Simultaneous directed evolution of coupled enzymes for efficient asymmetric synthesis of l-phosphinothricin

Development of growth selection system and pocket engineering of d-amino acid oxidase to enhance selective deamination activity toward d-phosphinothricin

D-amino acid oxidase (DAAO)-catalyzed selective oxidative deamination is a very promising process for synthesizing l-amino acids including l-phosphinothricin (l-PPT, a high-efficiency and broad-spectrum herbicide). However, the wild-type DAAO's low activity toward unnatural substrates like d-phosphinothricin (d-PPT) hampers its application. Herein, a DAAO from Caenorhabditis elegans (CeDAAO) was screened and engineered to improve the catalytic potential on d-PPT. First, we designed a novel growth selection system, taking into account the intricate relationship between the growth of Escherichia coli (E. coli) and the catalytic mechanism of DAAO. The developed system was used for high-throughput screening of gene libraries, resulting in the discovery of a variant (M6) with significantly increased catalytic activity against d-PPT. The variant displays different catalytic properties on substrates with varying hydrophobicity and hydrophilicity. Analysis using Alphafold2 modeling and molecular dynamic simulations showed that the reason for the enhanced activity was the substrate-binding pocket with enlarged size and suitable charge distribution. Further QM/MM calculations revealed that the crucial factor for enhancing activity lies in reducing the initial energy barrier of the reductive half reaction. Finally, a comprehensive binding-model index to predict the enhanced activity of DAAO toward d-PPT, and an enzymatic deracemization approach was developed, enabling the efficient synthesis of l-PPT with remarkable efficiency.

Dynamic and Static Regulation of Nicotinamide Adenine Dinucleotide Phosphate: Strategies, Challenges, and Future Directions in Metabolic Engineering

Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is a crucial cofactor in metabolic networks. The efficient regeneration of NADPH is one of the limiting factors for productivity in biotransformation processes. To date, many metabolic engineering tools and static regulation strategies have been developed to regulate NADPH regeneration. However, traditional static regulation methods often lead to the NADPH/NADP+ imbalance, causing disruptions in cell growth and production. These methods also fail to provide real-time monitoring of intracellular NADP(H) or NADPH/NADP+ levels. In recent years, various biosensors have been developed for the detection, monitoring, and dynamic regulate of the intracellular NADP(H) levels or the NADPH/NADP+ balance. These NADPH-related biosensors are mainly used in the cofactor engineering of bacteria, yeast, and mammalian cells. This review analyzes and summarizes the NADPH metabolic regulation strategies from both static and dynamic perspectives, highlighting current challenges and potential solutions, and discusses future directions for the advanced regulation of the NADPH/NADP+ balance.

Design of a cofactor self-sufficient whole-cell biocatalyst for enzymatic asymmetric reduction via engineered metabolic pathways and multi-enzyme cascade

NAD(P)H-dependent oxidoreductases are crucial biocatalysts for synthesizing chiral compounds. Yet, the industrial implementation of enzymatic redox reactions is often hampered by an insufficient supply of expensive nicotinamide cofactors. Here, a cofactor self-sufficient whole-cell biocatalyst was developed for the enzymatic asymmetric reduction of 2-oxo-4-[(hydroxy)(-methyl)phosphinyl] butyric acid (PPO) to L-phosphinothricin (L-PPT). The endogenous NADP+ pool was significantly enhanced by regulating Preiss-Handler pathway toward NAD(H) synthesis and, in the meantime, introducing NAD kinase to phosphorylate NAD(H) toward NADP+. The intracellular NADP(H) concentration displayed a 2.97-fold increase with the strategy compared with the wild-type strain. Furthermore, a recombinant multi-enzyme cascade biocatalytic system was constructed based on the Escherichia coli chassis. In order to balance multi-enzyme co-expression levels, the strategy of modulating rate-limiting enzyme PmGluDH by RBS strengths regulation successfully increased the catalytic efficiency of PPO conversion. Finally, the cofactor self-sufficient whole-cell biocatalyst effectively converted 300 mM PPO to L-PPT in 2 h without the need to add exogenous cofactors, resulting in a 2.3-fold increase in PPO conversion (%) from 43% to 100%, with a high space-time yield of 706.2 g L-1 d-1 and 99.9% ee. Overall, this work demonstrates a technological example for constructing a cofactor self-sufficient system for NADPH-dependent redox biocatalysis.

