A Bacillus licheniformis pectin acetylesterase is specific for homogalacturonans acetylated at O-3

One-pot biocatalysis of potato rhamnogalacturonan and the role of its deacetylation in efficient inhibition of colon cancer cells and hydrogel mediated colon-targeted drug delivery

Deacetylation of potato rhamnogalacturonan (PRG) by rhamnogalacturonan acetyl esterase (CtPae12B) was explored for enhanced hydrolysis of PRG by rhamnogalacturonan lyase (CtRGLf) and the effects of deacetylated PRG were studied in enhancing inhibition of colon-cancer cells and formation of colon-targeting drug delivery material. Pre-treatment of PRG with CtPae12B resulted in increased relative activity of CtRGLf. CtPae12B removed acetyl groups from both O-2 and O-3 positions of D-galactopyranosyluronic acid residues of PRG, resulting in 98 % deacetylation. PRG displayed 21.9 % degree of acetylation and 7.7 % degree of methylation. TLC and ESI-MS analysis of CtRGLf hydrolysed PRG showed unsaturated RG di-saccharide as the smallest product, with m/z 322. Deacetylated PRG-oligosaccharides displayed higher, 50 % inhibition of colon-cancer HCT-116 cells (with shrunken and globular morphology) than 35 % inhibition by acetylated PRG-oligosaccharides. FESEM and BET analysis of CtPae12B-treated PRG showed porous structure and significantly higher total surface area and pore volume than non-enzyme treated PRG. Higher drug entrapment efficiency and lower drug release rate of CtPae12B-treated PRG hydrogel (0.0033 min-1 at pH 1.2 and 0.009 min-1 at pH 7.4), than non-enzyme treated PRG hydrogel, (0.0057 min-1 at pH 1.2 and 0.02 min-1 at pH 7.4), showed it to be a potential biomaterial for sustainable colon-targeted drug delivery.

Ancient Origin of Acetyltransferases Catalyzing O-acetylation of Plant Cell Wall Polysaccharides

Members of the domain of unknown function 231/trichome birefringence-like (TBL) family have been shown to be O-acetyltransferases catalyzing the acetylation of plant cell wall polysaccharides, including pectins, mannan, xyloglucan and xylan. However, little is known about the origin and evolution of plant cell wall polysaccharide acetyltransferases. Here, we investigated the biochemical functions of TBL homologs from Klebsormidium nitens, a representative of an early divergent class of charophyte green algae that are considered to be the closest living relatives of land plants, and Marchantia polymorpha, a liverwort that is an extant representative of an ancient lineage of land plants. The genomes of K. nitens and Marchantia polymorpha harbor two and six TBL homologs, respectively. Biochemical characterization of their recombinant proteins expressed in human embryonic kidney 293 cells demonstrated that the two K. nitens TBLs exhibited acetyltransferase activities acetylating the pectin homogalacturonan (HG) and hence were named KnPOAT1 and KnPOAT2. Among the six M. polymorpha TBLs, five (MpPOAT1 to 5) possessed acetyltransferase activities toward pectins and the remaining one (MpMOAT1) catalyzed 2-O- and 3-O-acetylation of mannan. While MpPOAT1,2 specifically acetylated HG, MpPOAT3,4,5 could acetylate both HG and rhamnogalacturonan-I. Consistent with the acetyltransferase activities of these TBLs, pectins isolated from K. nitens and both pectins and mannan from M. polymorpha were shown to be acetylated. These findings indicate that the TBL genes were recruited as cell wall polysaccharide O-acetyltransferases as early as in charophyte green algae with activities toward pectins and they underwent expansion and functional diversification to acetylate various cell wall polysaccharides during evolution of land plants.

