The development of environmentally benign, biodegradable materials is considered an important way to address “white pollution”. Importantly, organic acid is one of the crucial monomers for preparing biodegradable materials. In recent years, the synthesis of organic acids through green and efficient methods has attracted much attention. As the most promising carbon source recognized for renewability and affordability, lignocellulose is considered a promising carbon source for the biochemical industry. Converting lignocellulose into organic acid is critical to preparing biodegradable materials and achieving carbon neutrality, which meets the requirements of the green and sustainable development strategy. Hence, researchers are focusing their investigations on the lignocellulose biorefinery. To date, innovations in synthetic biology have significantly advanced organic acid manufacturing. For example, the yield of succinic acid has exceeded 150 g/L, which facilitates the formation and development of the bio-based biodegradable materials industry. In this paper, various lignocellulose pretreatment technologies were reviewed, including physical, chemical, biological, physicochemical, and other emerging pretreatment methods. To realize the goal of efficient utilization of lignocellulose, the refining processes of lignocellulose were also reviewed, including detoxification of inhibitors, reductive catalytic fractionation, consolidated bioprocessing, and other methods. After the pretreatment and refining process, lignocellulose is transformed to sugars and aromatic compounds, which can be utilized for producing various organic acid compounds, such as succinic acid, 3-hydroxypropionic acid, cis,cis-muconic acid, 2,5-furandicarboxylic acid, 2-pyrone-4,6-dicarboxylic acid. Next, using the optimization of production of these organic acid compounds as examples, several synthetic biology strategies were summarized, including constructing biosynthetic pathways, optimizing regulatory elements, enlarging the substrates spectrum, and other strategies for improving cell production capacity. Finally, the development trends of the biodegradable materials industry are summarized and prospected. The development of emerging pretreatment and consolidated bioprocessing to facilitate the efficiency of lignocellulose utilization were discussed. Improving the robustness of microbial cell factories and designing the systematic lignocellulose conversion pathways could further optimize the performances of organic acid synthesis. The insights given in this review could facilitate further development on the industrial production of biodegradable materials, towards addressing the global energy crisis and “white pollution”.

Lignin is a major component of lignocellulose, accounting for 15%-30% on a dry weight basis, with an annual yield estimated to be 20 billion tonnes. Lignin is a heterogenous aromatic polymer of phenylpropanoids linked by various C—C and C—O bonds. It is an integral component of the secondary cell wall from terrestrial plants, providing plants with rigidness and fending off microbial pathogens. The abundance and renewability of lignin has recently attracted ample interest in valorizing this readily available polymer. However, the complex nature of lignin presents a significant challenge for lignin breakdown and utilization, and at present the majority of lignin is simply burned as a fuel. Among the different methods, biological utilization of lignin has emerged as a highly attractive approach, since it proceeds under mild conditions and is generally considered environmentally friendly, especially considering that environmental sustainability is trending worldwide. This review comprises three major sections. First, we will summarize key enzymes that nature has created to break down lignin, including laccase, manganese peroxidase, lignin peroxidase, dye-decolorizing peroxidase, and versatile peroxidase etc. Relevant enzymes and their catalytic mechanisms will also be briefly discussed. Second, we will review key reactions in priming and processing lignin derived aromatics before they enter microbial metabolic pathways: O-demethylation, hydroxylation, decarboxylation, and ring opening, as well as representative enzymes involved and their catalytic mechanisms. Finally, we will present engineering efforts toward biological valorization of lignin and lignin derived aromatics, which is largely driven by synthetic biology approaches. Biological valorization of lignin is undoubtedly a field full of potential, however its realization still faces several major hurdles, such as low conversion efficiency and long processing time. Nevertheless, as synthetic biology is developing rapidly, harnessing the power of genetic and metabolic engineering to improve the efficiency of lignin breakdown and utilization, microbial tolerance to toxic aromatics, and redox balance will certainly be a promising path forward, paving the way for industrial application in the near future.

