Glycosyltransferases (GTs) catalyze the transfer of a sugar moiety from an activated donor sugar onto saccharide and nonsaccharide acceptors. A sequence-based classification spreads GTs in many families thus reflecting the variety of molecules that can be used as acceptors. In contrast, this enzyme family is characterized by a more conserved three-dimensional architecture. Until recently, only two different folds (GT-A and GT-B) have been identified for solved crystal structures. The recent report of a structure for a bacterial sialyltransferase allows the definition of a new fold family. Progress in the elucidation of the structures and mechanisms of GTs are discussed in this review. To accommodate the growing number of crystal structures, we created the 3D-Glycosyltransferase database to gather structural information concerning this class of enzymes.

PA-IIL is a fucose-binding lectin from Pseudomonas aeruginosa that is closely related to the virulence factors of the bacterium. Previous structural studies have revealed a new carbohydrate-binding mode with direct involvement of two calcium ions (Mitchell E, Houles C, Sudakevitz D, Wimmerova M, Gautier C, Perez S, Wu AM, Gilboa-Garber N, Imberty A. Structural basis for selective recognition of oligosaccharides from cystic fibrosis patients by the lectin PA-IIL of Pseudomonas aeruginosa. Nat Struct Biol 2002;9:918-921). A combination of thermodynamic, structural, and computational methods has been used to study the basis of the high affinity for the monosaccharide ligand. A titration microcalorimetry study indicated that the high affinity is enthalpy driven. The crystal structure of the tetrameric PA-IIL in complex with fucose and calcium was refined to 1.0 A resolution and, in combination with modeling, allowed a proposal to be made for the hydrogen-bond network in the binding site. Calculations of partial charges using ab initio computational chemistry methods indicated that extensive delocalization of charges between the calcium ions, the side chains of the protein-binding site and the carbohydrate ligand is responsible for the high enthalpy of binding and therefore for the unusually high affinity observed for this unique mode of carbohydrate recognition.