Acetylation of homogalacturonan and rhamnogalacturonan-I is catalyzed by a suite of trichome birefringence-like proteins

Plant cell wall polysaccharides, including xylan, mannan, xyloglucan, and pectins, are often acetylated and members of the domain of unknown function 231 (DUF231)/trichome birefringence-like (TBL) family have been shown to be O-acetyltransferases mediating the acetylation of xylan, mannan, and xyloglucan. However, little is known about the O-acetyltransferases responsible for pectin acetylation. In this report, we biochemically characterized a suite of Arabidopsis DUF231/TBL proteins for their roles in pectin acetylation. We generated 24 TBL recombinant proteins in mammalian cells and demonstrated that 10 of them were able to transfer acetyl groups from acetyl-CoA onto the pectins homogalacturonan (HG) or rhamnogalacturonan-I (RG-I), and thus were named pectin O-acetyltransferase 1 to 10 (POAT1 to 10). It was found that POAT2,4,9,10 specifically acetylated HG and POAT5,6 acetylated RG-I, whereas POAT1,3,7,8 could act on both HG and RG-I. The acetylation of HG and RG-I by POATs was further corroborated by hydrolysis with pectin acetylesterases and by nuclear magnetic resonance spectroscopy. In addition, mutations of the conserved GDS and DXXH motifs in POAT3 and POAT8 were shown to lead to a loss of their ability to acetylate HG and RG-I. Furthermore, simultaneous RNA interference downregulation of POAT1,3,6,7,8 resulted in reduced cell expansion, impaired plant growth, and decreased pectin acetylation. Together, our findings indicate that these POATs are pectin O-acetyltransferases involved in acetylation of the pectin polysaccharides HG and RG-I.

Carbohydrate esterases involved in deacetylation of food components by the human gut microbiota

Non-carbohydrate modifications such as acetylations are widespread in food stuffs as well as they play important roles in diverse biological processes. These modifications meet the gut environment and are removed from their carbohydrate substrates by the resident microbiota. Among the most abundant modifications are O-acetylations, contributing to polysaccharides physico-chemical properties such as viscosity and gelling ability, as well as reducing accessibility for glycosyl hydrolases, and thus hindering polysaccharide degradation. Of particular note, O-acetylations increase the overall complexity of a polymer, thus requiring a more advanced degrading machinery for microbes to utilize it. This minireview describes acetylesterases from the gut microbiota that deacetylate various food polysaccharides, either as natural components of food, ingredients, stabilizers of microbial origin, or as part of microbes for food and beverage preparations. These enzymes include members belonging to at least 8 families in the CAZy database, as well as a large number of biochemically characterized esterases that have not been classified yet. Despite different structural folds, most of these acetylesterases have a common acid-base mechanism and belong to the SGNH hydrolase superfamily. We highlight examples of acetylesterases that are highly specific to one substrate and to the position of the acetyl group on the glycosyl residue of the carbohydrate, while other members that have more broad substrate specificity. Current research aimed at unveiling the functions and regioselectivity of acetylesterases will help providing fundamental mechanistic understanding on how dietary components are utilized in the human gut and will aid developing applications of these enzymes to manufacture novel industrial products.

A thermotolerant and pH stable rhamnogalacturonan acetylesterase (CtPae12B), a family 12 carbohydrate esterase from Clostridium thermocellum with broad substrate specificity

The gene encoding rhamnogalacturonan acetylesterase, CtPae12B from Clostridium thermocellum was cloned, expressed, purified and biochemically characterized. Purified CtPae12B was soluble and exhibited homogenous single band. Phylogenetically it was most closely related to an RGAE, YesT from B. subtilis. CtPae12B production was maximum with LB medium. CtPae12B showed optimal temperature, 65 °C and thermostability with half-life, 5.1 h at 80 °C. CtPae12B was alkaliphilic with optimal pH, 8.0, while it displayed stability at both acidic and alkaline pH ranges. Inhibition of CtPae12B activity by PMSF showed the importance of nucleophilic serine in the catalytic triad. The metal ions, chemical or chelating agents used, did not enhance CtPae12B activity, which was also corroborated by protein melting study. The enzymatic activity of CtPae12B remained unaffected by 5 M urea. CtPae12B showed broad substrate specificity as it displayed activity against a range of synthetic substrates showing highest V_max, 770 U/mg and K_m, 1.2 mM with β-D-gluco pentaacetate. CtPae12B could deacetylate both pectic and xylan substrates showing highest V_max, 770 U/mg and K_m, 13.4 mg/mL with potato rhamnogalacturonan and V_max, 105 U/mg and K_m, 7.1 mg/mL with acetylated birchwood xylan. The thermostability, pH stability and broad substrate specificity of CtPae12B makes it a versatile enzyme for industrial applications.

