Unusual substrate specificity in GH family 12: structure-function analysis of glucanases Bgh12A and Xgh12B from Aspergillus cervinus, and Egh12 from Thielavia terrestris

A novel GH12 xyloglucanase from the white rot fungus Abortiporus biennis, synergistically enhances lignocellulose saccharification by commercial cellulases

Xyloglucan is a complex, highly substituted plant biomass polysaccharide, which is largely overlooked in the design of enzyme cocktails for lignocellulose saccharification, due to its presence in specific plant tissues only, and its low content. Thus, the microbial mechanisms for its degradation have not been thoroughly studied. However, in the frame of the biorefinery concept, xyloglucan monomers also have to be utilized for the design of efficient bioprocesses. Moreover, in plant tissues, xyloglucan often covers cellulose fibrils, impeding the access of cellulases. In order to shed light on the enzymatic degradation of xyloglucan, a novel GH12 family xyloglucanase was studied, from the basidiomycete Abortiporus biennis. The enzyme was heterologously produced in Pichia pastoris, purified and characterized. AbiXeg12a is a 28 kDa glycoprotein, with relatively strict substrate specificity, since it is only active in xyloglucan and β-glucan. The main hydrolysis products are the oligomers XXXG, XLXG/XXLG, XLLG and the optimum activity conditions are pH 4.5 and 55 °C. The enzyme contributes to the saccharification of corn bran and apple pulp by a commercial cellulase preparation, increasing the release of reducing sugars by up to 39 % and 18 %, respectively, while the addition of AbiXeg12a can minimize the enzyme load of the reaction, at least for apple pulp, without loss in reducing sugar yield. Overall, the importance of xyloglucanases on the saccharification of xyloglucan-containing substrates was demonstrated in this study. The results could contribute to the design of more efficient, tailor-made enzyme cocktails for the saccharification and subsequent valorization of lignocellulose.

Enhancement of the catalytic activity of thermostable Endo-1,4-β-glucanase B (TnCelB) from Thermotoga neapolitana by error-prone PCR

Endo-1,4-β-glucanase plays a crucial role in converting cellulose from lignocellulosic biomass into fermentable sugars for biofuel production. However, its commercial utility is hindered by poor catalytic performance under extreme conditions. This study enhanced the catalytic activity of the endo-1,4-β-glucanase (TnCelB) from Thermotoga neapolitana through error-prone PCR directed evolution. After screening >4000 colonies, three mutants with enhanced activity were obtained. Mutants TnCelBY88F, TnCelBA233T, and TnCelBW219R displayed 52.14 U/mg, 44.90 U/mg, and 34.70 U/mg of specific activity on CMC, respectively, which is 1.9, 1.7, and 1.3 times higher than that of the wild-type (26.74 U/mg), correspondingly. Likewise, their enzyme activity on barley β-D-glucan increased by 3.5, 2.2, and 1.8 times, respectively. Notably, TnCelBY88F maintained over 90 % activity after 60 mins at high temperatures (80-100 °C), indicating an exceptional thermostability. Protein docking revealed that TnCelBY88F had higher binding affinity, aligned with kinetic studies. TnCelB was capble of released more non-oxidized sugars from the hydrolysis of regenerated amorphous cellulose (RAC) by synergy with auxiliary action family 10 (AA10), which is potential in development of efficient lignocellulosic saccharification. This study can provide useful insights for the future engineering of other endoglucanases in the glycoside hydrolases family 12.

Effects of N-linked glycosylation on the enzymatic properties of GH12 bifunctional enzymes from Aspergillus terreus expressed in Pichia pastoris

In industry, bioenergy, food process, and feed application, endoglucanases are highly valuable for lignocellulose degradation with high catalytic activity under high temperatures. The glycoside hydrolase family 12 endoglucanase (AtEglD) from Aspergillus terreus can efficiently hydrolyze both β-glucan and xyloglucan of barley with an optimal temperature of 55 °C under pH 5.0. To enhance the industrial potential of AtEglD, the rational design of its N-glycosylation sites is imperative. The genes encoding AtEglD (N-glycosylation site at Asn65), along with two mutants: D168S (N-glycosylation site at Asn166) and N65Q (which lacks an N-glycosylation site) were successfully expressed and characterized. AtEglD exhibits reduced activity at 60 °C whereas, the N65Q mutant exhibited enhanced activity, maintaining substantial activity even after 90 min incubation. In barley-β-glucan, its specific activity reached 3204.27 U·mg-1, representing 2.73 times increase compared to AtEglD (1175.35 U·mg-1), while the catalytic efficiency was measured at 779.00 S-1·mM-1, indicating a 74.4 % enhancement relative to AtEglD (447.34 S-1·mM-1). For xyloglucan, N65Q demonstrated a significantly greater affinity compared to AtEglD, with 36.0 % increase in catalytic efficiency. Intriguingly, the D168S mutant exhibited a marked reduction in both specific activity and catalytic efficiency across both substrates. The structure analysis of AtEglD revealed that the N65 residues are far away from the catalytic domain, while the N166 residues are close to the catalytic site. It is implied that N-glycosylation proximal to the catalytic site maybe constrict the substrate-binding channel, thereby diminishing substrate recognition. These findings underscore the pivotal role of N-glycosylation site variations of GH12 endoglucanase in modulating enzyme characteristics.

