A revised architecture of primary cell walls based on biomechanical changes induced by substrate-specific endoglucanases.
Park, Y. B. & Cosgrove, D. J. (2012). Plant Physiology, 158(4), 1933-1943.
Xyloglucan is widely believed to function as a tether between cellulose microfibrils in the primary cell wall, limiting cell enlargement by restricting the ability of microfibrils to separate laterally. To test the biomechanical predictions of this “tethered network” model, we assessed the ability of cucumber (Cucumis sativus) hypocotyl walls to undergo creep (long-term, irreversible extension) in response to three family-12 endo-β-1,4-glucanases that can specifically hydrolyze xyloglucan, cellulose, or both. Xyloglucan-specific endoglucanase (XEG from Aspergillus aculeatus) failed to induce cell wall creep, whereas an endoglucanase that hydrolyzes both xyloglucan and cellulose (Cel12A from Hypocrea jecorina) induced a high creep rate. A cellulose-specific endoglucanase (CEG from Aspergillus niger) did not cause cell wall creep, either by itself or in combination with XEG. Tests with additional enzymes, including a family-5 endoglucanase, confirmed the conclusion that to cause creep, endoglucanases must cut both xyloglucan and cellulose. Similar results were obtained with measurements of elastic and plastic compliance. Both XEG and Cel12A hydrolyzed xyloglucan in intact walls, but Cel12A could hydrolyze a minor xyloglucan compartment recalcitrant to XEG digestion. Xyloglucan involvement in these enzyme responses was confirmed by experiments with Arabidopsis (Arabidopsis thaliana) hypocotyls, where Cel12A induced creep in wild-type but not in xyloglucan-deficient (xxt1/xxt2) walls. Our results are incompatible with the common depiction of xyloglucan as a load-bearing tether spanning the 20- to 40-nm spacing between cellulose microfibrils, but they do implicate a minor xyloglucan component in wall mechanics. The structurally important xyloglucan may be located in limited regions of tight contact between microfibrils.
Production optimization and expression of pectin releasing enzyme from Aspergillus oryzae PO.
Chen, J., Yang, R., Chen, M., Wang, S., Li, P., Xia, Y., Zhou, L., Xie, J. & Wei, D. (2014). Carbohydrate Polymers, 101, 89-95.
Protopectinase is an enzyme that solubilizes protopectin forming highly polymerized soluble pectin. Protopectinase activity was detected from Aspergillus oryzae PO isolated from soil of persimmon orchard. Response surface methodology of Box–Behnken Design with three fermentation variables (temperature, NaNO3 and apple pomace concentration) was used to optimize protopectinase production of A. oryzae PO, and protopectinase activity was improved to 270.0 U/ml. Endo-polygalacturonase belonged to A-type PPase from A. oryzae PO was cloned and expressed in Pichia pastoris GS115. The endo-polygalacturonase expression was 0.418 mg/ml and the specific activity of purified recombinant endo-polygalacturonase was 7520 U/mg toward polygalacturonic acid. The optimal temperature and pH of recombinant endo-polygalacturonase were 45°C and 5.0, respectively. The recombinant endo-polygalacturonase activity was enhanced by the presence of Mg2+, while Ca2+, Ni2+ Mn2+, Cu2+ and SDS strongly inhibited the enzyme activity. The apparent Km value and Vmax value were 5.59 mg/ml and 1.01 µmol/(min ml), respectively.
Using AFM and force spectroscopy to determine pectin structure and (bio) functionality.
Morris, V. J., Gromer, A., Kirby, A. R. J., Bongaerts, R. J. M. & Patrick Gunning, A. (2011). Food Hydrocolloids, 25(2), 230-237.