Reduced photosynthesis in Arabidopsis thaliana atpme17.2 and atpae11.1 mutants is associated to altered cell wall composition

The cell wall is a complex and dynamic structure that determines plants' performance by constant remodeling of its compounds. Although cellulose is its major load-bearing component, pectins are crucial to determine wall characteristics. Changes in pectin physicochemical properties, due to pectin remodeling enzymes (PRE), induce the rearrangement of cell wall compounds, thus, modifying wall architecture. In this work, we tested for the first time how cell wall dynamics affect photosynthetic properties in Arabidopsis thaliana pectin methylesterase atpme17.2 and pectin acetylesterase atpae11.1 mutants in comparison to wild-type Col-0. Our results showed maintained PRE activities comparing mutants with wild-type and no significant differences in cellulose, but cell wall non-cellulosic neutral sugars contents changed. Particularly, the amount of galacturonic acid (GalA) - which represents to some extent the pectin cell wall proportion - was reduced in the two mutants. Additionally, physiological characterization revealed that mutants presented a decreased net CO₂ assimilation (A_N ) because of reductions in both stomatal (g_s ) and mesophyll conductances (g_m ). Thus, our results suggest that atpme17.2 and atpae11.1 cell wall modifications due to genetic alterations could play a significant role in determining photosynthesis.

TBL10 is required for O-acetylation of pectic rhamnogalacturonan-I in Arabidopsis thaliana

O-Acetylated pectins are abundant in the primary cell wall of plants and growing evidence suggests they have important roles in plant cell growth and interaction with the environment. Despite their importance, genes required for O-acetylation of pectins are still largely unknown. In this study, we showed that TRICHOME BIREFRINGENCE LIKE 10 (AT3G06080) is involved in O-acetylation of pectins in Arabidopsis (Arabidopsis thaliana). The activity of the TBL10 promoter was strong in tissues where pectins are highly abundant (e.g. leaves). Two homozygous knock-out mutants of Arabidopsis, tbl10-1 and tbl10-2, were isolated and shown to exhibit reduced levels of wall-bound acetyl esters, equivalent of ~50% of the wild-type level in pectin-enriched fractions derived from leaves. Further fractionation revealed that the degree of acetylation of the pectin rhamnogalacturonan-I (RG-I) was reduced in the tbl10 mutant compared to the wild type, whereas the pectin homogalacturonan (HG) was unaffected. The degrees of acetylation in hemicelluloses (i.e. xyloglucan, xylan and mannan) were indistinguishable between the tbl10 mutants and the wild type. The mutant plants contained normal trichomes in leaves and exhibited a similar level of susceptibility to the phytopathogenic microorganisms Pseudomonas syringae pv. tomato DC3000 and Botrytis cinerea; while they displayed enhanced tolerance to drought. These results indicate that TBL10 is required for O-acetylation of RG-I, possibly as an acetyltransferase, and suggest that O-acetylated RG-I plays a role in abiotic stress responses in Arabidopsis.

Biological Function(s) and Application (s) of Pectin and Pectin Degrading Enzymes

Pectin is an integral part of plant cell wall and since centuries pectin extracted from plants is widely used in food and fruit juice processing. Moreover, in last half century, the applications have also invaded into many bio-processing applications such as pharmaceutical, bioenergy, textile, paper and tea processing. In these growing industries, the use of pectinases has grown with a significant amount i.e. approximately 10 % of total global enzyme market comes from pectinases. Herein comprehensive analyses of information related to structure and function of pectin in plant cell wall as well as structural classes of pectins have been discussed. The major function of pectin is in cementing the cellulose and hemicelluloses network, cell-cell adhesion and plant defence. Keeping the wide use of pectin in food industry and growing need of environment friendly technology for pectin extraction has accelerated the demand of pectin degrading enzymes (PDEs). PDEs are from three enzyme classes: carbohydrate esterases from CE8 and CE12 family, glycoside hydrolases from GH28 family and lyases from PL1, 2, 3, 9 and 10. We have reviewed the literature related to abundance and structure-function of these abovementioned enzymes from bacteria. From the current available literature, we found very limited information is present about thermostable PDEs. Hence, in future it could be a topic of study to gain the insight about structure-function of enzymes together with the expanded role of thermostable enzymes in development of bioprocesses based on these enzymes.