Seventeen O-acetylated N-glycans and six O-acetylation sites of Myozyme identified using liquid chromatography-tandem mass spectrometry

Experimentally Determined Diagnostic Ions for Identification of Peptide Glycotopes

The analysis of the structures of glycans present on glycoproteins is an essential component for determining glycoprotein function; however, detailed glycan structural assignment on glycopeptides from proteomics mass spectrometric data remains challenging. Glycoproteomic analysis by mass spectrometry currently can provide significant, yet incomplete, information about the glycans present, including the glycan monosaccharide composition and in some circumstances the site(s) of glycosylation. Advancements in mass spectrometric resolution, using high-mass accuracy instrumentation and tailored MS/MS fragmentation parameters, coupled with a dedicated definition of diagnostic fragmentation ions have enabled the determination of some glycan structural features, or glycotopes, expressed on glycopeptides. Here we present a collation of diagnostic glycan fragments produced by traditional positive-ion-mode reversed-phase LC-ESI MS/MS proteomic workflows and describe the specific fragmentation energy settings required to identify specific glycotopes presented on N- or O-linked glycopeptides in a typical proteomics MS/MS experiment.

Structural identification and quantification of unreported sialylated N-glycans in bovine testicular hyaluronidase by LC-ESI-HCD-MS/MS

Bovine testicular hyaluronidase (BTH), which accelerates the absorption and dispersion of drugs by decomposing hyaluronan in subcutaneous tissues, has been used in medical applications, including local anesthesia, ophthalmology, and dermatosurgery. The requirement of N-glycans for the activity of human hyaluronidase has been reported, and BTH has greater activity than human hyaluronidase. However, the N-glycan characteristics of BTH are unclear. From a commercial BTH source containing additional proteins, purified BTH (pBTH) was obtained using size exclusion chromatography, and the structures and quantities of its N-glycans were analyzed using liquid chromatography (LC)-electrospray ionization-higher energy collisional dissociation (HCD)-tandem mass spectrometry (MS/MS). In pBTH, 32 N-glycans were identified, with 12 sialylations (39.0% of total N-glycan content), nine core-fucosylations (31.5%), six terminal galactosylations (14.6%), five high-mannosylations (13.7%), and four bisecting N-acetylglucosamine structures (7.8%). The presence of sialylated glycopeptides in pBTH was confirmed by nano-LC-HCD-MS/MS analysis. The absolute quantity of all N-glycans was calculated as 1.4 pmol (0.6 pmol for sialylation) in pBTH (1.0 pmol). The sialylation level (related to half-life, thermal stability, resistance to proteolysis, and solubility) was 24.4 times higher than that of human hyaluronidase. The hyaluronan degradation activity of de-sialylated pBTH decreased to 41.2 ± 4.2%, showing that sialylated N-glycans were required for pBTH activity as well. This is the first study to identify and quantify 32 N-glycans of pBTH and investigate their structural roles in its activity. The presence of larger amounts of sialylated N-glycans in pBTH than in human hyaluronidase suggests a greater utilization of pBTH.

Identification, quantification, and structural role of N-glycans in two highly purified isoforms of sheep testicular hyaluronidase

Animal-derived hyaluronidase, which hydrolyzes the polysaccharide hyaluronic acid, has been used in medical applications despite its limited purity. Additionally, the N-glycan characterization of sheep testicular hyaluronidase (STH) and its structural role remain poorly understood. In this study, STH was purified from the commercially available STH preparation (containing at least 14 impurity proteins) using heparin-affinity chromatography followed by size exclusion chromatography. The structure and quantity of N-glycans of STH were investigated using liquid chromatography-electrospray ionization-high energy collision dissociation-tandem mass spectrometry. Two isoforms, H3S1 and H3S2, of STH were obtained (purity >98 %) with a yield of 3.4 % and 5.1 %, respectively. Fourteen N-glycans, including nine core-fucosylated N-glycans (important for the stability and function of glycoproteins), were identified in both H3S1 and H3S2, with similar quantities of each N-glycan. The amino acid sequences of the proteolytic peptides of H3S1 and H3S2 were compared with those reported in STH. The hyaluronic acid-degrading activity of deglycosylated H3S1 and H3S2 was reduced to 70.8 % and 71.1 % compared to that (100 %) of H3S1 and H3S2, respectively. This is the first report of N-glycan characterization of two highly purified isoforms of STH. These H3S1 and H3S2 will be useful for medical use without unwanted effects of partially purified STH.

