²H enrichment distribution of hepatic glycogen from ²H₂O reveals the contribution of dietary fructose to glycogen synthesis

Dietary fructose can benefit or hinder glycemic control, depending on the quantity consumed, and these contrasting effects are reflected by alterations in postprandial hepatic glycogen synthesis. Recently, we showed that ²H enrichment of glycogen positions 5 and 2 from deuterated water (²H₂O) informs direct and indirect pathway contributions to glycogenesis in naturally feeding rats. Inclusion of position 6(S) ²H enrichment data allows indirect pathway sources to be further resolved into triose phosphate and Krebs cycle precursors. This analysis was applied to six rats that had fed on standard chow (SC) and six rats that had fed on SC plus 35% sucrose in their drinking water (HS). After 2 wk, hepatic glycogenesis sources during overnight feeding were determined by ²H₂O administration and postmortem analysis of glycogen ²H enrichment at the conclusion of the dark period. Net overnight hepatic glycogenesis was similar between SC and HS rodents. Whereas direct pathway contributions were similar (403 ± 71 μmol/g dry wt HS vs. 578 ± 76 μmol/g dry wt SC), triose phosphate contributions were significantly higher for HS compared with SC (382 ± 61 vs. 87 ± 24 μmol/g dry wt, P < 0.01) and Krebs cycle inputs lower for HS compared with SC (110 ± 9 vs. 197 ± 32 μmol/g dry wt, P < 0.05). Analysis of plasma glucose ²H enrichments at the end of the feeding period also revealed a significantly higher fractional contribution of triose phosphate to plasma glucose levels in HS vs. SC. Hence, the ²H enrichment distributions of hepatic glycogen and glucose from ²H₂O inform the contribution of dietary fructose to hepatic glycogen and glucose synthesis.

Glycoside hydrolase family 9 processive endoglucanase from Clostridium phytofermentans: heterologous expression, characterization, and synergy with family 48 cellobiohydrolase

The glycoside hydrolase family 9 cellulase (Cel9) from Clostridium phytofermentans has a multi-modular structure and is essential for cellulose hydrolysis. In order to facilitate production and purification of Cel9, recombinant Cel9 was functionally expressed in Escherichia coli. Cel9 exhibited maximum activity at pH 6.5 and 65 degrees C on carboxymethyl cellulose in a 10-min reaction period. The hydrolysis products on regenerated amorphous cellulose (RAC) were cellotetraose (a major product), cellotriose, cellobiose and glucose, and 71-80% of the reducing sugars produced by Cel9 were in soluble form, suggesting that Cel9 was a processive endoglucanase. The highest synergy between C. phytofermentans Cel9 and C. phytofermentans cellobiohydrolase Cel48 on Avicel was about 1.8 at a ratio of about 1:5. Cel9 alone was sufficient to solublize filter paper while Cel48 was not; however, it enhanced the solublization process along with Cel9 synergistically. This study provided useful information for understanding of the cellulose hydrolysis mechanism of this cellulolytic bacterium with potential industrial importance.

Prebiotic synthesis of simple sugars by an interstellar formose reaction

The prebiotic possibilities for the synthesis of interstellar carbohydrates through a protic variant of the formose reaction under gas phase conditions were studied. Ab initio calculations were used to evaluate potential mechanisms. Based on considerations of barrier heights and temperature variations in the Interstellar Medium the plausibility of extended sugar synthesis will be discussed.

The sugar model: autocatalytic activity of the triose-ammonia reaction

Reaction of triose sugars with ammonia under anaerobic conditions yielded autocatalytic products. The autocatalytic behavior of the products was examined by measuring the effect of the crude triose-ammonia reaction product on the kinetics of a second identical triose-ammonia reaction. The reaction product showed autocatalytic activity by increasing both the rate of disappearance of triose and the rate of formation of pyruvaldehyde, the product of triose dehydration. This synthetic process is considered a reasonable model of origin-of-life chemistry because it uses plausible prebiotic substrates, and resembles modern biosynthesis by employing the energized carbon groups of sugars to drive the synthesis of autocatalytic molecules.

Synthesis of globo- and isoglobotriosides bearing a cinnamoylphenyl tag as novel electrophilic thiol-specific carbohydrate reagents

The galactosyl donor, 4,6-di-O-acetyl-2,3-di-O-benzyl-D-galactopyranosyl trichloroacetimidate, was efficiently coupled with regioselectively benzylated lactoside acceptors under standard conditions to stereoselectively afford the corresponding globotrioside and isoglobotrioside derivatives in very good yields. These glycosides were smoothly functionalized with a 6-(p-cinnamoylphenoxy)-hexyl tether tag as novel electrophilic thiol-specific carbohydrate reagents. Immobilization of the globotrioside conjugate to Thiopropyl Sepharose 6B for purification of B-subunit of Shiga toxin (StxB) and coupling of a model cysteine-containing protein (StxB-Z(n)-Cys) to the isoglobotrioside conjugate were both performed with high efficiency.