A combination fermentation strategy for simultaneously increasing cellular NADP(H) level, biomass, and enzymatic activity of glufosinate dehydrogenase in Escherichia coli

Oxidoreductase is one of the most important biocatalysts for the synthesis of various chiral compounds. However, their whole-cell activity is frequently affected by an insufficient supply of expensive nicotinamide cofactors. This study aimed to overcome such shortcomings by developing a combination fermentation strategy for simultaneously increasing intracellular NADP(H) level, biomass, and glufosinate dehydrogenase activity in E. coli. The results showed that the feeding mode of NAD(H) synthesis precursor and lactose inducer had essential effects on the accumulation level of intracellular NADPH. Adding 40 mg L^-1 of L-aspartic acid to the medium increased the intracellular NADP(H) concentration by 36.3%. Under the pH-stat feeding mode and adding 0.4 g L^-1 h^-1 lactose, the NADP(H) concentration, biomass, and GluDH activity in the 5-L fermenter reached 445.7 μmol L^-1, 21.7 gDCW L^-1, and 8569.3 U L^-1, respectively. As far as we know, this is the highest reported activity of GluDH in the fermentation broth. Finally, the 5000-L fermenter was successfully scaled up to use this fermentation approach. The combination fermentation strategy might serve as a useful approach for the high-activity fermentation of other NADPH-dependent oxidoreductases.

Autoinduction Expression Modules for Regulating Gene Expression in Bacillus subtilis

Although quorum sensing (QS) promoters that can autonomously activate gene expression have been identified and engineered in Bacillus subtilis, researchers focus on quantifying individual promoters while ignoring the interaction between other genetic regulatory elements. Here, we constructed the autoinduction expression modules consisting of promoters responsive to QS ComQXPA, ribosome binding sites (RBSs), and terminators. Using superfolder green fluorescent protein (sfGFP) as a reporter gene, three individual element libraries were generated from 945 promoters, 12,000 RBSs, and 425 terminators by random mutation, de novo design, and database mining strategies, respectively. Then, the efficiency of three libraries in regulating gene expression was further enhanced by engineering the core region of each optimal element. After hybridizing the element libraries, the generated expression modules exhibited a 627-fold range in regulating gene expression without significantly affecting the autoinduction initiation. Here, the hybrid modules with broad expression strength may benefit the application of QS-based autoinduction systems in B. subtilis.

Targeting the gut microbiota for cancer therapy

Growing evidence suggests that the gut microbiota modulates the efficacy and toxicity of cancer therapy, most notably immunotherapy and its immune-related adverse effects. The poor response to immunotherapy in patients treated with antibiotics supports this influential role of the microbiota. Until recently, results pertaining to the identification of the microbial species responsible for these effects were incongruent, and relatively few studies analysed the underlying mechanisms. A better understanding of the taxonomy of the species involved and of the mechanisms of action has since been achieved. Defined bacterial species have been shown to promote an improved response to immune-checkpoint inhibitors by producing different products or metabolites. However, a suppressive effect of Gram-negative bacteria may be dominant in some unresponsive patients. Machine learning approaches trained on the microbiota composition of patients can predict the ability of patients to respond to immunotherapy with some accuracy. Thus, interest in modulating the microbiota composition to improve patient responsiveness to therapy has been mounting. Clinical proof-of-concept studies have demonstrated that faecal microbiota transplantation or dietary interventions might be utilized clinically to improve the success rate of immunotherapy in patients with cancer. Here, we review recent advances and discuss emerging strategies for microbiota-based cancer therapies.

Expression of L-phosphinothricin synthesis enzymes in Pichia pastoris for synthesis of L-phosphinothricin

With the ban of highly toxic herbicides, such as paraquat and glyphosate, phosphinothricin (PPT) is becoming the most popular broad-spectrum and highly effective herbicide. The current PPT products in the market are usually a racemic mixture with two configurations, the D-type and L-type, of which only the L-PPT has the herbicidal activity. The racemic product is not atom economic, more toxic and may cause soil damage. Asymmetric synthesis of L-PPT has become a research focus in recent years, while biological synthesis methods are preferred for its character of environmental friendly and requiring less reaction steps when being compared to the chemical methods. We have developed a biological synthesis route to produce optically pure L-PPT from D,L-PPT in two steps using 2-carbonyl-4- (hydroxymethyl phosphonyl) butyric acid as the intermediate. In this study, we expressed the glutamate dehydrogenase and glucose dehydrogenase using Pichia pastoris as the first time. After a series of optimization, the total L-PPT yield reached 84%. The developed synthesis system showed a high potential for future industrial application. Compare to the previous plasmid-carrying-E. coli expression system, the established method may avoid antibiotic usage and provided an alternative way for industrial synthesis of optically pure L-PPT.

A light-controlled biocatalytic system for precise regulation of enzymatic decarboxylation

A light-controlled biocatalytic one-pot system is developed, which enables precise regulation of gene expression and photocatalysis by illumination and yields high conversion and stereoselectivity.