Glycosyltation is the most complex form of posttranslational modification. polysaccharaides attached to membrane-bound proteins can facilitate cell-cell (or membrane-membrane, at least) communication by interacting with corresponding membrane-bound receptors.
Fundamental "glycomics" questions:
- Which proteins are modified with what kinds of glycans (50% of eykaryotic proteomes)?
- what are the functional consequences?
- How does the glycosylation vary with changes in physiology?
Glycosyltransferases use 9 (in higher mammals) monosaccharide building blocks (derived from metabolized sugars from diet) to glycosylize proteins in the endoplasmic reticulum and golgi compartment. There are essentially two forms of glycosylation: N-linked glycans and O-linked glycans. Both forms contain a "conserved core" with a GalNAc structure. This conserved core is linked to the N-group of asparagine for the N-linked form of glycans and to the oxygen of a serine or threonine in O-linked glycans.
Want to design inhibitors for O-linked glycans (the perturbing agent). (Diphosphate sugars are bad because they're charged and polar and prevent the compound from getting into the cell.) Built a 1338 uridine-based library to screen for ppGalNAcT inhibitors. Found two. Both had broad spectrum inhibitory activity. Importantly, none of the compounds inhibited any of the other glycosyltransferases.
It's hopefully self evident why you would want to visualize the presence and location of a molecule in a living system. You can usually fuse GFP onto a protein to track it, but what about the non-proteins components of the cell? Bioorthogonal reporters for particular glycan chains are needed and have been designed (the term "bioorthogonal" indicates the reporter should not interfere with normal biological activity).
Note, cancer cells have aberrent glycosylation... many possess lots of sialic acid glycan chains. How do you detect them? Modify the metabolic precursor (ManNAz) in an orthogonal way that can be detected with a reporter (synthesized in or ex vivo). For instance, radiolabeled and self-quencing "smart" probes have been developed.