Synthesis of Sialyl TN Glycopeptides - Enzymatic Sialylation by α2,6-Sialyltransferase fromPhotobacterium damsela

Chemoenzymatic Synthesis of SARS-CoV-2 Homogeneous O-Linked Glycopeptides for Exploring Their Inhibition Functions

Harnessing highly conserved peptides derived from the receptor binding domain (RBD) of spike (S) protein to construct peptide-based inhibitors is one of the most effective strategies to fight against the ever-mutating coronavirus SARS-CoV-2. But how the O-glycosylation affects their inhibition abilities has not been intensively explored. Herein, an intrinsic O-glycosylated peptide P_320-334 derived from RBD was screened and homogeneous O-linked glycopeptides containing Tn (GalNAcα1-O-Ser/Thr), T (Galβ1-3GalNAcα1-O-Ser/Thr), sialyl-Tn (sTn, Siaα2-6GalNAcα1-O-Ser/Thr), and sialyl-T (sT, Siaα2-3Galβ1-3GalNAcα1-O-Ser/Thr) structures were first synthesized via chemoenzymatic strategies. Compared with the unglycosylated peptide, the binding of sT-P_320-334 to hACE2 was enhanced to 133% and the inhibition capacity against RBD-hACE2 binding of sTn- and sT-P_320-334 was significantly increased up to 150-410%. Thus, our results suggest the sialic acid residue on the terminal of short O-glycan structures might strengthen the inhibition capacities of these peptide-based inhibitors, which might provide novel optimization directions for the inhibitor design.

Chemo-enzymatic synthesis of the ALG1-CDG biomarker and evaluation of its immunogenicity

Congenital disorders of glycosylation (CDG) are a growing group diseases that result from defects in genes involved in glycan biosynthesis pathways. One tetrasaccharide, i.e., Neu5Ac-α2, 6-Gal-β1, 4-GlcNAc-β1, 4-GlcNAc, was recently reported as the biomarker of ALG1-CDG, the disease caused by ALG1 deficiency. To develop a novel diagnostic method for ALG1-CDG, chemo-enzymatic synthesis of the tetrasaccharide biomarker linked to phytanyl phosphate and the biomarker's immune stimulation were investigated in this study. The immunization study using liposomes bearing phytanyl-linked tetrasaccharide revealed that they stimulated a moderate immune response. The induced antibody showed strong binding specificity for the ALG1-CDG biomarker, indicating its potential in medical applications.

Redox-Controlled Site-Specific α2-6-Sialylation

The first bacterial α2-6-sialyltransferase cloned from Photobacterium damselae (Pd2,6ST) has been widely applied for the synthesis of various α2-6-linked sialosides. However, the extreme substrate flexibility of Pd2,6ST makes it unsuitable for site-specific α2-6-sialylation of complex substrates containing multiple galactose and/or N-acetylgalactosamine units. To tackle this problem, a general redox-controlled site-specific sialylation strategy using Pd2,6ST is described. This approach features site-specific enzymatic oxidation of galactose units to mask the unwanted sialylation sites and precisely controlling the site-specific α2-6-sialylation at intact galactose or N-acetylgalactosamine units.

Stereoselective synthesis of a 9-O-sulfo Neu5Gc-capped O-linked oligosaccharide found on the sea urchin egg receptor

The first total synthesis of a serine bearing α2→5-O_glycolyl-linked oligoNeu5Gc found on sea urchin egg cell surfaces has been accomplished.

Human Neuraminidase Isoenzymes Show Variable Activities for 9- O-Acetyl-sialoside Substrates

Recognition of terminal sialic acids is central to many cellular processes, and structural modification of sialic acid can disrupt these interactions. A prominent, naturally occurring, modification of sialic acid is 9- O-acetylation (9- O-Ac). Study of this modification through generation and analysis of 9- O-Ac sialosides is challenging because of the lability of the acetate group. Fundamental questions regarding the role of 9- O-Ac sialic acids remain unanswered, including what effect it may have on recognition and hydrolysis by the human neuraminidase enzymes (hNEU). To investigate the substrate activity of 9- O-acetylated sialic acids (Neu5,9Ac₂), we synthesized an acetylated fluorogenic hNEU substrate 2'-(4-methylumbelliferyl)-9- O-acetyl-α-d- N-acetylneuraminic acid. Additionally, we generated a panel of octyl sialyllactosides containing modified sialic acids including variation in linkage, 9- O-acetylation, and C-5 group (Neu5Gc). Relative rates of substrate cleavage by hNEU were determined using fluorescence spectroscopy and electrospray ionization mass spectrometry. We report that 9- O-acetylation had a significant, and differential, impact on sialic acid hydrolysis by hNEU with general substrate tolerance following the trend of Neu5Ac > Neu5Gc ≫ Neu5,9Ac₂ for NEU2, NEU3, and NEU4. Both NEU2 and NEU3 had remarkably reduced activity for Neu5,9Ac₂ containing substrates. Other isoenzymes appeared to be more tolerant, with NEU4 even showing increased activity on Neu5,9Ac₂ substrates with an aryl aglycone. The impact of these minor structural changes to sialic acid on hNEU activity was unexpected, and these results provide evidence of the substantial influence of 9- O-Ac modifications on hNEU enzyme substrate specificity. Furthermore, these findings may implicate hNEU in processes governed by 9- O-acetyltransferases and -esterases.