Sialic acid O-acetylation patterns and glycosidic linkage type determination by ion mobility-mass spectrometry

O-acetylation is a common modification of sialic acids that has been implicated in a multitude of biological and disease processes. A lack of analytical methods that can determine exact structures of sialic acid variants is a hurdle to determine roles of distinct O-acetylated sialosides. Here, we describe a drift tube ion mobility-mass spectrometry approach that can elucidate exact O-acetylation patterns as well as glycosidic linkage types of sialosides isolated from complex biological samples. It is based on the use of a library of synthetic O-acetylated sialosides to establish intrinsic collision cross section (CCS) values of diagnostic fragment ions. The CCS values were used to characterize O-acetylated sialosides from mucins and N-linked glycans from biologicals as well as equine tracheal and nasal tissues. It uncovered contrasting sialic acid linkage types of acetylated and non-acetylated sialic acids and provided a rationale for sialic acid binding preferences of equine H7 influenza A viruses.

Glycoproteomics of a Single Protein: Revealing Tens of Thousands of Myozyme Glycoforms by Hybrid HPLC-MS Approaches

Characterization of highly glycosylated biopharma-ceuticals by mass spectrometry is challenging because of the huge chemical space of coexistent glycoforms present. Here, we report the use of an array of HPLC-mass spectrometry-based approaches at different structural levels of released glycan, glycopeptide, and hitherto unexplored intact glycoforms to scrutinize the biopharmaceutical Myozyme, containing the highly complex lysosomal enzyme recombinant acid α-glucosidase. The intrinsic heterogeneity of recombinant acid α-glucosidase glycoforms was unraveled using a novel strong anion exchange HPLC-mass spectrometry approach involving a pH-gradient of volatile buffers to facilitate chromatographic separation of glycoforms based on their degree of sialylation, followed by the acquisition of native mass spectra in an Orbitrap mass spectrometer. Upon considering the structures of 60 different glycans attached to seven glycosylation sites in the intact protein, the large set of interdependent data acquired at different structural levels was integrated using a set of bioinformatic tools and allowed the annotation of intact glycoforms unraveling more than 1,000,000 putative intact glycoforms. Detectable isoforms also included several mannose-6-phosphate variants, which are essential for directing the drug toward its target, the lysosomes. Finally, for the first time, we sought to validate the intact glycoform annotations by integrating experimental data on the enzymatically dissected proteoforms, which reduced the number of glycoforms supported by experimental evidence to 42,104. The latter verification clearly revealed the strengths but also intrinsic limitations of this approach for fully characterizing such highly complex glycoproteins by mass spectrometry.

Precision Characterization of Site-Specific O-Acetylated Sialic Acids on N-Glycoproteins

O-Acetylation is a common modification of sialic acid, playing a significant role in glycoprotein stability, immune response, and cell development. Due to the lack of efficient methods for direct analysis of O-acetylated sialoglycopeptides (O-AcSGPs), the majority of identified O-acetylated sialic acids (O-AcSia) until now had no glycosite/glycoprotein information. Herein, we introduced a new workflow for precise interpretation of O-AcSGPs with probability estimation by recognizing the characteristic B and Y ions of O-AcSias. With further optimization of mass spectrometry parameters, the method allowed us to identify a total of 171 unique O-AcSGPs in mouse serum. Although the majority of these O-AcSGPs were at a relatively low abundance compared with their non-O-acetylated states, they were mainly involved in peptidase/endopeptidase inhibitor activities. The method paves the way for large-scale structural and functional analyses of site-specific O-AcSias in various complex samples as well as further identification of many other similar chemical modifications on glycoproteins.

Fragmentation stability and retention time-shift obtained by LC-MS/MS to distinguish sialylated N-glycan linkage isomers in therapeutic glycoproteins

