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    Research advances in biosynthesis of natural product drugs within the past decade
    FENG Jin, PAN Haixue, TANG Gongli
    Synthetic Biology Journal    2024, 5 (3): 408-446.   DOI: 10.12211/2096-8280.2023-092
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    Natural products have long been considered as an important source for potential drugs. In history, natural products and their structural analogs have contributed substantially to the treatment of various diseases, especially cancers and infectious diseases. After a long history of applications, people have gradually begun to explore active ingredients in natural products that truly exert therapeutic effects, and discovered a series of functional compounds, such as morphine, quinine, ephedrine, etc. Over the past two hundred years, the discovery and research of natural products has undergone tremendous changes, from traditional identification and isolation methods to multidisciplinary approaches in the modern genomic era. Strategies for discovering natural products and tools for their prediction have been developed continuously. Although many novel and active natural products have been mined and discovered in the past two decades, considering the huge reserve of natural products in nature, a large number of genes or gene clusters encoding key enzymes for the biosynthesis of natural products have not yet been characterized, and both terrestrial and marine natural product resources are to be explored. Compared with traditional chemically synthesized molecules, natural products possess diverse skeletons for structural complexity, which have shown remarkable advantages in the discovery of new drugs. While there are still many challenges in discovering new drugs from natural products, such as the effective mining of molecules with new structural features, identification and isolation of functional natural products with trace abundance, derivatization of natural product analogs for exploring connections between their structures and activities, and the complete synthesis of complicated active natural products at large scales, etc., the emergence of novel analytical technologies and mining strategies is expected to substantially renovate natural product discovery. This review comments on the natural product drugs and semisynthetic drugs derived from natural products approved by the U.S. Food and Drug Administration within the past decade from January 2014 to October 2023, and provides an overview on the research progress on the biosynthesis of these natural products and their precursors. In addition, important progress in the biosynthesis of some drugs approved by FDA before is also briefly summarized. An in-depth understanding of the biosynthetic pathways and mechanisms underlying their efficacy is expected to provide valuable insights for the discovery and research of more new drugs in the future.

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    Enzyme engineering in the age of artificial intelligence
    KANG Liqi, TAN Pan, HONG Liang
    Synthetic Biology Journal    2023, 4 (3): 524-534.   DOI: 10.12211/2096-8280.2023-009
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    Enzymes have garnered significant attention in both research and industry due to their unparalleled specificity and functionality, and thus opportunities remain for enhancing their physichemical properties and fitness to improve catalytic performance. The primary objective of enzyme engineering is to optimize the fitness of targeted enzymes through various strategies for their modifications, even redesigning. This review provides a comprehensive overview for progress made in enzyme engineering, with a focus on artificial intelligence (AI)-guided design methodology. Several key strategies have been employed in enzyme engineering, including rational design, directed evolution, semi-rational design, and AI-guided design. Rational design relies on an extensive knowledge based on encompassing protein structures and catalytic mechanisms, allowing for purposeful manipulations of enzyme properties. Directed evolution, on the other hand, involves the generation of a library of random variants for subsequent high-throughput screening to identify beneficial mutations. Semi-rational design combines rational design and directed evolution, resulting in a smaller, yet more targeted, library of variants, which mitigates high cost associated with extensive screening of large libraries developed through directed evolution. In recent years, AI technologies, particularly deep neural networks, have emerged as a promising approach for enzyme engineering, and AI-guided methods leverage a vast amount of information regarding protein sequences, multiple sequence alignments, and protein structures to learn key features for correlations. These learned features can then be applied to various downstream tasks in enzyme engineering, such as predicting mutations with beneficial effect, optimizing protein stability, and enhancing catalytic activity. Herewith, we delves into advancements and successes in each of these strategies for enzyme engineering, highlighting the growing impact of AI-guided design on the process. By offering a detailed examination of the current state of enzyme engineering, we aim at providing valuable insight for researchers and engineers to further advance the development and optimization of enzymes for more applications.

