In the workflow of global N-glycosylation analysis, endoglycosidase-mediated removal of glycans from glycoproteins is an essential and rate-limiting step. Peptide-N-glycosidase F (PNGase F) is the most appropriate and efficient endoglycosidase for the removal of N-glycans from glycoproteins prior to analysis. Due to the high demand for PNGase F in both basic and industrial research, convenient and efficient methods are urgently needed to generate PNGase F, preferably in the immobilized form to solid phases. However, there is no integrated approach to implement both efficient expression, and site-specific immobilization of PNGase F. Herein, efficient production of PNGase F with a glutamine tag in Escherichia coli and site-specific covalent immobilization of PNGase F with this special tag via microbial transglutaminase (MTG) is described. PNGase F was fused with a glutamine tag to facilitate the co-expression of proteins in the supernatant. The glutamine tag was covalently and site-specifically transformed to primary amine-containing magnetic particles, mediated by MTG, to immobilize PNGase F. Immobilized PNGase F could deglycosylate substrates with identical enzymatic performance to that of the soluble counterpart, and exhibit good reusability and thermal stability. Moreover, the immobilized PNGase F could also be applied to clinical samples, including serum and saliva.

The N-glycosylation of proteins is a typical post-translational modification. Compared with other monoclonal antibodies, N-glycosylation modification in cetuximab is more complicated. Because cetuximab contains two N-glycosylation sites, one is located on the antigen-binding fragment (Fab) and the other is on the crystallizable fragment (Fc) of the heavy chain (HC). Among the two, the glycosylation of the Fab segment is more complicated. As this segment is located in the hypervariable region (VH), it may affect the affinity of the antibody antigen and cause other issues. Therefore, it is necessary to study glycosylation modification at this site. This modification is particularly challenging, necessitating the development of specific glycan cutting technology and a stable glycan ratio analysis method. In this study, cetuximab expressed in Chinese hamster ovary (CHO) cell was used as the experimental research object. Based on the digestion with endo-β-N-acetylglucosaminidase F2 (Endo F2), an experimental method was developed that can quickly release Fab glycans. Qualitative and glycan ratio analyses were carried out by ultra performance liquid chromatography-high resolution mass spectrometry (UPLC-HRMS). The test was divided into two steps: in the first step, a non-denaturing (native state) glycosidase excision test was performed on the CHO-cetuximab drug substance. The drug substance was diluted to 1.0 mg/mL by adding ultrapure water, following which 1.0 μL of Endo F2 was directly added to 100 μL of the drug substance for enzyme digestion at 37 ℃. Through HRMS, the data were deconvoluted to obtain the accurate mass of the drug substance. The results showed that when the digestion time of Endo F2 was 5 min, the glycans in the Fab segment could be completely removed, whereas those in the Fc segment were not affected. Rapid enzyme cutting of the Fab glycans was realized; simultaneously, it was concluded that this method was also very specific for the removal of Fab glycans. In the second step, an accurate ratio analysis test was performed on Fab glycans excised from CHO-cetuximab. The released Fab glycans were precipitated with ice ethanol, the supernatant was centrifuged and spin-dried, and then labeled with para-aminobenzyl (2-AB). 2-AB labeling could make glycans have fluorescent detectable signals, and after reconstitution in 70% acetonitrile aqueous solution, was detected by UPLC coupled with a fluorescence detector (FLR). Good chromatographic peak separation was obtained using a hydrophilic interaction chromatography (HILIC) column. Thus, the test enabled stable glycan ratio analysis. The molecular weight results for three independent Endo F2 digestion cycles for 5 min showed that the masses after digestion were similar; subsequently, glycan ratio analysis was performed based on HILIC. The results of three independent glycan ratio analysis experiments were also similar, indicating that the rapid enzyme digestion of Endo F2 followed by glycan ratio analysis after 5 min of digestion yielded good stability and reliability. Data obtained by measuring the samples produced using two different processes employed by our company showed that there were distinct differences in the glycan profiles of the two processes, especially in terms of the sialic acid glycoforms. These results prove that the method developed in this study can accurately analyze the ratio of glycans. Monitoring the antibody production process is important and meaningful for the evaluation of the process.