Huge challenges, such as food security, energy security, climate change, dual-carbon target, and so on, motivate human society to seek disruptive and innovative solutions. In vitro biotransformation (ivBT), bridging the gap between whole-cell-based fermentation and enzyme-based biocatalysis, is an emerging biomanufacturing platform designed for the production of biocommodities (e.g., synthetic starch, healthy sweeteners, organic acids, etc.) and bioenergy. In ivBT, in vitro synthetic enzymatic biosystem (ivSEB) is its high-efficiency biocatalyst. Based on the Chinese philosophy that “Tao is simple”, ivSEB is the in vitro reconstruction of artificial (non-natural) enzymatic pathways with a number of natural enzymes, artificial enzymes, and/or (biomimetic or natural) coenzymes, and/or artificial membrane, without living cell’s constraints, such as cell duplication, bioenergetics, basic metabolisms, regulation, and so on. ivBT enables it to surpass the limitations of whole-cell fermentation and has multiple advantages, such as theoretical product yield, at least 10-time volumetric productivity, tolerance to toxic substrate/product, and so on. This review defines the concept of ivBT, presents its design principles, distinguishes it from other seemingly-like concepts, such as cell-free protein synthesis and cascade enzyme biocatalysis, introduces several representative examples, and discusses its challenges and opportunities. The development of ivBT is based on the linear strategy of “Design-Build-GoNG-Optimization”, leading to super-biomanufacturing machines that can meet national needs, such as food security and new energy system. To address food security, we propose two out-of-the-box solutions: (1) in vitro biotransformation of cellulose to starch, possibly increasing the starch supply by a factor of 10; (2) artificial starch synthesis from CO₂ by combining ivBT and chemical catalysis. Furthermore, the revolutionary production of starch could open a door to the starch-based carbohydrate economy, wherein starch is a high-density hydrogen carrier, more than 2.5 times that of compressed hydrogen, and an ultra-high electricity storage compound, more than 10 times of lithium-ion battery. In a word, ivBT featuring ultra-high energy efficiency and potentially-low-cost production could become a third industrial biomanufacturing platform and help solve huge challenges.

Phosphinothricin (PPT) is one of the top three herbicides, known for its broad spectrum, high herbicidal activity, and non-selectivity, with a highly optimistic market prospect. However, PPT exists in two enantiomers (D-PPT and L-PPT), with the herbicidal activity primarily stemming from L-PPT. Therefore, efficient synthesis of L-PPT with high optical purity is crucial. Pesticide manufacturing enterprises have attempted to develop chemical synthesis methods for L-PPT using approaches such as racemic compound splitting, asymmetric synthesis, natural amino acid chiral source method, and chiral auxiliary induction. However, due to challenges such as low stereo-selectivity, low product yield, and high production costs, large-scale production has not been achieved. Under the guidance of Academician Yin-Chu Shen, the “Father of Biopesticides in China”, our research group has conducted scientific research and industrial practice on the biosynthesis of L-PPT for over 20 years. In cooperation with multiple enterprises, we have developed more than ten process routes and technologies. Among them, five routes (racemic mixture derivatization-resolution route, racemic PPT-chiral separation route, generic compound cyanation followed by hydrolysis route, de novo synthesis from common chemicals route, and synthesis of homoserine followed by chemical synthesis) are discussed in detail in this review. Each route’s reconstruction, establishment of bioinorganic amine technology, creation of biocatalysts, high-density fermentation for enzyme production, product separation and purification, and reaction equipment are included. Notably, we developed the BioHPP^®, a biomanufacturing technology for the synthesis of highly optically pure L-PPT. Based on this technology, a ten-thousand-ton digital and intelligent production line for L-PPT was established. Utilizing smart sensors and actuators, real-time data collection, transmission, analysis, and feedback adjustment were achieved at over a thousand control points. This led to fully automated parameter collection and control, increasing production efficiency by 50% and reducing labor intensity by more than 70%, thereby realizing the bio-intelligent manufacturing of ten thousand tons of L-PPT. Based on the long-term accumulation of our research efforts, we summarize and analyze the mainstream production processes of D,L-PPT, detailing on the principles and methods of biomanufacturing technology and synthetic biology to construct the key synthesis system for L-PPT. We also compare the characteristics and key points of industrialization implementation of these routes in terms of substrate synthesis and selection, types of biocatalysts, use of amino donors, and separation and purification. It can be foreseeable that, with the aid of synthetic biology technology, an increasing number of high-optical-purity chiral pesticides will be produced on a large scale through biomanufacturing in the future.