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    Recent advances in photoenzymatic synthesis
    MING Yang, CHEN Bin, HUANG Xiaoqiang
    Synthetic Biology Journal    2023, 4 (4): 651-675.   DOI: 10.12211/2096-8280.2022-056
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    Biocatalysis has the advantages in terms of sustainability, efficiency, selectivities and evolvability, thus it plays a more and more important role in green and sustainable synthesis, both in industrial production and academic research. However, compared with the well-known privileged chemocatalysts, enzymes suffer from the relatively limited types of reactions it can catalyze, which is unable to meet the future needs of green biomanufacturing. On the other hand, photocatalysis has emerged as one of the most effective strategies for the generation of reactive chemical intermediates under mild conditions, thereby providing a fertile testing ground for inventing new chemistry. However, the light-generated organic intermediates, including radicals, radical ions, ions, as well as excited states, are highly reactive resulting in the difficulties of controlling the chemo- and stereo-selectivities. The integration of biocatalysis and photocatalysis created a cross-disciplinary area, namely photoenzymatic catalysis, which can not only provide a new solution to stereochemical control of photochemical transformations with the exquisite and tunable active site of enzymes, but also open a new avenue to expand the reactivity of enzymes with visible-light-excitation. In addition, photoenzymatic catalysis inherits the inherent advantages of biocatalysis and photocatalysis, such as mild reaction conditions, representing green and sustainable synthetic methods. We have witnessed the booming development of photoenzymatic catalysis during the past several years. In this review paper, the recent advances in this field are highlighted. According to the cooperative modes of photocatalysis and enzymes, this paper is divided into following four parts: photoredox-enabled cofactor regeneration systems, cascade/cooperative reactions combining enzymes with photocatalysts, unnatural transformations with photoactivable oxidoreductase, and artificial photoenzymes. In this paper, we summarize the representative works and emphasize on the catalytic mechanisms of photoenzymatic transformations as well as the strategies for realizing abiological transformations. At the end of this review, by analyzing the challenges of photoenzymatic synthesis, the future directions are prospected. We hope that this review can inspire the discovery of more novel photoenzymatic systems and ultimately spur the applications of photoenzymes in industrial productions of high value-added enantiopure chiral products.

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    Microbiome-based biosynthetic gene cluster data mining techniques and application potentials
    LAI Qilong, YAO Shuai, ZHA Yuguo, BAI Hong, NING Kang
    Synthetic Biology Journal    2023, 4 (3): 611-627.   DOI: 10.12211/2096-8280.2022-075
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    Biosynthetic gene cluster (BGC) is an important type of gene set, which is commonly found in the genomes of various organisms, and plays important metabolic and regulatory roles. In terms of linear gene structure, the set of genes in a BGC is usually located in close proximity to each other in the genome, but for functions, genes in a BGC usually work synergistically and are responsible for a class of pathways that generate specific small molecules. Therefore, BGCs are vital in synthetic biology research as a highly promising source for elements. However, current BGC databases and analytical platforms are limited by the number and types of experimentally validated BGCs, as well as by the preliminary BGC data mining techniques. The establishment of data-driven systematic discovery of BGCs and their validation, as well as translational studies, are of great value in both fundamental research and practical applications. This article focuses on mining BGCs from big data with microbiome for synthetic biology research. We start with discussing the definition and significance of BGC mining, and summarize current data resources and methods for BGC mining: including MIBiG, antiSMASH and IMG-ABC for artificial intelligence (AI) enabled web services to accelerate BGC mining. Then, we compile a walk-through on how a typical BGC data mining could be conducted, with the history of BGC mining methods highlighted, which underlines the route build-up from traditional machine learning to deep learning. We also diagnose bottlenecks in BGC mining, and propose possible solutions. Furthermore, according to several BGC mining and validation experiments, we demonstrate the profound diversity and breadth of application scenarios with BGC discovery, as well as the importance of combining dry and wet lab experiments for validating newly discovered BGCs. Finally, we envision that the combination of advanced BGC mining methods and synthetic biology could broaden and deepen current synthetic biology research.