Endo-β-N-acetylglucosaminidase is used widely in the glycobiology studies and industries. In this study, a new endo-β-N-acetylglucosaminidase, designated as Endo SA, was cloned from Streptomyces alfalfae ACCC 40021 and expressed in Escherichia coli BL21 (DE3). The purified recombinant Endo SA exhibited the maximum activity at 35 ºC and pH 6.0, good thermo/pH stability and high specific activity (1.0×10⁶ U/mg). It displayed deglycosylation activity towards different protein substrates. These good properties make EndoSA a potential tool enzyme and industrial biocatalyst.
内切β-N-乙酰氨基葡萄糖苷酶广泛应用于糖生物学研究和工业生产。本研究从苜蓿链霉菌Streptomyces alfalfae ACCC 40021 中克隆并原核表达了一个新的内切β-N-乙酰氨基葡萄糖苷酶，该酶最适反应温度为35 ℃，最适pH 为6.0，具有良好的pH 稳定性、温度稳定性和高比活（1×10⁶ U/mg）的特性，可催化不同蛋白底物去糖基化，具有作为工具酶和生物催化剂的潜力。.

Peptide-N-glycosidase F (PNGase F) is the most frequently used enzyme to release N-glycan from glycoproteins in glycomics; however, the releasing process using PNGase F is tedious and can range in duration from hours to overnight. Recently, efforts have been made to accelerate this enzymatic reaction, and they include the use of microwave irradiation, ultrahigh pressure, enzyme immobilization, and other techniques. Here, we developed a novel method combining the oriented immobilization of PNGase F on magnetic particles and microwave-assisted enzymatic digestion techniques to achieve highly efficient release of N-glycans. The oriented immobilization of PNGase F on magnetic particles utilizes the affinity of its co-expressed His-tag towards iminodiacetic acid-Nickel modified magnetic particles. Compared with non-oriented immobilization, the oriented immobilization of PNGase F exhibits several advantages including tolerance to high temperature (52 °C) and the ability to retain strong activity after more than five reuses. When used in combination with microwave irradiation, efficient N-glycan removal from ribonuclease B was achieved within 5 min. The proposed strategy was also used to release glycan from fetuin and human serum and has proven to provide a promising deglycosylation method for the characterization of protein glycosylation.

Enzyme immobilization is widely applied in biocatalysis to improve stability and facilitate recovery and reuse of enzymes. However, high cost of supporting materials and laborious immobilization procedures has limited its industrial application and commercialization. In this study, we report a novel self-assembly immobilization system using bacteriophage T4 capsid as a nanocarrier. The system utilizes the binding sites of the small outer capsid protein, Soc, on the T4 capsid. Enzymes as Soc fusions constructed with regular molecular cloning technology expressed at the appropriate time during phage assembly and self-assembled onto the capsids. The proof of principle experiment was carried out by immobilizing β-galactosidase, and the system was successfully applied to the immobilization of an important glycomics enzyme, Peptide-N-Glycosidase F. Production of Peptide-N-Glycosidase F and simultaneous immobilization was finished within seven hours. Characterizations of the immobilized Peptide-N-Glycosidase F indicated high retention of activity and well reserved deglycosylation capacity. The immobilized Peptide-N-Glycosidase F was easily recycled by centrifugation and exhibited good stability that sustained five repeated uses. This novel system uses the self-amplified T4 capsid as the nanoparticle-type of supporting material, and operates with a self-assembly procedure, making it a simple and low-cost enzyme immobilization technology with promising application potentials.

Endo-β-N-acetylglucosaminidase from the methylotrophic yeast Ogataea minuta (Endo-Om) is a glycoside hydrolase family 85 enzyme that has dual catalytic activity in the hydrolysis and transglycosylation of complex N-glycans, in common with the enzymes from the eukaryotic species. In this study, we have conducted mutagenesis of Endo-Om at Trp295, to determine the effect on hydrolytic activity. Structural modeling predicted that Trp295 forms an important interaction with the α-1,3-linked mannose residue of the trimannosyl N-glycan core, rather than being directly involved in catalytic activity. Our results showed that an aromatic amino acid is required at position 295 for the hydrolytic activity of this enzyme. Notably, the tryptophan residue is highly conserved in eukaryotic endo-β-N-acetylglucosaminidases that show activity toward complex oligosaccharides. Accordingly, our results strongly suggested that Trp295 is involved in the recognition of oligosaccharide substrates by Endo-Om.