Effects of heterologous expression and N-glycosylation on the hyperthermostable endoglucanase of Pyrococcus furiosus

Hyperthermostable endoglucanases of glycoside hydrolase family 12 from the archaeon Pyrococcus furiosus (EGPf) catalyze the hydrolysis of β-1,4-glucosidic linkages in cellulose and β-glucan structures that contain β-1,3- and β-1,4-mixed linkages. In this study, EGPf was heterologously expressed with Aspergillus niger and the recombinant enzyme was characterized. The successful expression of EGPf resulted as N-glycosylated protein in its secretion into the culture medium. The glycosylation of the recombinant EGPf positively impacted the kinetic characterization of EGPf, thereby enhancing its catalytic efficiency. Moreover, glycosylation significantly boosted the thermostability of EGPf, allowing it to retain over 80% of its activity even after exposure to 100 °C for 5 h, with the optimal temperature being above 120 °C. Glycosylation did not affect the pH stability or salt tolerance of EGPf, although the glycosylated compound exhibited a high tolerance to ionic liquids. EGPf displayed the highest specific activity in the presence of 20% (v/v) 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), reaching approximately 2.4 times greater activity than that in the absence of [Bmim]Cl. The specific activity was comparable to that without the ionic liquid even in the presence of 40% (v/v) [Bmim]Cl. Glycosylated EGPf has potential as an enzyme for saccharifying cellulose under high-temperature conditions or with ionic liquid treatment due to its exceptional thermostability and ionic liquid tolerance. These results underscore the potential of N-glycosylation as an effective strategy to further enhance both the thermostability of highly thermostable archaeal enzymes and the hydrolysis of barley cellulose in the presence of [Bmim]Cl.

Discovery of two bifunctional/multifunctional cellulases by functional metagenomics

Cellulases are widely used in industry, and the usage in bioconversion of biofuels makes cellulases more valuable. In this study, two tandem genes that encoded cellulases ZF994-1 and ZF994-2, respectively, were identified on a cosmid from a soil metagenomic library. Phylogenetic analysis indicated that ZF994-1 and ZF994-2 belonged to glycoside hydrolase family 12 (GH12), and GH3, respectively. Based on the substrate specificity analysis, the recombinant ZF994-1 exhibited weak endoglucanase activity, moderate β-1,3-glucanase and β-1,4-mannanase activities, and strong β-glucosidase activity, while the recombinant ZF994-2 exhibited moderate endoglucanase activity and strong β-glucosidase activity. More than 45% β-glucosidase activity of the recombinant ZF994-1 retained in the buffer containing 3 M glucose, indicating the good tolerance against glucose. The recombinant ZF994-2 showed high activity in the presence of metal ions and organic reagents, exhibiting potential industrial applications.

Thermostable enzyme research advances: a bibliometric analysis

Thermostable enzymes are enzymes that can withstand elevated temperatures as high as 50 °C without altering their structure or distinctive features. The potential of thermostable enzymes to increase the conversion rate at high temperature has been identified as a key factor in enhancing the efficiency of industrial operations. Performing procedures at higher temperatures with thermostable enzymes minimises the risk of microbial contamination, which is one of the most significant benefits. In addition, it helps reduce substrate viscosity, improve transfer speeds, and increase solubility during reaction operations. Thermostable enzymes offer enormous industrial potential as biocatalysts, especially cellulase and xylanase, which have garnered considerable amount of interest for biodegradation and biofuel applications. As the usage of enzymes becomes more common, a range of performance-enhancing applications are being explored. This article offers a bibliometric evaluation of thermostable enzymes. Scopus databases were searched for scientific articles. The findings indicated that thermostable enzymes are widely employed in biodegradation as well as in biofuel and biomass production. Japan, the United States, China, and India, as along with the institutions affiliated with these nations, stand out as the academically most productive in the field of thermostable enzymes. This study's analysis exposed a vast number of published papers that demonstrate the industrial potential of thermostable enzymes. These results highlight the significance of thermostable enzyme research for a variety of applications.