degradation of misfolded glycoproteins (gpERAD). We previously showed that EDEM2 stably disulfide-bonded to the thioredoxin domain-containing protein TXNDC11 is responsible for the first step (George et al., 2020). Here, we show that EDEM3 and EDEM1 are responsible for the second step. Incubation of pyridylamine-labeled M8B with purified EDEM3 alone produced M7 (M7A and M7C), M6 and M5. EDEM1 showed a similar tendency, although much lower amounts of M6 and M5 were produced. Thus, EDEM3 is a major a1,2-mannosidase for the second step from M8B. Both EDEM3 and EDEM1 trimmed M8B from a glycoprotein efficiently. Our confirmation of the Golgi localization of MAN1B indicates that no other a1,2-mannosidase is required for gpERAD. Accordingly, we have established the entire route of oligosaccharide processing and the enzymes responsible.

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