Recent Advances in Structural Glycobiology
|Rapid progress in structural biology has great influence
on glycobiology. The crystal structure of a galactosyltransferase in complex
with sugar-nucleotide and an acceptor analogue reported in 2001 has provided
the structural basis of the formation of glycosidic bonds. Three-dimensional
structures of glycoside hydrases such as -mannosidase
have been reported one after another, some of which have been classified
into a novel fold. Knowledge on structural basis of interactions between
proteins and glycoconjugates has expanded: the crystal structure of human
P-selectin complexed with a cognate glycopeptide visualizes a mode of
binding of the selectin to the glycan as well as to the polypeptide containing
sulfated tyrosine residues. Crystal structures of MHC class I molecules
presenting glycopeptide and CD1 molecules presenting glycolipids have
also been reported recently. Inspection of these data underlines the importance
of structural studies not only of liberated glycans but also of those
conjugated with intrinsic carriers (lipids or polypeptides).
Although the biological importance of glycans expressed on proteins has been widely recognized, little is known about their specific roles from the structural aspect. This deficiency in our knowledge is largely due to the lack of an appropriate methodology to deal with glycoproteins as targets of structural biology. Carbohydrate moieties exhibit microheterogeneities and possess a significant degree of freedom in internal motion, which hampers crystallization or interpretation of electron density. Hence, X-ray crystallographic analyses of glycoproteins have so far been carried out with deglycosylated glycoproteins. Recently, crystallographic studies using recombinant glycoproteins produced in insect cells have been widely attempted.
Prior to conformational analyses of glycoproteins, their glycoforms must be determined in advance. Recent advances in HPLC mapping and mass spectrometric techniques allow us to determine the covalent structures of glycans of glycoproteins, which greatly facilitates the development of structural biology of glycoproteins. Now it is possible to perform NMR analyses of glycoproteins whose glycans are labeled with stable isotopes by metabolic labeling via biosynthetic pathways of mammalian cells or by in vitro enzymatic glycosylation onto isolated glycoproteins. This is opening up a new way for the elucidation of atomic resolution of the structure, dynamics, and interaction of glycoproteins in solution.
Structural biology of glycoconjugates is an unexplored field beyond structural genomics, which is highly thought of at present. It is essential to develop this field and decode the biological signals expressed by glycans.
(Graduate School of Pharmaceutical Sciences, Nagoya City University)
|Oct. 31, 2002|