Sialylated N-glycan isomers with α2-3 or α2-6 linkage(s) have distinctive roles in glycoproteins, but are difficult to distinguish. Wild-type (WT) and glycoengineered (mutant) therapeutic glycoproteins, cytotoxic T lymphocyte-associated antigen-4-immunoglobulin (CTLA4-Ig), were produced in Chinese hamster ovary cell lines; however, their linkage isomers have not been reported. In this study, N-glycans of CTLA4-Igs were released, labeled with procainamide, and analyzed by liquid chromatography-tandem mass spectrometry (MS/MS) to identify and quantify sialylated N-glycan linkage isomers. The linkage isomers were distinguished by comparison of 1) intensity of the N-acetylglucosamine ion to the sialic acid ion (Ln/Nn) using different fragmentation stability in MS/MS spectra and 2) retention time-shift for a selective m/z value in the extracted ion chromatogram. Each isomer was distinctively identified, and each quantity (>0.1%) was obtained relative to the total N-glycans (100%) for all observed ionization states. Twenty sialylated N-glycan isomers with only α2-3 linkage(s) in WT were identified, and each isomer's sum of quantities was 50.4%. Furthermore, 39 sialylated N-glycan isomers (58.8%) in mono- (3 N-glycans; 0.9%), bi- (18; 48.3%), tri- (14; 8.9%), and tetra- (4; 0.7%) antennary structures of mutant were obtained, which comprised mono- (15 N-glycans; 25.4%), di- (15; 28.4%), tri- (8; 4.8%), and tetra- (1; 0.2%) sialylation, respectively, with only α2-3 (10 N-glycans; 4.8%), both α2-3 and α2-6 (14; 18.4%), and only α2-6 (15; 35.6%) linkage(s). These results are consistent with those for α2-3 neuraminidase-treated N-glycans. This study generated a novel plot of Ln/Nn versus retention time to distinguish sialylated N-glycan linkage isomers in glycoprotein.

Efficient AuPd@GO-based electrochemical nanoprobe for sensitive detection of histone acetylase activity and its inhibitor

Histone acetylase (HAT p300), which has aroused great concern in fundamental research and clinical applications, serves as one class of significant tumor markers. In our work, a sensitive electrochemical immunoassay for testing HAT p300 based on both graphene-assisted supported AuPd nanomaterial (AuPd@GO composite) and a typical amperometric i-t technique with fast response is developed favorably. The AuPd@GO-based sensing mechanisms are distributed as follows: the HAT p300 derived acetylation reaction occurs at the customized peptide-immobilized electrode; the AuPd@GO composite acts as carrier to immobilize acetyl antibody, thus constructing a sandwich-type electrochemical immunosensor via an antigen and antibody interaction; importantly, a distinct electrochemical signal could be caught due to the AuPd@GO nanomaterial with a favorable electrocatalytic property to the commercialized 3,3,5',5'-tetramethyl benzidine solution (TMB). Taking advantage of AuPd@GO composite, the established immunosensor displays a wide linear range from 1 pM to 1000 nM, and the detection limit is 0.5 pM (S/N = 3) for HAT p300. Next, the biosensor is also used to analyze the inhibitor of HAT p300 successfully, which is promising for promoting the development of electrochemical HAT-related biodetection and drug discovery. Graphical abstract A sensitive electrochemical immunoassay for testing HAT p300 based on both graphene-assisted supported AuPd nanomaterial (AuPd@GO composite) and a typical amperometric i-t technique with fast response is developed favorably.

N-glycans of bovine submaxillary mucin contain core-fucosylated and sulfated glycans but not sialylated glycans

Bovine submaxillary mucin (BSM) is a heavily-glycosylated macromolecular (approximately 4 MDa) protein and is used in various biomaterial applications in light of its high viscosity and biocompatibility, in addition to use as a biochemical substrate or inhibitor as a result of its abundant O-glycans. Although it has been reported that N-glycosylation provides stability of human mucins, most BSM research has been focused on its O-glycans, while N-glycans have not been reported to date. In this study, a common N-glycan core component was detected by monosaccharide analysis of BSM, and the structures of the N-glycans and their relative quantities were determined by liquid chromatography-tandem mass spectrometry. Seventeen N-glycans comprising ten complex-type [Fucose_0~2Hexose_3~4N-acetylhexosamine_1~6Sulfate_0~1; 61.1% (the sum of the relative quantities of each N-glycan out of the total N-glycans)], two high-mannose-type (Hexose_5~6N-acetylhexosamine₂; 12.0%), and five paucimannose type (Fucose_0~1Hexose_3~4N-acetylhexosamine_2~3; 26.9%) were identified, but no hybrid-type or sialylated N-glycans were found. Additionally, these are less-branched structures compared to human mucins. Of these, ten glycans (77.2%), including two sulfated glycans (8.0%), were core fucosylated, which confer unique biological functions to glycoproteins. The N-glycosylation sites were identified from the analysis of glycopeptides from BSM. This study is the first confirmation of N-glycan attachment to BSM.