Mining and modification of Oryza sativa-derived squalene epoxidase for improved β-amyrin production in Saccharomyces cerevisiae

Silicon modulates nitrogen and secondary metabolism in Glycyrrhiza uralensis under drought and salt stress conditions

Glycyrrhiza uralensis Fisch (G. uralensis) is a key species for windbreak and sand fixation, possessing notable pharmacological and economic value. However, the yield of G. uralensis is considerably impacted due to its cultivation in arid, semi-arid, and salt-affected regions. Silicon (Si) has been reported to improve plant tolerance to drought and salt stress by regulating nitrogen and secondary metabolism. Herein, the effects of Si treatment on nitrogen and secondary metabolism of G. uralensis seedlings under drought (D), salt (S), and drought-salt (SD) stresses were investigated in combination with physiological and transcriptomic analyses. The results indicated that stress conditions significantly inhibited the growth of G. uralensis seedlings by suppressing nitrogen and secondary metabolism. Si treatment counteracted these inhibitions to some extent. Specifically, Si treatment increased soluble protein content by approximately 15% by regulating the nitrogen metabolism of G. uralensis under D stress. Furthermore, Si treatment elevated the content of glycyrrhetinic acid by about 89% under SD stress by increasing the content of primary metabolites and regulating the expression of enzymes involved in the biosynthesis of glycyrrhizic acid and liquiritin, including 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), squalene synthase (SQS), and β-amyrin synthase (β-AS). In summary, our findings suggest that Si could alleviate the adverse effects induced by drought and/or salt stresses on the growth of G. uralensis seedlings by regulating nitrogen metabolisms, which further triggered the accumulation of secondary metabolites, ultimately improving the stress resistance of cultivated G. uralensis seedlings. This work provides direction for Si to improve stress resistance.

Reengineering the Substrate Tunnel to Enhance the Catalytic Efficiency of Squalene Epoxidase

Squalene epoxidase plays a pivotal role in the biosynthesis of ergosterol, its derivatives, and other triterpenoid compounds by catalyzing the transformation of squalene into 2,3-oxidosqualene. However, its low catalytic efficiency remains a primary bottleneck for the microbial synthesis of triterpenoids. In this study, the catalytic activity of the squalene epoxidase from Saccharomyces cerevisiae was significantly improved by reshaping its substrate tunnel, resulting in a marked increase in the yield of the final product, ergosterol. First, the amino acid in the catalytic pocket of squalene epoxidase was replaced with alanine (Ala), effectively reducing the steric hindrance, and thus, enhancing the affinity of the enzyme with its substrate. Then, the V249H/L343A mutant was obtained by redesigning the substrate tunnel of dominant mutant L343A, thus, increasing the titer of ergosterol. The study also elucidated the mechanism behind the increased catalytic activity of the V249H/L343A mutant through substrate tunnel parameter analysis and molecular dynamics simulations. Finally, a titer of 3345 mg/L of ergosterol was achieved by strains containing V249H/L343A in a 5 L bioreactor, with a specific yield of 84 mg/g dry cell weight (DCW), marking a 64% increase compared with the titer achieved by wild type strains. This study established a strong foundation for improving the synthetic efficiency of ergosterol and other triterpenoid compounds.

Recent advancements in mogrosides: A review on biological activities, synthetic biology, and applications in the food industry

Mogrosides are low-calorie, biologically active sweeteners that face high production costs due to strict cultivation requirements and the low yield of monk fruit. The rapid advancement in synthetic biology holds the potential to overcome this challenge. This review presents mogrosides exhibiting antioxidant, anti-inflammatory, anti-cancer, anti-diabetic, and liver protective activities, with their efficacy in diabetes treatment surpassing that of Xiaoke pills (a Chinese diabetes medication). It also discusses the latest elucidated biosynthesis pathways of mogrosides, highlighting the challenges and research gaps in this field. The critical and most challenging step in this pathway is the transformation of mogrol into a variety of mogrosides by different UDP-glucosyltransferases (UGTs), primarily hindered by the poor substrate selectivity, product specificity, and low catalytic efficiency of current UGTs. Finally, the applications of mogrosides in the current food industry and the challenges they face are discussed.