Biosynthesis of 3'-O-methylisoorientin from luteolin by selecting O-methylation/C-glycosylation motif

Advances in glycosyltransferase-mediated glycodiversification of small molecules

Currently, numerous glycosides have been synthesized and used in clinical applications, neutraceuticals, cosmetics, and food processing. Structurally, a glycoside is composed of aglycone attaching to one or several sugar moieties so-called glycone. It is found that biochemical or biopharmaceutical properties of glycoside are mainly determined by its sugar part and thereby alternation of this glycone resulting in novel structure and characteristics as well. The use of traditional production methods of glycosides such as direct extraction and purification from plants, animals, or microorganisms is very challenging (laborious, time-consuming, technique, high price, low yield, etc.). Alternatively, the use of enzymatic methods for the biosynthesis of glycosides has become a highly promising tool. Particularly, the diverse structure of glycosides can be obtained using the promiscuous catalytic activity of glycosyltransferases (GT) mined from bioresources (plants, fungi, microorganisms, etc.). In addition, the exploration of GT catalytic promiscuity toward diverse aglycones, and glycones has indeed been interesting and played a key role in the production of novel glycosides. This review described the recent advances in glycosyltransferase-mediated glycodiversification of small molecules (flavonoids, steroids, terpenoids, etc.). Mostly, references were collected from 2014 to 2023.

Diversity and regioselectivity of O-methyltransferases catalyzing the formation of O-methylated flavonoids

Flavonoids and their methylated derivatives have immense market potential in the food and biomedical industries due to their multiple beneficial effects, such as antimicrobial, anti-inflammatory, and anticancer activities. The biological synthesis of flavonoids and their derivatives is often accomplished via the use of genetically modified microorganisms to ensure large-scale production. Therefore, it is pivotal to understand the properties of O-methyltransferases (OMTs) that mediate the methylation of flavonoids. However, the properties of these OMTs are governed by their: sources, substrate specificity, amino acid residues in the active sites, and the intricate mechanism. In order to obtain a clue for the selection of suitable OMTs for the biosynthesis of a target methylated flavonoid, we made a comprehensive review of the currently reported results, with a particular focus on their comparative regioselectivity for different flavonoid substrates. Additionally, the possible mechanisms for the diversity of this class of enzymes were explored using molecular simulation technology. Finally, major gaps in our understanding and areas for future studies were discussed. The findings of this study may be useful in selecting genes that encode OMTs and designing enzyme-based processes for synthesizing O-methylated flavonoids.

A growth selection system for sucrose synthases (SuSy): design and test

High throughput screening (HTS) methods of enzyme variants are essential for the development of robust biocatalysts suited for low impact, industrial scale, biobased synthesis of a myriad of compounds. However, for the majority of enzyme classes, current screening methods have limited throughput, or need expensive substrates in combination with sophisticated setups. Here, we present a straightforward, high throughput selection system that couples sucrose synthase activity to growth. Enabling high throughput screening of this enzyme class holds the potential to facilitate the creation of robust variants, which in turn can significantly impact the future of cost effective industrial glycosylation.

Advances in the study of the function and mechanism of the action of flavonoids in plants under environmental stresses

MAIN CONCLUSION

This review summarizes the anti-stress effects of flavonoids in plants and highlights its role in the regulation of polar auxin transport and free radical scavenging mechanism. As secondary metabolites widely present in plants, flavonoids play a vital function in plant growth, but also in resistance to stresses. This review introduces the classification, structure and synthetic pathways of flavonoids. The effects of flavonoids in plant stress resistance were enumerated, and the mechanism of flavonoids in plant stress resistance was discussed in detail. It is clarified that plants under stress accumulate flavonoids by regulating the expression of flavonoid synthase genes. It was also determined that the synthesized flavonoids are transported in plants through three pathways: membrane transport proteins, vesicles, and bound to glutathione S-transferase (GST). At the same time, the paper explores that flavonoids regulate polar auxin transport (PAT) by acting on the auxin export carrier PIN-FORMED (PIN) in the form of ATP-binding cassette subfamily B/P-glycoprotein (ABCB/PGP) transporter, which can help plants to respond in a more dominant form to stress. We have demonstrated that the number and location of hydroxyl groups in the structure of flavonoids can determine their free radical scavenging ability and also elucidated the mechanism by which flavonoids exert free radical removal in cells. We also identified flavonoids as signaling molecules to promote rhizobial nodulation and colonization of arbuscular mycorrhizal fungi (AMF) to enhance plant-microbial symbiosis in defense to stresses. Given all this knowledge, we can foresee that the in-depth study of flavonoids will be an essential way to reveal plant tolerance and enhance plant stress resistance.

Engineering of flavonoid 3'-O-methyltransferase for improved biosynthesis of chrysoeriol in Escherichia coli

O-Methylation catalyzed by O-methyltransferases (OMTs) is an important modification of flavonoids for improving the transport efficiency across membranes and metabolic stability in mammalian cells. Chrysoeriol, also known as 3'-O-methylated luteolin, is a methylated flavonoid compound with health-promoting activities. The generation of chrysoeriol from luteolin can be catalyzed by a rice-derived 3'-OMT named ROMT-9, which has a high regiospecificity and activity toward flavonoids in vitro. Herein, we explored the potential of ROMT-9 for in vivo biosynthesis of chrysoeriol in Escherichia coli and adopted semi-rational enzyme engineering guided by homology modeling and molecular docking to improve the bio-production. Two positive variants including L34Q and W284A were obtained which promoted chrysoeriol formation to more than 85 mg/L from 200 mg/L of luteolin in 24 h compared with a titer of 55 mg/L for the strain expressing the native enzyme. Further biochemical analysis confirmed that such improvement in production stemmed from a higher enzyme expression level for the L34Q variant and higher efficiency in substrate binding and catalysis for the W284A variant. This study provides some insights into the engineering of other flavonoid OMTs and will facilitate high-level biosynthesis of methylated flavonoids in engineered microorganisms. KEY POINTS: • Biosynthesis of chrysoeriol from luteolin in E. coli using ROMT-9 • Engineering of ROMT-9 for better bio-production • ROMT-9 variants promote production via better expression or better catalysis.

Phenolic C-glycoside synthesis using microbial systems

Plants produce different types of phenolic compounds. The majority of these compounds are glycosylated. Phenolic O-glycosides are also common. Recently, C-glycosylation of phenolic compounds has received attention because of the biological importance of phenolic C-glycosides. To date, three classes of C-glycosyltransferases (CGTs) have been characterized based on the type of sugar acceptor: flavonoid CGT, coumarin CGT, and xanthone CGT. Phylogenetic analysis of glycosyltransferases has revealed that CGTs form a distinct class that is clearly different from that of O-glycosyltransferases. The characterized CGTs have been introduced into microbial systems to synthesize phenolic C-glycosides. Here, we review recent progress in the development of CGTs and their application in the synthesis of phenolic C-glycosides using microbial systems.