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    Design of synthetic biology components based on artificial intelligence and computational biology
    WANG Sheng, WANG Zechen, CHEN Weihua, CHEN Ke, PENG Xiangda, OU Fafen, ZHENG Liangzhen, SUN Jinyuan, SHEN Tao, ZHAO Guoping
    Synthetic Biology Journal    2023, 4 (3): 422-443.   DOI: 10.12211/2096-8280.2023-004
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    The primary objective of synthetic biology is to conceptualize, engineer, and construct novel biological components, devices, and systems based on established principles and extant information or to reconfigure existing natural biological systems. The core concept of synthetic biology encompasses the design, modification, reconstruction, or fabrication of biological components, reaction systems, metabolic pathways and processes, and even the creation of cells and organisms with functions or living characteristics. This burgeoning field offers innovative technologies to address challenges with sustainable development in environment, resource, energy, and so on. Undeniably, synthetic biology has yielded significant progress in numerous fields, ranging from DNA recombination to gene circuit design, yet its full potential remains insufficiently explored, but the emergence and application of artificial intelligence (AI) definitely can facilitate the development of synthetic biology for more applications. From a synthetic biology perspective, essence for life is rooted in digitalization and designability. This article reviews current advances in computational biology, particularly AI for synthetic biology to be more efficient and effective, focusing on the development of biocatalysts, regulators, and sensors. De novo enzyme design has been successfully implemented by using Rosetta software, as AI exhibiting significant potential for generating innovative structures and protein sequences with diverse functions. Also, the reprogramming of natural enzymes for specific purposes is crucial for synthetic biology applications. By employing various force fields and sampling techniques, promiscuity and thermal stability can be modified to accommodate specific requirements rather than those with natural hosts. AI can be integrated into the life-cycle of synthetic biology through an active learning paradigm, which enables alterations in enzyme specificity, and demonstrates potential for accurately and rapidly predicting mutation effects, surpassing force-field-based methods. The rapidly decreasing cost of sequencing has facilitated the characterization of cis-regulators, primarily DNA and RNA, with high-throughput. Concurrently, more trans-regulators have been identified in sequenced genomes. The expanding wealth in big data serves as a driving force for AI. AI models have successfully predicted the strength of promoters, ribosome binding sites (RBSs), and enhancers, and generated artificial protomers and RBSs. Recent progress in RNA structure prediction is expected to aid the design of RNA elements. Sensors, vital for genetic circuits and other applications such as toxin detection, typically involve interactions among various molecules, including nucleic acids, proteins, small organic molecules, and metal ions. Consequently, sensor design necessitates the integration of diverse computational biology tools to balance accuracy and computational cost. As the pool of data keeps growing, we anticipate that AI will be increasingly applied to the design of more bio-parts.

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    Computational design and directed evolution strategies for optimizing protein stability
    RUAN Qingyun, HUANG Xin, MENG Zijun, QUAN Shu
    Synthetic Biology Journal    2023, 4 (1): 5-29.   DOI: 10.12211/2096-8280.2022-038
    Abstract4409)   HTML477)    PDF(pc) (2169KB)(7835)       Save

