Conserved domains of glycosyltransferases

Glycosyltransferases catalyze the synthesis of glycoconjugates by transferr ing a properly activated sugar residue to an appropriate acceptor molecule or aglycone for chain initiation and elongation. The acceptor can be a lipi d, a protein, a heterocyclic compound, or another carbohydrate residue. A c atalytic reaction is believed to involve the recognition of both the donor and acceptor by suitable domains, as well as the catalytic site of the enzy me. To elucidate the structural requirements for substrate recognition and catalytic reactions of glycosyltransferases, we have searched the databases for homologous sequences, identified conserved amino acid residues, and pr oposed potential domain motifs for these enzymes. Depending on the configur ation of the anomeric functional group of the glycosyl donor molecule and o f the resulting glycoconjugate, all known glycosyltransferases can be divid ed into two major types: retaining glycosyltransferases, which transfer sug ar residue with the retention of anomeric configuration, and inverting glyc osyltransferases, which transfer sugar residue with the inversion of anomer ic configuration. One conserved domain of the inverting glycosyltransferase s identified in the database is responsible for the recognition of a pyrimi dine nucleotide, which is either the UDP or the TDP portion of a donor suga r-nucleotide molecule. This domain is termed "Nucleotide Recognition Domain 1 beta," or NRD1 beta, since the type of nucleotide is the only common str ucture among the sugar donors and accepters. NRD1 beta is present in 140 gl ycosyltransferases. The central portion of the NRD1 beta domain is very sim ilar to the domain that is present in one family of retaining glycosyltrans ferases. This family is termed NRD1 alpha to designate the similarity and s tereochemistry of sugar transfer, and it consists of 77 glycosyltransferase s identified thus far. In the central portion there is a homologous region for these two families and this region probably has a catalytic function. A third conserved domain is found exclusively in membrane-bound glycosyltran sferases and is termed NRD2; this domain is present in 98 glycosyltransfera ses, All three identified NRDs are present in archaebacterial, eubacterial, viral, and eukaryotic glycosyltransferases, The present article presents t he alignment of conserved NRD domains and also presents a brief overview of the analyzed glycosyltransferases which comprise about 65% of all known su gar-nucleotide dependent (Leloir-type) and putative glycosyltransferases in different databases. A potential mechanism for the catalytic reaction is a lso proposed. This proposed mechanism should facilitate the design of exper iments to elucidate the regulatory mechanisms of glycosylation reactions, A mino acid sequence information within the conserved domain may be utilized to design degenerate primers for identifying DNA encoding new glycosyltrans ferases.