The recent explosion of proteomic research has created rich knowledge about the protein content of cells, tissues and whole organisms. The discovery of proteins with post-translational modifications (PTMs) has become an important frontier of proteomics studies, with more emphasis on the structures and functions of the proteins rather than interest only in sequence identifications in the early work. Emerging separation techniques coupled with mass spectrometry (MS) offer great capabilities in elucidating more information on proteins with PTMs. In addition, the development of methods that are capable of measuring the relative expression of proteins between two or more samples has become an essential aspect of systems biology, and has greatly facilitated biomarker discovery in various diseases.
Protein glycosylation has long been recognized as a common PTM. Glycosylated proteins are ubiquitous components of both extracellular matrices and cellular surfaces. There are four known categories of glycosylation: (1) the N
-glycosylation, where glycans are attached to asparagine residues in a consensus sequence N-X-S/T (X can be any amino acid except proline) via an N
-GlcNAc) residue; (2) the O
-glycosylation, where the glycans are attached to serine or threonine; (3) glycosylphosphatidylinositol anchors, which are attached to the carboxyl terminus of membrane-associated proteins and (4) C
-glycosylation, in which sugars are attached to tryptophan residues in some membrane-associated and secreted proteins [1
]. As the first two cases have been the most common forms of glycosylation, we will limit our discussion to only N
- and O
-glycosylation in this mini-review. Carbohydrates can have a great influence on the physicochemical properties of glycoproteins, affecting their folding, solubility, aggregation and propensity to degrade. Furthermore, glycan chains in glycoproteins play fundamental roles in many biological processes such as embryonic development, immune response and cell-to-cell interactions involving sugar–sugar- or sugar–protein-specific recognition [2
]. Consequently, aberrant glycosylation has now been implicated in many diseases, including hereditary disorders, immune deficiencies, neurodegenerative diseases, cardiovascular diseases and cancer [3
]. Many clinical biomarkers and therapeutic targets are glycoproteins, including Her2/neu (breast cancer), prostate-specific antigen (PSA, prostate cancer) and CA 125 (ovarian cancer) [4
In order to examine the disease-related glycosylation alteration, sensitive, fast and robust analytical methods are required. Although the identification of proteins is routinely performed in many laboratories, the study of glycoproteins remains challenging. Comprehensive reviews have been published in recent years covering the isolation and characterization of glycoproteins [6–9
]. Therefore, the focus of this review will be on recent development of glycoproteomics, with an emphasis on the quantitative aspect, and its application in tackling biological problems.