    Most natural proteins tend to be marginally stable, which allows them to gain flexibility for biological functions. However, marginal stability is often associated with protein misfolding and aggregation under stress conditions, presenting a challenge for protein research and applications such as proteins as biocatalysts and therapeutic agents. In addition, protein instability has been increasingly recognized as one of the major factors causing human diseases. For example, the formation of toxic protein aggregates is the hallmark of many neurodegenerative diseases, including Alzheimer's and Parkinson's diseases. Therefore, optimizing protein folding and maintaining protein homeostasis in cells are long-standing goals for the scientific community. Confronting these challenges, various methods have been developed to stabilize proteins. In this review, we classify and summarize various techniques for engineering protein stability, with a focus on strategies for optimizing protein sequences or cellular folding environments. We first outline the principles of protein folding, and describe factors that affect protein stability. Then, we describe two main approaches for protein stability engineering, namely, computational design and directed evolution. Computational design can be further classified into structure-based, phylogeny-based, folding energy calculation-based and artificial intelligence-assisted methods. We present the principles of several methods under each category, and also introduce easily accessible web-based tools. For directed evolution approaches, we focus on library-based, high-throughput screening or selection techniques, including cellular or cell-free display and stability biosensors, which link protein stability to easily detectable phenotypes. We not only introduce the applications of these techniques in protein sequence optimization, but also highlight their roles in identifying novel folding factors, including molecular chaperones, chemical chaperones, and inhibitors of protein aggregation. Moreover, we demonstrate the applications of protein stability engineering in biomedicine and pharmacotherapeutics, including identifying small molecules to stabilize disease-related, aggregation-prone proteins, obtaining conformation-fixed and stable antigens for vaccine development, and targeting protein stability as a means to control protein homeostasis. Finally, we look forward to the trends and prospects of protein stabilization technologies, and believe that protein stability engineering will lead to a better understanding of protein folding processes to facilitate the development of precision medicine.

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    Biological degradation and utilization of lignin
    LIU Kuanqing, ZHANG Yi-Heng P.Job
    Synthetic Biology Journal    2024, 5 (6): 1264-1278.   DOI: 10.12211/2096-8280.2023-062
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    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.

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    Research progress on recombinant collagen expression system
    PAN Jiahao, PAN Weisong, QIU Jian, XIE Donling, ZOU Qi, WU Chuan
    Synthetic Biology Journal    2023, 4 (4): 808-823.   DOI: 10.12211/2096-8280.2023-020
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    Collagen is the most abundant protein in mammals, and its production has been widely used in biomedicine, cosmetics, leather, biotechnology, etc. At present, collagen is generally divided into animal collagen and recombinant collagen. Although animal collagen is the main source of collagen, most of it comes from animal carcasses, and its collagen has been cross-linked and embedded in native tissues, which is more demanding on extraction and purification technology. In addition, pathogen contamination and allergy risks have become unavoidable problems for animal collagen. Recombinant collagen is a protein obtained by using human collagen cDNA fragments as the backbone gene, cloning the gene to the selected expression vector and converting it into an expression cell, and finally achieved by purification technology. Due to its single molecule, clear structure and easy control, recombinant collagen is the best alternative to replace animal collagen in biomedicine and tissue engineering. In this paper, the structure, category, biosynthesis mechanism and market scale of collagen are briefly described. Emphasis is placed on the construction strategies, advantages and limitations of different expression systems of recombinant collagen, including prokaryotic, yeast, plant, baculovirus and mammalian or human cell expression systems. Prokaryotes and yeast have a short cycle of producing recombinant collagen, but do not have a triple helix structure. The plant expression system produces recombinant collagen with a moderate cycle and a certain triple helix structure. The baculovirus-insect expression system and the mammalian expression system have a long cycle of recombinant collagen production and a complete triple helix structure. The practical application of recombinant collagen in ophthalmology, cartilage engineering, skin treatment and other biological medicine is described. Currently, the most commercially valuable use of collagen is subcutaneous injection of soluble protein to repair damaged skin. At the same time, collagen, as the main component of animal skin, can cross-link collagen in raw hides through chemical processes such as tanning, so that collagen becomes harder, more durable, and corrosion-resistant leather. By designing collagen scaffolds that are familiar with the natural cytoplasmic matrix environment, it can effectively reveal the pathogenesis of cell behavior and disease etiology. It is expected to provide suggestions on the research of recombinant collagen and future industrial development.