Crystal Structure of Truncated Fibrobacter succinogenes 1,3-1,4-β-d-Glucanase in Complex with β-1,3-1,4-Cellotriose

Fibrobacter succinogenes 1,3-1,4-beta-D-glucanase (Fsbeta-glucanase) catalyzes the specific hydrolysis of beta-1,4 glycosidic bonds adjacent to beta-1,3 linkages in beta-D-glucans or lichenan. This is the first report to elucidate the crystal structure of a truncated Fsbeta-glucanase (TFsbeta-glucanase) in complex with beta-1,3-1,4-cellotriose, a major product of the enzyme reaction. The crystal structures, at a resolution of 2.3 angstroms, reveal that the overall fold of TFsbeta-glucanase remains virtually unchanged upon sugar binding. The enzyme accommodates five glucose residues, forming a concave active cleft. The beta-1,3-1,4-cellotriose with subsites -3 to -1 bound to the active cleft of TFsbeta-glucanase with its reducing end subsite -1 close to the key catalytic residues Glu56 and Glu60. All three subsites of the beta-1,3-1,4-cellotriose adopted a relaxed C(1)4 conformation, with a beta-1,3 glycosidic linkage between subsites -2 and -1, and a beta-1,4 glycosidic linkage between subsites -3 and -2. On the basis of the enzyme-product complex structure observed in this study, a catalytic mechanism and substrate binding conformation of the active site of TFsbeta-glucanase is proposed.

Stability domains, substrate-induced conformational changes, and hinge-bending motions in a psychrophilic phosphoglycerate kinase. A microcalorimetric study

The cold-active phosphoglycerate kinase from the Antarctic bacterium Pseudomonas sp. TACII18 exhibits two distinct stability domains in the free, open conformation. It is shown that these stability domains do not match the structural N- and C-domains as the heat-stable domain corresponds to about 80 residues of the C-domain, including the nucleotide binding site, whereas the remaining of the protein contributes to the main heat-labile domain. This was demonstrated by spectroscopic and microcalorimetric analyses of the native enzyme, of its mutants, and of the isolated recombinant structural domains. It is proposed that the heat-stable domain provides a compact structure improving the binding affinity of the nucleotide, therefore increasing the catalytic efficiency at low temperatures. Upon substrate binding, the enzyme adopts a uniformly more stable closed conformation. Substrate-induced stability changes suggest that the free energy of ligand binding is converted into an increased conformational stability used to drive the hinge-bending motions and domain closure.

Tin-catalyzed conversion of trioses to alkyl lactates in alcohol solution

Tin chlorides, SnCl2 and SnCl4.5H2O are excellent catalysts for the reactions of trioses, dihydroxyacetone and glyceraldehyde with alcohols (MeOH, EtOH and nBuOH) to give alkyl lactates, whose reaction mechanism involves the intermediary formation of pyruvic aldehyde followed by its esterification, which is distinctively promoted by tin halides.

Triosidines: novel Maillard reaction products and cross-links from the reaction of triose sugars with lysine and arginine residues

