Trichoderma reesei CE16 acetyl esterase and its role in enzymatic degradation of acetylated hemicellulose

BACKGROUND

Trichoderma reesei CE16 acetyl esterase (AcE) is a component of the plant cell wall degrading system of the fungus. The enzyme behaves as an exo-acting deacetylase removing acetyl groups from non-reducing end sugar residues.

METHODS

In this work we demonstrate this exo-deacetylating activity on natural acetylated xylooligosaccharides using MALDI ToF MS.

RESULTS

The combined action of GH10 xylanase and acetylxylan esterases (AcXEs) leads to formation of neutral and acidic xylooligosaccharides with a few resistant acetyl groups mainly at their non-reducing ends. We show here that these acetyl groups serve as targets for TrCE16 AcE. The most prominent target is the 3-O-acetyl group at the non-reducing terminal Xylp residues of linear neutral xylooligosaccharides or on aldouronic acids carrying MeGlcA at the non-reducing terminus. Deacetylation of the non-reducing end sugar may involve migration of acetyl groups to position 4, which also serves as substrate of the TrCE16 esterase.

CONCLUSION

Concerted action of CtGH10 xylanase, an AcXE and TrCE16 AcE resulted in close to complete deacetylation of neutral xylooligosaccharides, whereas substitution with MeGlcA prevents removal of acetyl groups from only a small fraction of the aldouronic acids. Experiments with diacetyl derivatives of methyl β-d-xylopyranoside confirmed that the best substrate of TrCE16 AcE is 3-O-acetylated Xylp residue followed by 4-O-acetylated Xylp residue with a free vicinal hydroxyl group.

GENERAL SIGNIFICANCE

This study shows that CE16 acetyl esterases are crucial enzymes to achieve complete deacetylation and, consequently, complete the saccharification of acetylated xylans by xylanases, which is an important task of current biotechnology.

Mode of action of acetylxylan esterases on acetyl glucuronoxylan and acetylated oligosaccharides generated by a GH10 endoxylanase

BACKGROUND

Substitutions on the xylan main chain are widely accepted to limit plant cell wall degradability and acetylations are considered as one of the most important obstacles. Hence, understanding the modes of action of a range of acetylxylan esterases (AcXEs) is of ample importance not only to increase the understanding of the enzymology of plant decay/bioremediation but also to enable efficient bioconversion of plant biomass.

METHODS

In this study, the modes of action of acetylxylan esterases (AcXEs) belonging to carbohydrate esterase (CE) families 1, 4, 5 and 6 on xylooligosaccharides generated from hardwood acetyl glucuronoxylan were compared using MALDI ToF MS. Supporting data were obtained by following enzymatic deacetylation by (1)H NMR spectroscopy.

CONCLUSIONS

None of the used enzymes were capable of complete deacetylation, except from linear xylooligosaccharides which were completely deacetylated by some of the esterases in the presence of endoxylanase. A clear difference was observed between the performance of the serine-type esterases of CE families 1, 5 and 6, and the aspartate-metalloesterases of family CE4. The difference is mainly due to the inability of CE4 AcXEs to catalyze deacetylation of 2,3-di-O-acetylated xylopyranosyl residues. Complete deacetylation of a hardwood acetyl glucuronoxylan requires additional deacetylating enzyme(s).

GENERAL SIGNIFICANCE

The results contribute to the understanding of microbial degradation of plant biomass and outline the way to achieve complete saccharification of plant hemicelluloses which did not undergo alkaline pretreatment.

Influence of graded levels of brewers dried grains on pellet quality and performance in broiler chickens

The aim of the current study was to evaluate the effect of a gradual substitution of wheat and soy with brewers dried grains (BDG) on pellet quality, performance, and organ weights in broiler chickens. Five diets were formulated in which 0, 10, 20, 30, or 40% BDG replaced wheat and soy, with a concomitant gradual reduction in the calculated AME level. Each of the 5 experimental diets was fed to 12-d-old broiler chickens (Ross 308) kept in 6 pens, with 12 birds/pen. The birds had ad libitum access to feed and water until termination of the experiment at d 33. Feed intake was not affected by BDG inclusion and compensatory feed intake did not occur, perhaps having been neutralized by a significant (P < 0.001) reduction in the pellet durability index. The pellet durability index was 85% in the control diet (0% BDG) and decreased significantly (P < 0.001) to 68% in the diet with 40% BDG. Increased levels of BDG reduced BW gain significantly (P < 0.001) and led to a significant (P < 0.001) increase in the feed:gain ratio. The feed:gain ratio was significantly (P < 0.05) higher in birds fed 30 and 40% BDG compared with birds fed 0, 10, and 20% BDG. The apparent ileal digestibility values of protein and energy were significantly reduced by BDG inclusion (P < 0.01 and P < 0.001, respectively), whereas starch digestibility increased significantly (P < 0.001). The relative gizzard weight increased significantly (P < 0.001), whereas the relative cecal weights were not affected by BDG inclusion. To conclude, 10 to 20% inclusion of BDG supports acceptable growth and feed utilization, and favors the development of a well-functioning gizzard.

Enzymatic solubilization of brewers' spent grain by combined action of carbohydrases and peptidases

Brewers' spent grain (BSG), a high-volume coproduct from the brewing industry, primarily contains proteins, barley cell wall carbohydrates, and lignin. To create new possibilities for the exploitation of this large biomass stream, the solubilization of BSG by the combined action of carbohydrases (Depol 740 and Econase) and peptidase (Alcalase and Promod 439) was explored. Hydrolysis protocols were optimized with respect to temperature (influencing both microbial contamination and rate of enzymatic hydrolysis), pH, enzyme dose, order of enzyme addition, and processing time. On the basis of this approach, one- and two-step protocols are proposed taking 4-8 h and yielding combined or separate fractions of hydrolyzed oligosaccharides and liberated hydrolyzed protein. Optimized procedures resulted in the solubilization of >80% of the proteinaceous material, up to 39% of the total carbohydrates, and up to 42% of total dry matter in BSG. Of the original xylan present in BSG, 36% could be solubilized. Sequential and simultaneous treatments with the two enzyme types gave similar results. In sequential processes, the order of the carbohydrase and peptidase treatments had only minor effects on the outcome. Depol 740 released more pentoses than Econase and gave slightly higher overall dry matter solubilization yields.