Cellobiose

Plant cell-wall polysaccharides constitute the most abundant but recalcitrant organic carbon source in nature. Microbes residing in the digestive tract of herbivorous bilaterians are particularly efficient at depolymerizing polysaccharides into fermentable sugars and play a significant support role towards their host's lifestyle. Here, we combine large-scale functional screening of fosmid libraries, shotgun sequencing, and biochemical assays to interrogate the gut microbiota of the wood-feeding "higher" termite Globitermes brachycerastes. A number of putative polysaccharide utilization gene clusters were identified with multiple fibrolytic genes. Our large-scale functional screening of 50,000 fosmid clones resulted in 464 clones demonstrating plant polysaccharide-degrading activities, including 267 endoglucanase-, 24 exoglucanase-, 72 β-glucosidase-, and 101 endoxylanase-positive clones. We sequenced 173 functionally active clones and identified ~219 genes encoding putative carbohydrate-active enzymes (CAZymes) targeting cellulose, hemicellulose and pectin. Further analyses revealed that 68 of 154 contigs encode one or more CAZyme, which includes 35 examples of putative saccharolytic operons, suggesting that clustering of CAZymes is common in termite gut microbial inhabitants. Biochemical characterization of a representative xylanase cluster demonstrated that constituent enzymes exhibited complementary physicochemical properties and saccharolytic capabilities. Furthermore, diverse cellobiose-metabolizing enzymes include β-glucosidases, cellobiose phosphorylases, and phopho-6-β-glucosidases were identified and functionally verified, indicating that the termite gut micro-ecosystem utilizes diverse metabolic pathways to interconnect hydrolysis and central metabolism. Collectively, these results provide an in-depth view of the adaptation and digestive strategies employed by gut microbiota within this tiny-yet-efficient host-associated ecosystem.

BackgroundThe optimization of metabolic pathways is critical for efficient and economical production of biofuels and specialty chemicals. One such significant pathway is the cellobiose utilization pathway, identified as a promising route in biomass utilization. Here we describe the optimization of cellobiose consumption and ethanol productivity by simultaneously engineering both proteins of the pathway, the β-glucosidase (gh1-1) and the cellodextrin transporter (cdt-1), in an example of pathway engineering through directed evolution.ResultsThe improved pathway was assessed based on the strain specific growth rate on cellobiose, with the final mutant exhibiting a 47% increase over the wild-type pathway. Metabolite analysis of the engineered pathway identified a 49% increase in cellobiose consumption (1.78 to 2.65 g cellobiose/(L · h)) and a 64% increase in ethanol productivity (0.611 to 1.00 g ethanol/(L · h)).ConclusionsBy simultaneously engineering multiple proteins in the pathway, cellobiose utilization in S. cerevisiae was improved. This optimization can be generally applied to other metabolic pathways, provided a selection/screening method is available for the desired phenotype. The improved in vivo cellobiose utilization demonstrated here could help to decrease the in vitro enzyme load in biomass pretreatment, ultimately contributing to a reduction in the high cost of biofuel production.

A hypothetic gene (THA_1941) encoding a putative cellobiose phosphorylase (CBP) from Thermosipho africanus TCF52B has very low amino acid identities (less than 12%) to all known GH94 enzymes. This gene was cloned and over-expressed in Escherichia coli BL21(DE3). The recombinant protein was hypothesized to be a CBP enzyme and it showed an optimum temperature of 75 °C and an optimum pH of 7.5. Beyond its CBP activity, this enzyme can use cellobiose and long-chain cellodextrins with a degree of polymerization of greater than two as a glucose acceptor, releasing phosphate from glucose 1-phosphate. The catalytic efficiencies (k cat/K m) indicated that cellotetraose and cellopentaose were the best substrates for the phosphorolytic and reverse synthetic reactions, respectively. These results suggested that this enzyme was the first enzyme having both cellodextrin and cellobiose phosphorylases activities. Because it preferred cellobiose and cellodextrins to glucose in the synthetic direction, it was categorized as a cellodextrin phosphorylase (CDP). Due to its unique ability of the reverse synthetic reaction, this enzyme could be a potential catalyst for the synthesis of various oligosaccharides. The speculative function of this CDP in the carbohydrate metabolism of T. africanus TCF52B was also discussed.

BackgroundCellobiose 2-epimerases (CEs) are promising enzymes that catalyze the conversion of lactose to the bioactive disaccharides epilactose and lactulose. Low-temperature (≤8 °C) lactose bioconversion is essential in dairy applications to prevent microbial contamination. Unfortunately, most CEs are not particularly active at these low temperatures. Therefore, this work aims to discover and characterize novel cold-active CEs.ResultsTwo novel mesophilic CEs from Teredinibacter haidensis (ThCE) and Cellvibrio japonicus (CjCE) were characterized, which showed both epi- and isomerization activity additionally at low temperatures. ThCE showed a maximum epimerization activity (595.4 ± 4.4 nkat/mg protein) at pH 7.5 and 35 °C, while CjCE achieved maximum epimerization activity (887.4 ± 3.9 nkat/mg protein) at pH 9.5 and 40 °C. In addition, CjCE maintained an >80% activity across a wide pH (6-9.5) and temperature range (30-45 °C). Both CEs exhibited significant activity and stability at 8 °C. Specifically, ThCE and CjCE retained around 20 and 40% of their epimerization activity at 8 °C and showed half-lives of 96.3 and 72.2 days, respectively. Moreover, CjCE was also found to be capable of catalyzing the isomerization reaction at 8 °C. The isomerization activity of ThCE and CjCE increased ~370-fold (70.8 ± 2.7 nkat/mg protein) and ~230-fold (66.1 ± 0.3 nkat/mg protein), respectively, at lactose concentrations up to 600 mM (35 °C for ThCE and 40 °C for CjCE). This demonstrates how important it is to conduct investigations of CEs for lactose isomerization to lactulose up to the solubility limit of lactose.ConclusionsThis study identified two new CEs that exhibit interesting catalytic properties for possible application in the generation of disaccharides with prebiotic properties from lactose at low temperatures.