BIOORTHOGONAL REACTIONS FOR LABELING GLYCOCONJUGATES - Click Chemistry in Glycoscience: New Developments and Strategies - page 206

action of the multi-subunit oligosaccharide transferase complex [11, 12]. Subsequent
trimming and processing of the transferred oligosaccharide results in a Man 3 GlcNAc 2
core structure, which is transported to the medial stacks of the Golgi complex where
maturation of the oligosaccharide gives rise to extreme structural diversity [13-
16]. This complexity of N -glycan structures is largely based on the cell-specific
expression of a collection of glycosyl transferases that specify the extension of
oligosaccharide structures onto the trimmed Man 3 GlcNAc 2 core structure. The switch
from structural uniformity in the ER to diversification in the Golgi complex coincides
with a marked change in glycan function. In the early secretory pathway, the glycans
have a common role in the promotion of protein folding, quality control, and certain
sorting events. This is in contrast to their roles in the Golgi complex, in which
they are modified to perform a wide spectrum of functions displayed by the mature
glycoproteins.
Despite their importance, in most cases the precise roles that glycans play in
biological systems is not well understood because of their underpinning complexity.
Over the past decade, integrated approaches have been developed, which are broadly
termed glycomics, which are aimed at unraveling structure-function relationships
of complex carbohydrates. Key glycomics technologies include mass spectrometric
profiling of glycan structures isolated from cells and tissues [17-19], glyco-gene
microarray technology for measuring the expression levels of glycoenzymes and
glycan-binding proteins and screening for glycan-protein interactions using glycan
and lectin array technologies [20-25]. The diverse data sets generated by the use
of these technologies are beginning to provide an understanding of the fundamental
structure-function relationships of glycans. Critical components that enable this pro-
cess are bioinformatics platforms that store, integrate, process, and disseminate the
data in a meaningful way [26-28].
The use of tandem MS for glycomics is driven by the need to obtain structural
information of glycans present in serum/plasma or tissue samples to understand
metabolic or disease processes and discover new biomarkers [17-19]. Strategies
for acquisition and interpretation of multistage MS have been most fully developed
for permethylated glycans. The advantage to this approach is that tandem mass
spectrometric dissociation of a glycosidic bond leaves a site that lacks a methyl
group that is clearly indicated by mass. It is possible to differentiate some types of
positional isomers based on the formation of specific product ion types. However,
the assignment of glycan structures is very challenging due to the isobaric nature of
glycans, that is, different structures with the identical molecular weights.
Glycoarray technology is a key tool for glycomics that has as a distinct advantage
that only minute amounts of precious oligosaccharides are required while allowing
fast, quantitative, systematic identification and characterization of carbohydrate-
protein interactions [20-25]. In addition, a glycoarray format presents glycans in
a multivalent fashion, which is often an important requirement for high affinity
binding. In particular, the glycan array developed by the consortium of functional
glycomics (CFG) has found wide utility in profiling interactions with carbohydrate
binding proteins, growth factors, pathogen- and cancer-induced antibodies, viruses,
and bacteria [26, 27]. The saccharides of this array are prepared by chemoenzymatic