Increasing petroleum consumption and growing environmental concerns necessitate the sustainable production of chemicals and fuels from renewable resources. By utilizing renewable resources as raw materials and engineered microorganisms as the core tools, the bio-manufacturing of bio-based materials has become a hot research topic due to its green and low-carbon advantages. ω-Amino acids are a type of non-natural amino acids with amino and carboxyl groups located at the ends of the straight carbon chain. Self-cyclization of ω-amino acids produce lactams, which are the key monomers for the synthesis of polyamide materials, commonly known as nylon. Polyamide materials have wide applications and a huge global market over seven million tons per year. Nowadays, polyamide materials and their monomers are primarily produced through petrochemical routes with non-renewable resources. The research on biosynthesis of these materials and monomers is still in the early stages, but significant progress has been made in recent years. This review article systematically introduces the recent advances in the biosynthesis of ω-amino acids and lactams. To achieve the bio-manufacturing of bio-based polyamide materials, researchers have designed artificial biosynthetic pathways for ω-amino acids from renewable carbon sources such as glucose. The key enzymes for the cyclization of ω-amino acids to form lactams have been identified. By assembling the biosynthetic pathway in microbial chassis such as Escherichia coli and Corynebacterium glutamicum, production of ω-amino acids and lactams have been achieved. Furthermore, the metabolic flux was fine-tuned by regulating and optimizing the expression of key genes to improve the biosynthesis of ω-amino acids and lactams. Besides, biosensors of lactams have been developed to transfer the intracellular concentrations of lactams into easily detectable signals such as fluorescence. Such biosensors have been successfully used for high-throughput screening of ω-amino acid cyclization enzymes and dynamic regulation of biosynthetic pathway. These effects have resulted in the successful biosynthesis of C₄-C₆ ω-amino acids and lactams. Particularly, using glucose as a raw material, the production of valerolactam by fed-batch fermentation exceeded 70 g/L, with a productivity of about 1 g/(L·h), which approaches the level required for industrialization and commercialization. Finally, the review article discusses the current challenges faced in the biosynthesis of ω-amino acids and lactams, including the low yield of biosynthetic pathways, rate-limitations posed by key cyclization enzymes, and insufficient utilization of non-food carbon sources such as one-carbon compounds.

Production of chemicals using renewable bioresources and green biomanufacturing processes is highly important for sustainable bioeconomy. Diols are important bulk chemicals widely used in the production of polymers, cosmetics, fuels, food, and pharmaceutical industries due to their versatile functional properties. Currently, most of diols are produced mainly from fossil resources via energy-cost chemical approaches. The development of biosynthetic routes for the production of diols from renewable resources such as biomass and C₁ has garnered significant attention due to its potential in reducing the utilization of fossil resources and carbon dioxide emissions. Although biological production of 1,3-propanediol, 1,3-butanediol and 1,4-butanediol has been commercialized, the biosynthesis of other major diols remains challenging due to the absence of efficient natural biosynthetic pathways and low efficiency of the recombinant microbes. Recent development of metabolic engineering and synthetic biology enables the production of non-natural chemicals via artificial metabolic pathways and novel biological parts, significantly expanding the boundary of biomanufacturing. This review comprehensively explores recent advances in the microbial synthesis of diols, emphasizing the development of new pathways and engineering strategies for the biosynthesis of C₂ to C₅ diols. Especially, we focus on the innovative approaches include constructing non-natural synthetic pathways to achieve the biosynthesis of non-natural diols, or using alternative carbon sources such as lignocellulose through specific metabolic pathways to synthesize diols. Furthermore, this review also discusses the primary challenges and future perspectives in transforming these biosynthetic processes toward industrial applications. Key challenges involve the accessibility of low-cost and sustainable raw materials, the complexities in scaling up these processes, the development of extraction techniques that cater to specific downstream requirements, and the economic assessment of these processes to ensure profitability and sustainability. These advancements are essential for the economic and environmental viability of producing diols from renewable resources, thereby facilitating the transition to more sustainable industrial practices globally.