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    Research advances on paclitaxel biosynthesis
    LIU Xiaonan, LI Jing, ZHU Xiaoxi, XU Zishuo, QI Jian, JIANG Huifeng
    Synthetic Biology Journal    2024, 5 (3): 527-547.   DOI: 10.12211/2096-8280.2023-085
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    Paclitaxel (Taxol) is a natural broad-spectrum anticancer drug, which is well-known for its potent anticancer activity. Its production mainly relies on the extraction and purification from the rare Taxus plant, followed by chemical semi-synthesis. The limited natural resource for paclitaxel imposes a significant constraint on its production capacity. In recent years, with the complete decoding of the Taxus genome and the rapid development of synthetic biology, constructing recombinant cells through synthetic biology techniques has emerged as an effective method to address this challenge. Since paclitaxel biosynthesis involves more than 20 steps of complicated enzymatic reactions and about half of them are P450 enzyme-mediated hydroxylation reactions, the complete elucidation of its biosynthetic pathway remains elusive. Meanwhile, the production of paclitaxel by engineered microbes is still at the initial stage, and there are numerous by-products, which seriously compromise the efficient synthesis of paclitaxel. Therefore, this article reviews research progress related to paclitaxel synthesis pathways, Taxus omics analyses, construction of chassis cells, synthesis of key precursors, modifications of crucial enzymes, and catalytic mechanisms underlying paclitaxel biosynthesis. Special attention is given to the recent breakthrough in elucidating the formation of oxetane ring and the discovery of Taxane 1-β- and 9-α-hydroxylases. Recent advances in the study of the catalytic mechanism of Taxadiene-5-α-hydroxylase and significant progress in engineering tobacco and yeast chassis will also be commented. Furthermore, challenges and future prospects involved in the paclitaxel synthetic biology research are discussed, such as the issues of low enzyme catalytic efficiency, significant product promiscuity, unknown specific reaction sequences, and the biosynthesis of critical paclitaxel intermediates, aiming to enhance the understandings of paclitaxel biosynthetic pathways and catalytic mechanisms for greener and more efficient production of paclitaxel.

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    Recent research progress in non-canonical biosynthesis of terpenoids
    CHENG Xiaolei, LIU Tiangang, TAO Hui
    Synthetic Biology Journal    2024, 5 (5): 1050-1071.   DOI: 10.12211/2096-8280.2024-006
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    Terpenoids are a class of natural products with important physiological functions and significant biological activities that are widely found in nature and have a wide range of applications in the food, medical, and daily chemical industries. In the biosynthetic pathway of terpenoids, terpene synthases often determine the type and novelty of the terpene carbon skeleton, and tailoring enzymes, such as cytochrome P450 enzymes, can carry out a variety of post-modifications, ultimately resulting in terpenoids with a rich diversity of structures and functions. In recent years, with the development of genome-sequencing technology and synthetic biology, a large number of terpene biosynthetic enzymes of plant and microbial origin have been characterized, which, excitingly, include non-canonical terpene synthases that can also catalyze the generation of unique cyclized skeletons. Meanwhile, the use of combinatorial biosynthetic strategies has led to the creation of many novel and unnatural terpenoids, further enriching the kingdom of terpenoids. Here, we review the recent advances in non-canonical terpene cyclases and combinatorial biosynthetic pathways over the past five years, with a view to shedding light on the discovery and biosynthesis of novel terpenes in the future. Firstly, the newly discovered novel enzymes with terpene cyclization functions are reviewed, containing a new subclass of type Ⅰ terpene synthases, non-squalene triterpene synthases, UbiA-type terpene cyclases, cytochrome P450 oxygenases, methyltransferases, vanadium-dependent haloperoxidases, and haloacid dehalogenase, along with their sequences, functions, and possible cyclization mechanisms, which can contribute to our understanding of terpenoid biosynthetic enzymes and the discovery of novel terpenoids. This review then describes the combinatorial biosynthesis of non-canonical terpenoids. By combining terpene synthases with methyltransferases or natural/artificial cytochrome P450 oxygenases, a series of unnatural terpenoids containing non-canonical C11 and C16 backbones, or with unusual structural modifications, were produced. This could inspire the structural innovation studies of terpenoids in the future. The discovery of novel enzymes and the construction of novel combinatorial biosynthetic pathways will further broaden the structural diversity and chemical space of terpenoids, which is expected to provide more potential novel terpenoids for clinical drug development.