A Photobacterium sp. α2-6-sialyltransferase (Psp2,6ST) mutant with an increased expression level and improved activities in sialylating Tn antigens

In order to improve the catalytic efficiency of recombinant Photobacterium sp. JT-ISH-224 α2-6-sialyltransferase Psp2,6ST(15-501)-His6 in sialylating α-GalNAc-containing acceptors for the synthesis of tumor-associated carbohydrate antigens sialyl Tn (STn), protein crystal structure-based mutagenesis studies were carried out. Among several mutants obtained by altering the residues close to the acceptor substrate binding pocket, mutant A366G was shown to improve the sialyltransferase activity of Psp2,6ST(15-501)-His6 toward α-GalNAc-containing acceptors by 21-115% without significantly affecting its sialylation activity to β-galactosides. Furthermore, the expression level was improved from 18-40 mg L(-1) for the wild-type enzyme to 72-110 mg L(-1) for the A366G mutant. In situ generation of CMP-sialic acid in a one-pot two-enzyme system was shown effective in overcoming the high donor hydrolysis of the enzyme. Mutant A366G performed better than the wild-type Psp2,6ST(15-501)-His6 for synthesizing Neu5Acα2-6GalNAcαOSer/Thr STn antigens.

One-pot multi-enzyme (OPME) chemoenzymatic synthesis of sialyl-Tn-MUC1 and sialyl-T-MUC1 glycopeptides containing natural or non-natural sialic acid

A series of STn-MUC1 and ST-MUC1 glycopeptides containing naturally occurring and non-natural sialic acids have been chemoenzymatically synthesized from Tn-MUC1 glycopeptide using one-pot multienzyme (OPME) approaches. In situ generation of the sialyltransferase donor cytidine 5'-monophosphate-sialic acid (CMP-Sia) using a CMP-sialic acid synthetase in the presence of an extra amount of cytidine 5'-triphosphate (CTP) and removal of CMP from the reaction mixture by flash C18 cartridge purification allow the complete consumption of Tn-MUC1 glycopeptide for quantitative synthesis of STn-MUC1. A Campylobacter jejuni β1-3GalT (CjCgtBΔ30-His6) mutant has been found to catalyze the transfer of one or more galactose residues to Tn-MUC1 for the synthesis of T-MUC1 and galactosylated T-MUC1. Sialylation of T-MUC1 using Pasteurella multocida α2-3-sialyltransferase 3 (PmST3) with Neisseria meningitidis CMP-sialic acid synthetase (NmCSS) and Escherichia coli sialic acid aldolase in one pot produced ST-MUC1 efficiently. These glycopeptides are potential cancer vaccine candidates.

Efficient chemoenzymatic synthesis of sialyl Tn-antigens and derivatives

An N-terminal and C-terminal truncated recombinant α2-6-sialyltransferase cloned from Photobacterium sp. JH-ISH-224, Psp2,6ST(15-501)-His(6), was shown to be an efficient catalyst for one-pot three-enzyme synthesis of sialyl Tn (STn) antigens and derivatives containing natural and non-natural sialic acid forms.

Trans-sialidase activity of Photobacterium damsela alpha2,6-sialyltransferase and its application in the synthesis of sialosides

Trans-sialidases catalyze the transfer of a sialic acid from one sialoside to an acceptor to form a new sialoside. alpha2,3-Trans-sialidase activity was initially discovered in the parasitic protozoan Trypanosoma cruzi, and more recently was found in a multifunctional Pasteurella multocida sialyltransferase PmST1. alpha2,8-Trans-sialidase activity was also described for a multifunctional Campylobacter jejuni sialyltransferase CstII. We report here the discovery of the alpha2,6-trans-sialidase activity of a previously reported recombinant truncated bacterial alpha2,6-sialyltransferase from Photobacterium damsela (Delta15Pd2,6ST). This is the first time that the alpha2,6-trans-sialidase activity has ever been identified. Kinetic studies indicate that Delta15Pd2,6ST-catalyzed trans-sialidase reaction follows a ping-pong bi-bi reaction mechanism. Cytidine 5'-monophosphate, the product of sialyltransferase reactions, is not required by the trans-sialidase activity of the enzyme but enhances the trans-sialidase activity modestly as a non-essential activator. Using chemically synthesized Neu5AcalphapNP and LacbetaMU, alpha2,6-linked sialoside Neu5Acalpha2,6LacbetaMU has been obtained in one-step in high yield using the trans-sialidase activity of Delta15Pd2,6ST. In addition to the alpha2,6-trans-sialidase activity, Delta15Pd2,6ST also has alpha2,6-sialidase activity. The multifunctionality is thus a common feature of many bacterial sialyltransferases.

N-Terminal 112 amino acid residues are not required for the sialyltransferase activity of Photobacterium damsela alpha2,6-sialyltransferase

Photobacterium damsela alpha2,6-sialyltransferase was cloned as N- and C- His-tagged fusion proteins with different lengths (16-497 aa or 113-497 aa). Expression and activity assays indicated that the N-terminal 112 amino acid residues of the protein were not required for its alpha2,6-sialyltransferase activity. Among four truncated forms tested, N-His-tagged Delta15Pd2,6ST(N) containing 16-497 amino acid residues had the highest expression level. Similar to the Delta15Pd2,6ST(N), the shorter Delta112Pd2,6ST(N) was active in a wide pH range of 7.5-10.0. A divalent metal ion was not required for the sialyltransferase activity, and the addition of EDTA and dithiothreitol did not affect the activity significantly.

Efficient chemoenzymatic synthesis of biotinylated human serum albumin-sialoglycoside conjugates containing O-acetylated sialic acids

Sialyl Tn (STn) and sialyl lactoside derivatives containing O-acetylated sialic acid residues have been chemoenzymatically synthesized using a one-pot three-enzyme system and conjugated to biotinylated human serum albumin (HSA) using an adipic acid para-nitrophenyl ester coupling reagent. This approach provides an efficient and general protocol for preparing carbohydrate-protein conjugates containing base-sensitive groups.