Pectin is an integral component of non-graminaceous plant cell walls. It is believed to form an interconnected network structure independent of the cellulose-xyloglucan network structure. Pectin gels are often used as a model for the pectin network structure within the plant cell wall. Atomic force microscopy studies of calcium-induced gel precursors, and fragments released from gels, suggest that association leads to a branched fibrous structure within the gels. Enzymatic de-esterification of high-methoxyl pectin in the presence of calcium ions can induce gelation of the pectin. Thus pectin gel networks may provide a model for a self-assembled network structure within the middle lamella region of the plant cell wall. The pectin network in plant cell walls is a source of soluble and insoluble fibre. In addition to the health benefits associated with the dietary fibre aspects of pectin new health claims are emerging. Recently published in vitro and in vivo animal studies, and human studies, suggest that oral consumption of a modified form of pectin may have anti-cancer properties. These studies suggest that the modified pectin may act on a range of cancers at several stages of progression of the cancer. It has been hypothesised that this generic action is due to the modification allowing release of bioactive fragment(s) which are claimed to bind specifically to and inhibit the action of the mammalian lectin galectin 3 (Gal3). Gal3 is a key regulator of cellular homeostasis and plays important roles in several stages of cancer metastasis. Studies using force spectroscopy, flow cytometry and fluorescence microscopy suggest that the bioactive fragments of pectin may be pectin-derived galactans.
Delayed utilization of some fast-fermenting soluble dietary fibers by human gut microbiota when presented in a mixture.
Tuncil, Y. E., Nakatsu, C. H., Kazem, A. E., Arioglu-Tuncil, S., Reuhs, B., Martens, E. C. & Hamaker, B. R. (2017). Journal of Functional Foods, 32, 347-357.
Delivering fibers more distally could be important to prevent or treat colonic diseases such as cancer and ulcerative colitis. Here, we hypothesized that fermentation of fast-fermenting soluble fibers by the colonic microbiota is delayed when they are presented in a mixture due to hierarchical utilization of fibers. A series of in vitro fermentation studies was performed using fecal microbiota obtained from three healthy donors using single dietary fibers [arabinoxylan, chondroitin sulfate (CS), galactomannan (GM), polygalacturonic acid (PGA), xyloglucan (XG)] and a mixture containing an equal amount of each. Substrate disappearance analysis, as measured by GC–MS, revealed that CS, PGA, and XG utilization was delayed when present in the mixture. 16S rRNA sequencing showed certain fibers consistently increased specific genera in the microbiota of all donor groups. Mixing different types of fermentable dietary fibers might be a logical strategy for delivering fibers into more distal regions of the colon.
Reciprocal Prioritization to Dietary Glycans by Gut Bacteria in a Competitive Environment Promotes Stable Coexistence.
Tuncil, Y. E., Xiao, Y., Porter, N. T., Reuhs, B. L., Martens, E. C. & Hamaker, B. R. (2017). mBio, 8(5), e01068-17.
When presented with nutrient mixtures, several human gut Bacteroides species exhibit hierarchical utilization of glycans through a phenomenon that resembles catabolite repression. However, it is unclear how closely these observed physiological changes, often measured by altered transcription of glycan utilization genes, mirror actual glycan depletion. To understand the glycan prioritization strategies of two closely related human gut symbionts, Bacteroides ovatus and Bacteroides thetaiotaomicron, we performed a series of time course assays in which both species were individually grown in a medium with six different glycans that both species can degrade. Disappearance of the substrates and transcription of the corresponding polysaccharide utilization loci (PULs) were measured. Each species utilized some glycans before others, but with different priorities per species, providing insight into species-specific hierarchical preferences. In general, the presence of highly prioritized glycans repressed transcription of genes involved in utilizing lower-priority nutrients. However, transcriptional sensitivity to some glycans varied relative to the residual concentration in the medium, with some PULs that target high-priority substrates remaining highly expressed even after their target glycan had been mostly depleted. Coculturing of these organisms in the same mixture showed that the hierarchical orders generally remained the same, promoting stable coexistence. Polymer length was found to be a contributing factor for glycan utilization, thereby affecting its place in the hierarchy. Our findings not only elucidate how B. ovatus and B. thetaiotaomicron strategically access glycans to maintain coexistence but also support the prioritization of carbohydrate utilization based on carbohydrate structure, advancing our understanding of the relationships between diet and the gut microbiome.