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    Advances and applications of droplet-based microfluidics in evolution and screening of engineered microbial strains
    TU Ran, LI Shixin, LI Haoni, WANG Meng
    Synthetic Biology Journal    2023, 4 (1): 165-184.   DOI: 10.12211/2096-8280.2021-105
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    Microbial strains are perquisites for biomanufacting through microbial culture and fermentation. However, most strains usually need to be engineered to improve their performances for industrial applications. Therefore how to efficiently screen and isolate robust strains is a critical step of strain engineering. As an advanced high-throughput screening technology, droplet-based microfluidics developed with micro-chips can generate highly independent and uniform micro- or nano-liter droplets, in which single cells can be encapsulated, inoculated, detected, and analyzed for strain engineering. It is especially useful in the evolution of microbial strains for producing extracellular products. In this review, we first introduce the basic components of the droplet-based microfluidic system and the main steps involved in the strain screening. We then summarize key factors for the application of the droplet-based microfluidic technology in strain engineering, such as the signal sources of droplet detection, the difficulties of handling droplet screening, and the scopes of droplet sorting instruments. Based on the instruments used for the droplet sorting, we group the application cases into two types either via fluorescence-activated droplet sorting (FADS) using microfluidic equipment or via fluorescence-activated cell sorting (FACS) using flow cytometry instrument. While FADS using single-layer water-in-oil droplet can be further classified into cellular signature, fluorescent reporter protein, and substrate-based reaction according to the signal sources, FACS can be divided into double-layers water-in-oil-in-water (W/O/W) droplet or microgel droplet according to the droplet property. Finally, we outline challenges and prospects for the droplet microfluidic technology, and provide some guidelines for its applications in synthetic biology. Compared with traditional screening methods such as shaking flask or microplate with a throughput of hundreds to thousands of samples per day in milli- or micro-liter volume, the droplet-based microfluidic technology can achieve millions of samples per day in pico- or nano-liter volume, resulting in an increase of thousand-folds in screening speed and cost-saving for million-folds. By integrating with an automated station, the droplet-based microfluidic technology can be further improved for its screening efficiencies and application potentials in microbial synthetic biology.

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    Research progress in synthesis of astaxanthin by microbial fermentation
    ZHOU Qiang, ZHOU Dawei, SUN Jingxiang, WANG Jingnan, JIANG Wankui, ZHANG Wenming, JIANG Yujia, XIN Fengxue, JIANG Min
    Synthetic Biology Journal    2024, 5 (1): 126-143.   DOI: 10.12211/2096-8280.2023-065
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    Astaxanthin is a value-added terpene with strong antioxidant activity as well as other physiological functions, such as anti-cancer, enhancing immunity, eye protection, and cardio-cerebrovascular protection. Natural astaxanthin mainly comes from algae and aquatic crustaceans such as lobster shell. Astaxanthin presents with stereoisomerism and geometric isomerism, which have different biological activities and applications. Currently, astaxanthin in the market is obtained primarily through natural extraction from Haematococcus pluvialis or Xanthophyllomyces dendrorhous and chemical synthesis as well. While H. pluvialis has a long growth cycle and high light demand, leading to low biomass productivity and extraction rate for high production cost of astaxanthin, X. dendrorhous has a low astaxanthin yield and is easy to degenerate, making them challenging for the large-scale commercial production. The chemical synthesis of astaxanthin involves multiple reactions with complicated processes, producing mixed isomers and various byproducts, which consequently compromises its antioxidant capacity. Moreover, the assimilation and utilization of chemically synthesized astaxanthin in vivo is poor compared to its natural product, making it not suitable for being used by human being. With the continuous development of synthetic biology, microbial fermentation has been developed as an effective way for the commercial production of astaxanthin to better meet consumer demand. At present, astaxanthin-producing microorganisms include bacteria, fungi, and algae. This review introduces astaxanthin's structure, properties, production methods, and processes for its extraction and purification, with an emphasis on natural and engineered biosynthetic pathways. The latest progress in the production of astaxanthin by different microorganisms such as H. pluvialis, Yarrowia lipolytica and Escherichia coli is summarized, along with strategies for increasing astaxanthin production through genetic engineering and fermentation process optimization. Future metabolic engineering strategies are proposed, such as over-expression of astaxanthin synthesis genes, promoters with higher substitution intensity, subcellular localization, metabolic pathway optimization, etc, to increase astaxanthin yield for wide usage in food, medical, cosmetic and feed industries.

