Specificity of α- and β-d-galactosidase towards analogs of d-galactopyranosides modified at C-4 or C-5

Identification of acacia gum fermenting bacteria from pooled human feces using anaerobic enrichment culture

Commercial acacia gum (AG) used in this study is a premium-grade free-flowing powder. It is a gummy exudate composed of arabinogalactan branched polysaccharide, a biopolymer of arabinose and galactose. Also known as food additive, acacia gum (E414), which is presently marketed as a functional dietary fiber to improve overall human gut health. The health effects may be related to the luminal pH regulation from the short-chain fatty acids (SCFA) production. Studies suggested that amylolytic and butyrogenic pathways are the major factors determining the SCFA outcome of AG in the lower gut. However, the primary bacteria involved in the fermentation have not been studied. This study aimed to investigate the putative primary degraders of acacia gum in the gut ecosystem. Isolation and identification of gum-fermenting bacteria were performed through enrichment culture fermentation. The experiment was conducted in an anaerobic chamber for 144 h in three stages. The study was conducted in triplicate using an anaerobic chamber system. This culture system allows specific responses to support only bacteria that are responsible for gum fermentation among the gut microbiota. Five bacterial strains were isolated and found to be gum-fermenting bacteria. Based on the 16s RNA sequence, the isolates matched to butyrate-producing Escherichia fergusonii, ATCC 35469.

Linearization of carbohydrate derived polycyclic frameworks

We report an easy access to carbohydrate derived diverse tricyclic skeletons which could be beneficial for exploring polyfunctionalized chiral alicycles.

LacZ β-galactosidase: structure and function of an enzyme of historical and molecular biological importance

This review provides an overview of the structure, function, and catalytic mechanism of lacZ β-galactosidase. The protein played a central role in Jacob and Monod's development of the operon model for the regulation of gene expression. Determination of the crystal structure made it possible to understand why deletion of certain residues toward the amino-terminus not only caused the full enzyme tetramer to dissociate into dimers but also abolished activity. It was also possible to rationalize α-complementation, in which addition to the inactive dimers of peptides containing the "missing" N-terminal residues restored catalytic activity. The enzyme is well known to signal its presence by hydrolyzing X-gal to produce a blue product. That this reaction takes place in crystals of the protein confirms that the X-ray structure represents an active conformation. Individual tetramers of β-galactosidase have been measured to catalyze 38,500 ± 900 reactions per minute. Extensive kinetic, biochemical, mutagenic, and crystallographic analyses have made it possible to develop a presumed mechanism of action. Substrate initially binds near the top of the active site but then moves deeper for reaction. The first catalytic step (called galactosylation) is a nucleophilic displacement by Glu537 to form a covalent bond with galactose. This is initiated by proton donation by Glu461. The second displacement (degalactosylation) by water or an acceptor is initiated by proton abstraction by Glu461. Both of these displacements occur via planar oxocarbenium ion-like transition states. The acceptor reaction with glucose is important for the formation of allolactose, the natural inducer of the lac operon.

Quantifying the electronic effects of carbohydrate hydroxy groups by using aminosugar models

Methyl amino-deoxy-glycosides with α- and β-gluco, α-galacto, or α-manno stereochemistry with the amino functionality in each of the four possible non-anomeric positions have been synthesized and their pK(a) values determined by titration. These model compounds were chosen because they are the amino derivatives of the most common glycosyl acceptors. From this study it was possible to evaluate the electron density at each of the given positions in the carbohydrate and compare them. Some general trends were observed: The basicity of the amino groups decreases in the order 6-NH(2)>3-NH(2)>2-NH(2)>4-NH(2) (referring to the position). The basicity of a of an amino-deoxy-sugar generally increases when one or more substituents on the sugar ring are axial. The basicity decreases when the amine is antiperiplanar to an oxygen atom. These findings are in agreement with the observations obtained from glycosylation chemistry and the regioselective protection of sugars.

Binding of modified fragments of the Shigella dysenteriae type 1 O-specific polysaccharide to monoclonal IgM 3707 E9 and docking of the immunodeterminant to its modeled Fv

The O-specific polysaccharide (O-SP) of Shigella dysenteriae type 1 has been shown by others to have the structure-->3)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->2)-alp ha-D- Galp-(1-->3)-alpha-D-GlcpNAc-(1-->. We have shown in the past that IgM 3707 E9, an anti S. dysenteriae type 1 O-SP monoclonal antibody, binds specifically to the -alpha-L-Rhap-(1-->2)-alpha-D-Galp-determinant of the polysaccharide. In this report we show that determinant to have hydrogen bonds, necessary for binding to the antibody, involving positions 3, 4 and 6 of the galactopyranosyl residue. The hydroxyl groups of the rhamnopyranosyl moiety of the immunodeterminant appear not to partake in hydrogen-bond interactions with the antibody. A model is presented of the Fv of IgM 3707 E9 based on our previously established cDNA-sequence and two known, highly homologous immunoglobulin crystal structures. The methyl glycoside of the immunodeterminant alpha-L-rhamnopyranosyl-(1-->2)-alpha-D-galactopyranose is docked to the combining area of the Fv.

Synthesis of specifically monofluorinated ligands related to the O-polysaccharide of Shigella dysenteriae type 1

The synthesis is reported of galactopyranose nucleophiles monofluorinated at positions 3, 4, or 6 and protected by 4,6-O-benzylidene, 3,6-di-O-benzyl, or 3,4-O-isopropylidene groups, respectively. The condensation of these nucleophiles with 2,3,4-tri-O-benzoyl-alpha-L-rhamnosyl bromide gave, after deprotection, the disaccharide analogues of methyl O-alpha-L-rhamnopyranosyl-(1-->2)-alpha-D-galactopyranoside, monofluorinated at position 3, 4, or 6 of the galactoside residue.