Enzymatic synthesis of Salmonella O-specific polysaccharide analogs from modified polyprenyl pyrophosphate sugar acceptors

Glycobiotechnology: enzymes for the synthesis of nucleotide sugars

Complex carbohydrates, as constituting part of glycoconjugates such as glycoproteins, glycolipids, hormones, antibiotics and other secondary metabolites, play an active role in inter- and intracellular communication. The aim of "glycobiotechnology" as an upcoming interdisciplinary research field is to develop highly efficient synthesis strategies, including in vivo and in vitro approaches, in order to bring such complex molecules into analytical and therapeutic studies. The enzymatic synthesis of glycosidic bonds by Leloir-glycosyltransferases is an efficient strategy for obtaining saccharides with absolute stereo- and regioselectivity in high yields and under mild conditions. There are, however, two obstacles hindering the realization of this process on a biotechnological scale, namely the production of recombinant Leloir-glycosyltransferases and the availability of enzymes for the synthesis of nucleotide sugars (the glycosyltransferase donor substrates). The present review surveys some synthetic targets which have attracted the interest of glycobiologists as well as recombinant expression systems which give Leloir-glycosyltransferase activities in the mU and U range. The main part summarizes publications concerned with the complex pathways of primary and secondary nucleotide sugars and the availability and use of these enzymes for synthesis applications. In this context, a survey of our work will demonstrate how enzymes from different sources and pathways can be combined for the synthesis of nucleotide deoxysugars and oligosaccharides.

Genetic characterization of the O4 polysaccharide gene cluster from Escherichia coli

The Escherichia coli O4 serotype is among those commonly isolated from urinary tract infections. In order to study the genetics of the O-antigen, the O4 biosynthesis genes from a uropathogenic E. coli have previously been cloned into E. coli K-12. A subclone, GH58, has been identified which reacts with antisera against the O4 serotype. In contrast to the wild-type parental strain, lipopolysaccharide (LPS) from this clone is devoid of rhamnose and does not cross-react with O18 antisera. The recombinant plasmid from GH58, pGH58, was used to transform the rfb deletion strain HU1190. The resultant strain agglutinates in O4 antisera, but produces unpolymerized LPS. Escherichia coli K-12 strains HB101 and RC712 containing pGH58 produce polymerized LPS, indicating that the genetic background of the host can influence the LPS encoded by recombinant molecules. A cosmid, pGH84, has been identified which encompasses the entire pGH58 gene sequences and includes an additional 34 kilobases of DNA. HU1190 containing this cosmid agglutinates in O4 antisera and produces a polymerized LPS. By constructing several deletion subclones of pGH84, we have localized the genes necessary for polymerized LPS to a 5.5 kb ClaI-BamHI fragment. P1 transductants that make polymerized and unpolymerized O4 LPS have also been identified.

Synthesis of oligosaccharide fragments of the repeating unit of Salmonella kentucky O-specific polysaccharide and conversion of the oligosaccharides into the glycosyl phosphates

alpha-D-Man-(1----2)-alpha-D-Man-(1----3)-D-Gal, a structural fragment of the main chain of Salmonella serogroups C2 and C3 O-specific polysaccharides, and the isomer with the central residue beta have been synthesised, as have some oligosaccharides related to the structure of the O-specific polysaccharide of S. kentucky (serogroup C3), namely, alpha-D-Glc-(1----4)-D-Gal, alpha-D-Man-(1----3)-[alpha-D-Glc-(1----4)]-D-Gal, and alpha-D-Man-(1----2)-alpha-D-Man-(1----3)-[alpha-D-Glc-(1----4)]-D-Gal, and the isomers with the D-Glc unit beta. Each oligosaccharide was converted into the alpha-glycosyl phosphate.

Biosynthesis of the O9 antigen of Escherichia coli. Synthetic glycosyldiphosphomoraprenols as probes for requirement of mannose acceptors

Synthetic monosaccharide derivatives (alpha-glucosyl, beta-glucosyl, alpha-mannosyl) and disaccharide derivatives (alpha-mannosyl-1,2-alpha-glucosyl, alpha-mannosyl-1,3-alpha-glucosyl, alpha-mannosyl-1,4-alpha-glucosyl, alpha-mannosyl-1,6-alpha-glucosyl) of diphosphomoraprenol were used as putative mannose acceptors in the biosynthesis of Escherichia coli O9 antigen. Membranes of E. coli O9 derived from the rfe mutant F 1357 were reconstituted with these compounds and then incubated with different concentrations of GDP-[14C]mannose. Of the monosaccharide derivatives tested, only alpha-glucodiphosphomoraprenol was a mannose acceptor and the only disaccharide derivative which accepted mannose was alpha-mannosyl-1,3-alpha-glucosyldiphosphomoraprenol. The alpha-glucosyl derivative accepted only one mannose unit at 4 microM GDP-[14C]mannose, and above 50 microM GDP-[14C]mannose about 25% of the product had a minimum size of about 30 mannose units. The alpha-mannosyl-1,3-alpha-glucosyl derivative was only a mannose acceptor at a GDP-[14C]mannose concentration of 50 microM and higher, and the product had a minimum size of about 30 mannose units. The results are discussed with respect to requirement of mannose acceptors.

Chemically synthesized galactosyl ficaprenyl diphosphate as an intermediate in the biosynthesis of the Salmonella O-antigenic polysaccharide

The membrane fraction from a mutant of Salmonella anatum deficient in UDPgalactose-4-epimerase, utilized synthetic ficaprenyl alpha-D-galactosyl diphosphate as a substrate in the biosynthesis of the O-polysaccharide portion of lipopolysaccharide which has a mannosylrhamnosylgalactose repeating sequence. The galactosyl lipid was prepared by chemical synthesis from D-galactose and ficaprenol extracted from Ficus elastica. Membrane preparations catalyzed the transfer of rhamnose from TDP-rhamnose onto membrane-bound ficaprenyl galactosyl diphosphate forming rhamnosylgalactosyl ficaprenyl diphosphate; the reaction was dependent on the prior insertion of the synthetic glycosyl-lipid into the membrane, and was proportional to incubation time up to 4 min at 29 degrees C. When both TDP-rhamnose and GDP-mannose were added, the product formed was O-polysaccharide. These results indicate that the chemically synthesized ficaprenyl galactosyl diphosphate can be an active substrate for the in vitro synthesis of the Salmonella O-polysaccharide.