The role of the highly reactive triose sugars glyceraldehyde and glyceraldehyde-3-phosphate in protein cross-linking and other amino acid modifications during the Maillard reaction was investigated. From the incubation of glyceraldehyde with N (alpha)-acetyl-L-lysine and N (alpha)-acetyl-L-arginine, we isolated four new Maillard reaction pyridinium compounds named 'triosidines'. Two of them, 'lys-hydroxy-triosidine' [1-(5-amino-5-carboxypentyl)-3-[(5-amino-5-carboxypentylamino)methyl]-5-hydroxypyridinium] and 'arg-hydroxy-triosidine' [2-(4-amino-4-carboxybutylamino)-8-(5-amino-5-carboxypentyl)-6-hydroxy-3,4-dihydro-pyrido[2,3-d]pyrimidin-8-ium] are fluorescent, UV-active Lys-Lys and Lys-Arg cross-links respectively. Their structures were identified by NMR and MS. In addition, two UV-active lysine adducts, 'trihydroxy-triosidine' [1-(5-amino-5-carboxypentyl)-3,4-dihydroxy-5-(hydroxymethyl)pyridinium] and 'triosidine carbaldehyde' [1-(5-amino-5-carboxypentyl)-3-formylpyridinium] were tentatively identified by MS. All structures involve six sugar-derived carbons as part of the heterocyclic ring. Of the two novel cross-links, only arg-hydroxy-triosidine was formed by glyceraldehyde-3-phosphate, an intermediate metabolite of the glycolytic pathway. Lys-hydroxy-triosidine and arg-hydroxy-triosidine were detected in human and porcine corneas treated with glyceraldehyde. The HPLC-fluorescence identification was confirmed by MS. Triosidines were also formed from dihydroxyacetone, a widely used artificial sun-tanning agent. Triosidines are expected to be useful tools in tissue engineering, where the utilization of highly reactive sugars is needed to stabilize the loose matrix. In addition, they are expected to be present in selected biological conditions, such as on consumption of a high fructose diet, and syndromes associated with high glyceraldehyde excretion, such as Fanconi Syndrome, fructose-1,6-diphosphatase deficiency and tyrosinaemia.

Recognition of cello-oligosaccharides by a family 17 carbohydrate-binding module: an X-ray crystallographic, thermodynamic and mutagenic study

The crystal structure of the Clostridium cellulovorans carbohydrate-binding module (CBM) belonging to family 17 has been solved to 1.7 A resolution by multiple anomalous dispersion methods. CBM17 binds to non-crystalline cellulose and soluble beta-1,4-glucans, with a minimal binding requirement of cellotriose and optimal affinity for cellohexaose. The crystal structure of CBM17 complexed with cellotetraose solved at 2.0 A resolution revealed that binding occurs in a cleft on the surface of the molecule involving two tryptophan residues and several charged amino acids. Thermodynamic binding studies and alanine scanning mutagenesis in combination with the cellotetraose complex structure allowed the mapping of the CBM17 binding cleft. In contrast to the binding groove characteristic of family 4 CBMs, family 17 CBMs appear to have a very shallow binding cleft that may be more accessible to cellulose chains in non-crystalline cellulose than the deeper binding clefts of family 4 CBMs. The structural differences in these two modules may reflect non-overlapping binding niches on cellulose surfaces.

Biochemical characterization and identification of catalytic residues in alpha-glucuronidase from Bacillus stearothermophilus T-6

Alpha-D-glucuronidases cleave the alpha-1,2-glycosidic bond of the 4-O-methyl-D-glucuronic acid side chain of xylan, as a part of an array of xylan hydrolyzing enzymes. The alpha-D-glucuronidase from Bacillus stearothermophilus T-6 was overexpressed in Escherichia coli using the T7 polymerase expression system. The purification procedure included two steps, heat treatment and gel filtration chromatography, and provided over 0.3 g of pure enzyme from 1 L of overnight culture. Based on gel filtration, the native protein is comprised of two identical subunits. Kinetic constants with aldotetraouronic acid as a substrate, at 55 degrees C, were a Km of 0.2 mM, and a specific activity of 42 U x mg(-1) (kcat = 54.9 s(-1)). The enzyme was most active at 65 degrees C, pH 5.5-6.0, in a 10-min assay, and retained 100% of its activity following incubation at 70 degrees C for 20 min. Based on differential scanning calorimetry, the protein denatured at 73.4 degrees C. Truncated forms of the enzyme, lacking either 126 amino acids from its N-terminus or 81 amino acids from its C-terminus, exhibited low residual activity, indicating that the catalytic site is located in the central region of the protein. To identify the potential catalytic residues, site-directed mutagenesis was applied on highly conserved acidic amino acids in the central region. The replacements Glu392-->Cys and Asp364-->Ala resulted in a decrease in activity of about five orders of magnitude, suggesting that these residues are the catalytic pair.

The sugar model: catalytic flow reactor dynamics of pyruvaldehyde synthesis from triose catalyzed by poly-l-lysine contained in a dialyzer

The formation of pyruvaldehyde from triose sugars was catalyzed by poly-l-lysine contained in a small dialyzer with a 100 molecular weight cut off (100 MWCO) suspended in a much larger triose substrate reservoir at pH 5.5 and 40 degrees C. The polylysine confined in the dialyzer functioned as a catalytic flow reactor that constantly brought in triose from the substrate reservoir by diffusion to offset the drop in triose concentration within the reactor caused by its conversion to pyruvaldehyde. The catalytic polylysine solution (400 mM, 0.35 mL) within the dialyzer generated pyruvaldehyde with a synthetic intensity (rate/volume) that was 3400 times greater than that of the triose substrate solution (12 mM, 120 mL) outside the dialyzer. Under the given conditions the final yield of pyruvaldehyde was greater than twice the weight of the polylysine catalyst. During the reaction the polylysine catalyst was poisoned presumably by reaction of its amino groups with aldehyde reactants and products. Similar results were obtained using a dialyzer with a 500 MWCO. The dialyzer method of catalyst containment was selected because it provides a simple and easily manipulated experimental system for studying the dynamics and evolutionary development of confined autocatalytic processes related to the origin of life under anaerobic conditions.