Hydrogenases are the most important enzymes in biological hydrogen production and hydrogen energy utilization. They are widely distributed, oxygen-sensitive, multiunit complexed metal enzymes. In vitro synthetic enzymatic biosystems (ivSEB) is a type of in vitro biotransformation (ivBT) technology, which is an emerging biomanufacturing powerhouse that combines microbial fermentation with enzymatic biocatalysis, allowing for novel and efficient hydrogen production, also breaking the Thauer limit and achieving a yield of hydrogen close to the theoretical value of chemistry (1 mole of glucose to produce 12 moles of hydrogen in maximum). It represents the future direction of biological hydrogen production. However, the recombinant expression of hydrogenase is the main bottleneck limiting the wide application of ivSEB for hydrogen production technology. Hydrogenases are widely distributed in all life domains, but are oxygen-sensitive and mostly consist of metalloproteins with multi-subunits, bearing [Fe] only, [NiFe] or [FeFe] dinuclear core in their catalytic center. Oxygen not only inhibits the activity of hydrogenase, but also affects the transcription of the enzyme-encoding gene and post-translational process of the enzymes. As a result, the levels of recombinant hydrogenase are usually low and the enzymatic activities are also incomparable to the native enzymes, often leading to high production costs due to the strict anaerobic purification procedures. In order to meet the requirements of industrial hydrogen production, hydrogenases must possess excellent catalytic properties, such as a high catalytic turnover number, great thermal stability, and the ability to tolerate trace amounts of oxygen. This review summarizes the studies on the structural and catalytic characterizations of hydrogenases, including their classification, oxygen resistance mechanisms, and progress in recombinant expression. Additionally, the evolution of natural electron transfer chains and the design of artificial routes, which can improve hydrogen production efficiency and reduce costs, are briefly discussed. The review also discussed the progress in the studies on the mechanisms of hydrogenases’ tolerance toward oxygen, the strategies for microbial expression of recombinant hydrogenases as well as the optimization of the artificial electron transfer chains adapted for the production of hydrogen using ivSEB, in expectations of promoting the applications of hydrogenases involved ivSEB, from renewable energy storage, anaerobic artificial respiration, to clean hydrogenation or dehydrogenation in biocatalysis.

Chiral amino acids represent a crucial class of chiral building blocks with significant value in food, medicine, chemical industry, and agriculture. The market scale of pharmaceuticals, pesticides, food, and chemical industries relying on chiral amino acids is substantial and has been attracting increasing attention. The pursuit of efficient, environmentally friendly, and cost-effective synthesis of chiral amino acids has long been a goal for scientists. Commonly used preparation methods for chiral amino acids fall into four following categories: protein hydrolysis, fermentation, chemical synthesis, and enzyme-catalyzed synthesis. Among these, enzyme-catalyzed synthesis has demonstrated great potential due to its mild reaction conditions, high stereo-selectivity, simplicity of steps, and wide application range. In recent years, with the rapid development of bioinformatics, protein engineering, and computational biology, there has been an increasing number of high-performance enzyme preparations developed, leading to a steady increase in the diversity of enzymes and the gradual diversification of catalyzed reactions, further promoting the wide application of enzyme-catalyzed synthesis of chiral amino acids. The enzyme-catalyzed synthesis of chiral amino acids can be categorized into three groups: asymmetric synthesis, deracemization synthesis, and kinetic resolution. Kinetic resolution, due to its theoretical yield of only 50% and low atom economy, is not suitable for industrial applications. In contrast, asymmetric synthesis and deracemization synthesis with theoretical yield of 100% find wider industrial application. This article reviews the application of enzymatic asymmetric synthesis and deracemization synthesis in the synthesis of chiral amino acids. It includes the development and modification of key enzyme such as amino acid dehydrogenase, transaminase, ammonia lyase, aldolase, amino acid oxidase, and amino acid deaminase, as well as their application in the synthesis of high-value chiral amino acids such as phosphinothricin, tert-leucine, and intermediate of sitagliptin. Additionally, it summarizes the main challenges faced in the field of enzymatic synthesis of chiral amino acids, such as the lack of key enzyme components, and low enantioselectivity, narrow substrate spectra, low catalytic activity, poor stability, limited reaction conditions of wild-type enzymes. Finally, it looks ahead to the application of cutting-edge technologies such as automated experimental devices, machine learning, and artificial intelligence in the field of enzyme modification, as well as the development of more efficient and environmentally friendly catalytic processes through reactor design and reaction process control. These endeavors collectively aim to facilitate the broader industrial application of enzymatic synthesis for chiral amino acids.

