Preparation of a novel soluble inducer by cellobiase-release microcapsules and its application in cellulase production

Heterologous expression of a highly thermostable L-asparaginase from Thermococcus zilligii in Aspergillus niger for efficient reduction of acrylamide in French fries

L-asparaginase (L-ASNase) can hydrolyze L-asparagine, a precursor to acrylamide, thereby reducing toxic acrylamide formation in fried foods. Currently, commercial L-ASNases are primarily produced by wild-type (WT) filamentous fungi; however, these enzymes often exhibit rapid activity loss during high-temperature processing due to limited thermal stability. In this study, we screened a thermostable L-ASNase gene from thermophile bacteria and expressed it in Aspergillus niger to reduce acrylamide content in French fries. Initially, four genes encoding thermostable L-ASNases were selected and integrated into the A. niger genome via non-homologous end joining. Among these, the L-ASNase gene tzi from Thermococcus zilligii was successfully expressed in A. niger, yielding an extracellular activity of 114 U·mg-1. The recombinant enzyme (An-Tzi) displayed the same optimal temperature and pH as its WT counterpart but exhibited superior catalytic efficiency, likely due to the efficient post-translational modifications in A. niger. To further enhance expression, the tzi gene was integrated into the amylase (amyA) locus of the A. niger genome using the CRISPR-Cas9 system, resulting in increased activity of 128 U·mg-1. Additionally, various lengths of the highly expressed glucoamylase (glaA) protein from A. niger AG11 were fused to the N-terminus of the Tzi. Notably, fusing the 500-amino-acid catalytic domain of glaA led to a substantial 3.3-fold increase in enzyme activity. Despite the metabolic stress induced by high-level expression of glaA, supplementing the culture medium with metal ions and sophorose resulted in an extracellular activity of 486.74 U·mg-1, the highest reported yield of L-ASNase in shake flasks. Finally, applying the An-Tzi to French fries achieved a 32 % greater reduction in acrylamide compared to the commercial enzyme. Overall, the recombinant A. niger strain expressing thermostable An-Tzi demonstrates significant potential for industrial applications targeting acrylamide reduction in fried and baked foods.

Efficient production of sophorose from glucose and its potentially industrial application in cellulase production

Sophorose is the most effective inducer for cellulase production by Trichoderma reesei. Currently, the biosynthesis of sophorose is very inefficient, resulting in that unavailable for cellulase production in industry. In this study, CoGH1A, a multifunctional thermophilic glycoside hydrolase, was employed for sophorose production. Under the optimized conditions, the sophorose yield was 37.86 g/L with a productivity of 9.47 g/L/h which is by far the highest productivity. Meanwhile, the Fe3O4-CS-THP-CoGH1A nanoparticles were constructed to realize the recycling of CoGH1A. After 5 cycles of catalysis, Fe3O4-CS-THP-CoGH1A retained about 83.90 % enzyme activity. Finally, the mixtures of glucose and disaccharides (MGDC) obtained after being catalyzed by CoGH1A was used for cellulase production. As a result, the cellulase productivity achieved 188.38 FPU/L/h in 120 h. These results indicated that sophorose could be efficiently produced from glucose via transglycosylation by CoGH1A, making it possible to be industrially used as the inducer to improving the cellulase productivity.

A novel strategy for efficient disaccharides synthesis from glucose by β-glucosidase

Oligosaccharides have important therapeutic applications. A useful route for oligosaccharides synthesis is reverse hydrolysis by β-glucosidase. However, the low conversion efficiency of disaccharides from monosaccharides limits its large-scale production because the equilibrium is biased in the direction of hydrolysis. Based on the analysis of the docking results, we hypothesized that the hydropathy index of key amino acid residues in the catalytic site is closely related with disaccharide synthesis and more hydrophilic residues located in the catalytic site would enhance reverse hydrolysis activity. In this study, positive variants TrCel1bI177S, TrCel1bI177S/I174S, and TrCel1bI177S/I174S/W173H, and one negative variant TrCel1bN240I were designed according to the Hydropathy Index For Enzyme Activity (HIFEA) strategy. The reverse hydrolysis with TrCel1bI177S/I174S/W173H was accelerated and then the maximum total production (195.8 mg/mL/mg enzyme) of the synthesized disaccharides was increased by 3.5-fold compared to that of wild type. On the contrary, TrCel1bN240I lost reverse hydrolysis activity. The results demonstrate that the average hydropathy index of the key amino acid residues in the catalytic site of TrCel1b is an important factor for the synthesis of laminaribiose, sophorose, and cellobiose. The HIFEA strategy provides a new perspective for the rational design of β-glucosidases used for the synthesis of oligosaccharides.