Structural basis of functional mimicry between carbohydrate and peptide ligands of con A

Crystallographic studies have shown independent binding sites for sugar and peptide ligands of concanavalin A, although they were considered functional mimics based on biochemical experiments. The topological correlation of 12-residue peptide with different carbohydrate ligands of concanavalin A showed similarity between trimannose and the YPY region of the peptide establishing structural mimicry. Molecular docking of trimannose and the YPY motif on the reciprocal binding sites revealed equivalent interactions and energetics implying that the peptide-binding sites may constitute additional sugar-binding subsites of concanavalin A. The binding of a mannose-rich neoglycoprotein with significantly higher affinity compared with that of the methyl alpha-d-mannopyranoside is consistent with this interpretation.

A rectifying ATP-regulated solute channel in the chloroplastic outer envelope from pea

Phosphorylated carbohydrates are the main photoassimilated export products from chloroplasts that support the energy household and metabolism of the plant cell. Channels formed by the chloroplastic outer envelope protein OEP21 selectively facilitate the translocation of triosephosphate, 3-phosphoglycerate and phosphate, central intermediates in the source-sink relationship between the chloroplast and the cytosol. The anion selectivity and asymmetric transport properties of OEP21 are modulated by the ratio between ATP and triosephosphates, 3-phosphoglycerate and phosphate in the intermembrane space. Conditions that lead to export of triosephosphate from chloroplasts, i.e. photosynthesis, result in outward-rectifying OEP21 channels, while a high ATP to triosephosphate ratio, e.g. dark metabolism, leads to inward-rectifying OEP21 channels with a less pronounced anion selectivity. We conclude that solute exchange between plastids and cytosol can already be regulated at the level of the organellar outer membrane.

Carbohydrate fluxes into alginate biosynthesis in Azotobacter vinelandii NCIB 8789: NMR investigations of the triose pools

The metabolite flux of carbohydrates through primary catabolism and into hexose resynthesis has been investigated for alginic acid biosynthesis in Azotobacter vinelandii. To do these studies, we fed the microorganism a variety of 13C-labeled glucose precursors, including [U-13C6]glucose. The incorporations of the precursors were determined by 1D 13C-NMR, 2D 13C-DQF-COSY, and inverse triple-quantum correlation experiments. The results clearly show that the entire catabolism of hexose is through the Entner-Doudoroff (E-D) pathway and that the triose pools are in equilibrium. Reentry into gluconeogenesis prior to alginate synthesis occurs totally from the glyceraldehyde 3-phosphate generated by the E-D pathway. The obligatory intermediacy of triose intermediates in alginate biosynthesis was proved. The experiments and results presented in this paper constitute a new method for distinguishing the E-D pathway from glycolysis in bacteria.

Conformation of the glucotriose unit in the lipid-linked oligosaccharide precursor for protein glycosylation

The conformation of the glucotriose unit of the protein glycosylation precursor Glc3Man9GlcNAc2 was assessed by deuterium exchange studies on the model tetrasaccharide alpha Glc----2 alpha Glc----3 alpha Glc----3 alpha Man----OCH2CH2CH3 dissolved in deuterated dimethyl sulfoxide. The hydroxyl proton on C-2 of the nonreducing end glucose and on C-4 of the glucose attached to mannose both show dramatic isotope shifts indicative of a strong hydrogen bond between these two hydroxyl groups. Such a hydrogen bond requires a fixed conformation of the glucotriose unit that brings these hydroxyl groups within 3 A of each other, a conformation that is supported by molecular modeling based on hard-sphere exo-anomeric (HSEA) calculations. The temperature dependence of the hydroxyl proton chemical shifts supports the postulated hydrogen bond, and the torsional angles between the three glucose units derived from the HSEA calculations are consistent with results from related studies on other saccharides. The results support a model for biochemical function in which the glucotriose unit could modulate the activity of the oligosaccharyltransferase by binding in a fixed conformation to a specific effector site in the enzyme.