Structure-Function of Galactosyltransferase sub-family members The oligosaccharide moieties (glycans) of glycoproteins, glycolipids and proteoglycans (glycoconjugates) are involved in cell-cell interactions and cell adhesion, host-pathogen interactions, immune defense, innate and acquired immunity and the recruitment of neutrophils to sites of tissue damage. They are also the recognition molecules for lectins and, together with chaperones they play an important role in the in vivo folding and transport of glycoproteins across cellular compartments. The carbohydrate moieties of proteoglycans, the glycosaminoglycans, take part in a wide range of biological functions. For example, they are involved in the formation of an extra-cellular matrix to which growth factors bind with a high degree of specificity and thereby regulate growth factor activity. Owing to the conformational flexibility of oligosaccharides, the torsional angles of a disaccharide unit, particularly around the 1-6-linkage, adjust in such a way that the side groups of the oligosaccharides orient themselves in a manner that bring about specific protein-carbohydrate or protein-protein interactions. Branched oligosaccharides cross-link proteins and generate homo- and hetero-dimerization of glycoproteins and infinite networks of protein-carbohydrate complexes, resulting in the modulation of various cell responses. Profound changes occur in the glycan structures during cellular development, differentiation, and tumorigenesis. Defective glycan synthesis has been shown to have serious pathological consequences and result in several human diseases. ? ? Our laboratory has been investigating the structure and function of glycosyltransferases, the enzymes which synthesize the oligosaccharides moieties of glycoconjugates, and has also been performing the conformational analysis of oligosaccharides by molecular dynamics simulations. The glycosyltransferases transfer a monosaccharide moiety of an activated sugar donor to an acceptor molecule, many requiring a metal ion cofactor. The majority of these enzymes are anchored in the Golgi compartment of eukaryotic cell as type II membrane proteins. They have a short N-terminal cytoplasmic domain, a membrane-spanning region, as well as a stem and a C-terminal catalytic domain that face the Golgi-lumen. The reaction catalyzed by these enzymes follows sequential ordered mechanism in which the metal ion and sugar-nucleotide bind first to the enzyme, followed by the acceptor. After the glycosyl moiety of the sugar-nucleotide donor is transferred to the acceptor, the saccharide product is ejected out, followed by the release of the nucleotide and the metal ion. The sugar transfer occurs either with the retention or inversion of the configuration at the anomeric carbon atom of the sugar donor. The structural studies on beta-1,4-galactosylransferase (b4Gal-T1), either free or complexed with substrates, first from our laboratory and later from other laboratories on other glycosyltransferases, have revealed that upon binding the donor substrate, one or two flexible loops undergo a marked conformational change, from an open to a closed conformation, simultaneously creating an acceptor binding site on the enzyme that did not exist before. Thus, the loop acts as a lid covering the bound donor substrate. After completion of the transfer of the glycosyl unit to the acceptor, the saccharide product is ejected; the loop reverts to its native conformation to release the remaining nucleotide moiety. In the case of b4Gal-T1, this conformational change also creates the binding site for cellular proteins like alpha-lactalbumin, Ovalbumin, former being a protein specific to mammary gland. The interaction of alpha-lactalbumin with galactosyltransferase enzyme changes the acceptor specificity of the enzyme towards glucose to synthesize lactose during lactation. ? ?
