Structures of sugars have long
fascinated chemists and biologists, beginning with Emil Fischer's
landmark efforts to decipher the isoforms of hexoses more than a
century ago .
Since then, even with modern techniques, biologists have been outpaced
by the difficulty of obtaining a glycosylation profile - the specific
complement of glycoconjugates present - of even a single cell. To
illustrate that there is no simple task in carbohydrate analysis,
Figure 1 shows a few biologically significant glycoconjugates. Even the addition of a single N-acetylglucosamine moiety to a protein to give the O-GlcNAc modification, which regulates numerous biochemical pathways by acting in a yin-yang manner with phosphorylation  (Figure 1c),
is complicated by its occurrence on hundreds of different cytosolic and
nuclear proteins, and on multiple sites within a single protein. The
various biological activities of glycosphingolipids, relatively simple
sugar-bearing biomolecules exemplified by the ganglioside GM3 (Figure 1d), demonstrate that very subtle changes to sialic acid (N-acetylneuraminic acid or Sia), an unusual nine-carbon sugar found in more than 50 different chemically distinct forms ,
can regulate apoptosis, senescence, and proliferation, thereby
highlighting the need for careful analysis of fine structural details.
to larger glycoconjugates, prions are glycosylated proteins that
possess only two sites where oligosaccharides attach (Figure 1a).
Even so, any one of several dozen different sugar chains can reside at
either site; consequently, prions exist as hundreds of distinct
entities. The discovery of the influence of carbohydrates on prion
infectivity and on the development of spongiform encephalopathies [14,15]
underlines the importance of fully defining structural heterogeneity of
this kind. As a final example, the heavily glycosylated cell-surface
glycoprotein CD34 (Figure 1a),
found on hematopoietic cells and epithelial cells, serves as a
developmental marker for hematopoietic cells, mediates leukocyte
homing, and contributes to cancer metastasis. It bears 20 or more
separate oligosaccharide chains , implying that, if ten different oligosaccharide structures randomly occur at each site (a conservative estimate), 1020 different forms of CD34 can exist and each of the approximately 104 to 105 copies of this protein found in a typical cell has a reasonable probability of being unique.