Analysis of asparagine-linked oligosaccharides by sequential lectin affinity chromatography

N-glycoprotein enrichment by lectin affinity chromatography

Lectins are proteins that bind to sugars with varying specificities and several have been identified that show differential binding to structurally variable glycans attached to glycoproteins. Consequently, lectin affinity chromatography represents a valuable tool for glycoproteome studies, allowing enrichment of glycoproteins in samples prior to their identification by mass spectrometry (MS). From the perspective of plant scientists, lectin enrichment has proven useful for studies of the proteomes of the secretory pathways and cell wall, due to the high proportion of constituent proteins that are glycosylated. This chapter outlines a strategy to generate samples enriched with glycoproteins from bulk plant tissues prior to further characterization by MS, or other techniques.

Glycoproteomic analysis of embryonic stem cells: identification of potential glycobiomarkers using lectin affinity chromatography of glycopeptides

Numerous studies have recently focused on the identification of specific glycan biomarkers, given the important roles that protein linked glycans play, for example, during development and disease progression. The identification of protein glycobiomarkers, which are part of a very complex proteome, has involved the use of fractionation techniques such as lectin affinity chromatography. In this study, the glycoproteomic characterization of pluripotent murine embryonic stem cells (ES) and from ES cells that were differentiated into embroid bodies (EB) was performed using immobilized Concanavalin A (ConA). This procedure allowed the isolation of glycopeptides that express biantennary and hybrid N-linked structures (ConA2 fraction) as well as high mannose glycans (ConA3 fraction) that were abundant in both ES and EB stages. A total of 293 unique N-linked glycopeptide sequences (from 180 glycoproteins) were identified in the combined data sets from ES and EB cells. Of these glycopeptides, a total of 119 sequences were identified exclusively in only one of the lectin-bound fractions (24 in the ES-ConA2, 15 in the ES-ConA3, 16 in the EB-ConA2, and 64 in the EB-ConA3). Results from this study allowed the identification of individual N-glycosylation sites of proteins that express specific glycan types. The absence of some of these lectin-bound glycopeptides in a cell stage suggested that they were derived from proteins that were either expressed exclusively on a defined developmental stage or were expressed in both cell stages but carried the lectin-bound oligosaccharides in only one of them. Therefore, these lectin-bound glycopeptides can be considered as stage-specific glycobiomarkers.

Glycopeptide analysis by mass spectrometry

Glycosylation is one of the most important post-translational modifications found in nature. Identifying and characterizing glycans is an important step in correlating glycosylation structure to the glycan's function, both in normal glycoproteins and those that are modified in a disease state. Glycans on a protein can be characterized by a variety of methods. This review focuses on the mass spectral analysis of glycopeptides, after subjecting the glycoprotein to proteolysis. This analytical approach is useful in characterizing glycan heterogeneity and correlating glycan compositions to their attachment sites on the protein. The information obtained from this approach can serve as the foundation for understanding how glycan compositions affect protein function, in both normal and aberrant glycoproteins.

Solid-phase chemical tools for glycobiology

Techniques involving solid supports have played crucial roles in the development of genomics, proteomics, and in molecular biology in general. Similarly, methods for immobilization or attachment to surfaces and resins have become ubiquitous in sequencing, synthesis, analysis, and screening of oligonucleotides, peptides, and proteins. However, solid-phase tools have been employed to a much lesser extent in glycobiology and glycomics. This review provides a comprehensive overview of solid-phase chemical tools for glycobiology including methodologies and applications. We provide a broad perspective of different approaches, including some well-established ones, such as immobilization in microtiter plates and to cross-linked polymers. Emerging areas such as glycan microarrays and glycan sequencing, quantum dots, and gold nanoparticles for nanobioscience applications are also discussed. The applications reviewed here include enzymology, immunology, elucidation of biosynthesis, and systems biology, as well as first steps toward solid-supported sequencing. From these methods and applications emerge a general vision for the use of solid-phase chemical tools in glycobiology.

Barley beta-galactosidase: structure, function, heterogeneity, and gene origin

Barley (Hordeum vulgare) beta-galactosidase is composed of a large (45 kDa) and a small (33 kDa) polypeptide. N-terminal sequencing of the polypeptides and antibody reactivity data place the barley enzyme and heterodimeric plant beta-galactosidases from jack bean, maize, and wheat in family 35 of the glycosyl hydrolases. Sequence analysis indicates the existence of a subfamily of genes coding for polypeptide precursors that are cleaved to produce the two subunits in heterodimeric beta-galactosidases. The heterogeneity of the barley holoenzyme is related, but not restricted, to the N-glycosylation of the small polypeptide. Both polypeptides are essential for the catalytic activity of the enzyme.

Proteomics of glycoproteins based on affinity selection of glycopeptides from tryptic digests

Identification of glycoproteins in complex mixtures derived from either human blood serum or a cancer cell line was achieved in a process involving the steps of (1) reduction and alkylation, (2) proteolysis of all proteins in the mixture with trypsin, (3) affinity chromatographic selection of the glycopeptides with an immobilized lectin, (4) direct transfer of the glycopeptide fraction to a reversed-phase liquid chromatography (RPLC) column and further fractionation by gradient elution, (5) matrix-assisted laser desorption ionization mass spectrometry of individual fractions collected from the RPLC column, and (6) peptide identification based on a database search. The types of glycoproteins analyzed were; (1) N-type glycoproteins of known primary structure, (2) N-type glycoproteins of unknown structure, and (3) O-type glycoproteins glycosylated with a single N-acetylglucosamine. Identification of peptides from complex mixtures was greatly facilitated by either C-terminal sequencing with a carboxypeptidase mixture or by comparing chromatographic behavior and mass to standards, as in the case of a known protein. In addition, deglycosylation of peptides with N glycosidase F was necessary to identify N-type glycoproteins of unknown structure. The strength of this approach is that it is fast and targets specific molecular species or classes of glycoproteins for identification. The weakness is that it does not discriminate between glycoforms.

Glycoaffinity chromatography and biological recognition

The potential of bioaffinity chromatography as a tool for study of biological recognition mechanisms is gaining increasing recognition. Biochromatographic methods allow the separation of proteins according to both the structure of their polypeptidic chain and their post-translational modifications. Among the various post-translational modifications which proteins undergo, glycosylation has conducted to the development of original methods (glycotechnologies). This review discusses the applications of glycotechnologies in bioaffinity chromatography, and particularly the use of biochromatography to elucidate mechanisms involved in glycobiology.