In plant secretory pathways, the Golgi apparatus serves as the major sorting hub to receive de novo synthesized protein from the endoplasmic reticulum for further sorting to post-Golgi compartments or for residence in the cisternae of Golgi stacks. Meanwhile, Golgi functions as a pivotal biochemical factory to make modifications of N-glycans and to produce mature glycoproteins. Fluorescent tag-based confocal microscopy in combination with the brefeldin A drug or the genetic tools to disturb Golgi function have been shown as powerful approaches to analyze Golgi-mediated protein traffic in transiently expressed plant protoplasts or in stably expressed transgenic plants. Various endoglycosidases like Endo H and PNGase F have been widely used to monitor Golgi-mediated glycosylation of secretory proteins. Here, using fluorescently tagged Golgi-resident proteins and known glycosylated proteins as examples, we describe detailed protocols to analyze Golgi-mediated protein traffic and glycosylation in transiently expressed protoplasts derived from Arabidopsis suspension culture cells and in stably expressed transgenic plants.

Detailed characterization of glycoprotein structures requires determining both the sites of glycosylation as well as the glycan structures associated with each site. In this work, we developed an analytical strategy for characterization of intact N-glycopeptides in complex proteome samples. In the first step, tryptic glycopeptides were enriched using ZIC-HILIC. Secondly, a portion of the glycopeptides was treated with endoglycosidase H (Endo H) to remove high-mannose (Man) and hybrid N-linked glycans. Thirdly, a fraction of the Endo H-treated glycopeptides was further subjected to PNGase F treatment in 18O water to remove the remaining complex glycans. The intact glycopeptides and deglycosylated peptides were analyzed by nano-RPLC-MS/MS, and the glycan structures and the peptide sequences were identified by using the Byonic or pFind tools. Sequential digestion by endoglycosidase provided candidate glycosites information and indication of the glycoforms on each glycopeptide, thus helping to confine the database search space and improve the confidence regarding intact glycopeptide identification. We demonstrated the effectiveness of this approach using RNase B and IgG and applied this sequential digestion strategy for the identification of glycopeptides from the HepG2 cell line. We identified 4514 intact glycopeptides coming from 947 glycosites and 1011 unique peptide sequences from HepG2 cells. The intensity of different glycoforms at a specific glycosite was obtained to reach the occupancy ratios of site-specific glycoforms. These results indicate that our method can be used for characterizing site-specific protein glycosylation in complex samples. Graphical abstract Through integrating the information of intact glycopeptide, fragment ions filters and endoglycosidase digestion, the reliability of the identification could be significantly improved. We quantified the site-specific glycoforms occupancy ratios through the MS response signaling of each glycopeptide at the same time.

N-Glycosylation is a posttranslational modification commonly occurred in fungi and plays roles in a variety of enzyme functions. In this study, a xylanase (Af-XYNA) of glycoside hydrolase (GH) family 10 from Aspergillus fumigatus harboring three potential N-glycosylation sites (N87, N124 and N335) was heterologously produced in Pichia pastoris. The N-glycosylated Af-XYNA (WT) exhibited favorable temperature and pH optima (75°C and pH 5.0) and good thermostability (maintaining stable at 60°C). To reveal the role of N-glycosylation on Af-XYNA, the enzyme was deglycosylated by endo-β-N-acetylglucosaminidase H (DE) or modified by site-directed mutagenesis at N124 (N124T). The deglycosylated DE and mutant N124T showed narrower pH adaptation range, lower specific activity, and worse pH and thermal stability. Further thermodynamic analysis revealed that the enzyme with higher N-glycosylation degree was more thermostable. This study demonstrated that the effects of glycosylation at different degrees and sites were diverse, in which the glycan linked to N124 played a key role in pH and thermal stability of Af-XYNA.

Heterogeneity of glycan structures in native glycoconjugates always hampers precise studies on carbohydrate-involved biological functions. To construct homogeneous glycoconjugates from natural resource of homogeneous glycans is therefore a practical approach to solve this problem. We report here an optimized procedure for gram-scale production of sialylglycopeptide (SGP) containing a disialyl biantennary complex-type N-glycan from egg yolks. Our new procedure simplified the extraction process by treating the egg yolk powder with 40% acetone, avoiding massive emulsification, high-speed centrifugation, and sophisticated chromatography in reported methods. Subsequent semi-synthesis of the N-glycan core Man3GlcNAc oxazoline from SGP was accomplished for the first-time via glyco-trimming and successive oxazoline formation. This efficient semi-synthesis provides an alternative to the pure chemical approach that involves multi-step total synthesis and facilitates the application of endo-glycosidase-enabled chemoenzymatic synthesis of various homogeneous glycoconjugates.