Biomimetic compartmentalization immobilization of multi-enzyme system is a frontier for in vitro synthetic biology, focusing on the spatial and temporal separation of reactions. Compared with simple co-immobilization, biomimetic compartmentalization immobilization can form substrate channels and promote the transmission of intermediates for sequential or coupling reaction. By controlling the relative positions of the enzymes on carriers, this method improves system stability, productivity, as well as purity of product. In this review, we summarized the recent advances of carriers for biomimetic compartmentalization immobilization of multi-enzyme systems, including metal-organic frameworks (MOFs), polymer vesicles and polymer capsules. Metal-organic frameworks (MOFs) are porous coordination materials which are composed of metal ions as nodes and organic linkers. MOFs possess unique characteristics including high porosity, large specific surface area and tunable structure, which are suitable for multi-enzyme systems. The strategies involving the hierarchically porous MOFs, MOF-on-MOF and multi-MOF combinations construct compartmentalized environments for efficient catalytic reactions in vitro. Polymer vesicles are hollow nanostructures composed of amphiphilic block copolymers. The membrane structure of polymer vesicles, similar to the natural phospholipid bilayers, has good mechanical stability and biocompatibility for protecting enzyme molecules, and provides unique microenvironment for sequential reactions. Multiple small vesicles were encapsulated into the larger vesicles to form a “vesicle-in-vesicle” by mimicking the structure of cellular organelles. Polymer capsules with a core-shell spherical nanostructure are formed by the templating method, and have structural stability and excellent shape controllability. Multilayered core-shell structures created by layer-by-layer self-assembly are applied for compartmentalized immobilization of multi-enzyme. In the future, the integration of microfluidic technologies with biomimetic compartmentalization immobilization of multi-enzyme is expected to provide highly efficient and stable multi-enzyme catalytic systems for in vitro synthetic biology and green biomanufacturing.

One-carbon compounds are liquid or gaseous substances that can be naturally occurring or produced in industrial processes, offering the advantages of being abundant, cost-effective, and sustainable to produce. They are anticipated to serve as fundamental raw materials for the next phase of bio-manufacturing, encompassing easily transportable and storable liquid methanol, formic acid, and gaseous CO₂, CO, and CH₄. China is currently focusing on reducing carbon emissions and aims to progressively achieve the targets of carbon peak and carbon neutrality through diverse approaches. Amidst the flourishing landscape of bio-manufacturing, microorganisms are being genetically manipulated using synthetic biology techniques to efficiently harness one-carbon compounds for the creation of high-value products like lipids and single-cell protein. This initiative aims to reduce dependence on imported food and fossil resources, serving as a strategic measure to alleviate food and energy crises. This review presents a comprehensive overview of the most recent advancements in converting one-carbon compounds into valuable oils and single-cell proteins through the utilization of metabolic pathways, chassis genetic modification, and other methodologies involving methylotrophic microorganisms, acetogenic bacteria, yeast, and other microorganisms. It discusses pertinent studies on enhancing molecularly engineered strains through the fermentation process using one-carbon compounds and includes research cases focusing on the production of ultra-long-chain fatty acids. Furthermore, it collates industrial instances related to the conversion of one-carbon compounds from research institutions or companies. Lastly, by addressing the constraints in metabolic pathway design and genetic tools for utilizing one-carbon compound strains, as well as the energy conversion challenges between acetogenic bacteria and lipids-producing microorganisms, it offers foresight into the future opportunities and obstacles encountered in the bio-manufacturing of lipids and single-cell proteins. It suggests advancing inter-disciplinary, efficient systematic integration for fermentation within complex systemic bio-manufacturing processes, driving exploration on the biological conversion of one-carbon compounds, proposing novel solutions to current theoretical and practical challenges, and providing guidance for practical applications and industrial advancements.