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    Research progress of artificial intelligence in desiging protein structures
    CHEN Zhihang, JI Menglin, QI Yifei
    Synthetic Biology Journal    2023, 4 (3): 464-487.   DOI: 10.12211/2096-8280.2023-008
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    Proteins are essential to life as they carry out a great variety of biological functions. Protein sequences determine their three-dimensional structures, and therefore physiological functions. Proteins with specific functions have important applications in many fields such as biomedicine, where they are utilized in drug design and delivery. In the past, protein engineering and directed evolution are commonly used to improve the activity and stability of proteins. These methods, however, are both complex and expensive, as they require a large number of biological experiments for validation. Computational protein design (CPD) allows the design of amino acid sequences based on desired protein functions and structures, and more intriguingly, generation of proteins even not found in nature. Conventional CPD uses energy function and optimization algorithm to design protein sequences. In recent years, with the rapid development of artificial intelligence (AI) technique, the accumulation of big data and the development of high speed computing, AI has made great progresses in learning, and been successfully applied in CPD. In this review, based on the input constraints and sampling space size, we present a systematic overview of recent applications of AI in protein design from three aspects: fixed-backbone design, flexible-backbone design, and sequence structure generation. We focus on algorithms and protein feature encoding, present the effect of dataset size and architectural improvements on model performance in prediction, and showcase several enzymes, antibodies, and binding proteins that were successfully designed using these models. The advantages of AI compared with traditional CPD methods are also discussed. Finally, we highlight challenges in AI-aided protein design, and propose some strategies for solutions.

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    Optimization and development of CRISPR/Cas9 systems for genome editing
    TENG Xiaolong, SHI Shuobo
    Synthetic Biology Journal    2023, 4 (1): 67-85.   DOI: 10.12211/2096-8280.2022-047
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    As an emerging technology developed within recent years, CRISPR/Cas9 exhibits fast, efficient, and precise gene editing and regulation capabilities in various organisms and tissues, and these advantages make it widely used in research with fundamental sciences and applied technologies as well such as synthetic biology. This review first briefly introduces the discovery history, classification, and mechanism of CRISPR/Cas9. The system of CRISPR/Cas9 usually contains a single guide RNA (gRNA) molecule for targeting a specific sequence, and a Cas9 endonuclease for catalyzing a double-strand break (DSB) in the sequence (target DNA strands). The recognition and cleavage of target DNA strictly require the presence of a protospacer adjacent motif (PAM) in the target sequence. The DSB(s) can be repaired by various DNA repair mechanisms, which allow various gene editing such as gene integration, gene replacement, and gene knockout. Due to limitations of CRISPR/Cas9, such as PAM dependence and high off-target rate, researchers have developed various fused or engineered Cas9 proteins and gRNAs that play significant roles in fulfilling various purposes. These Cas9 variants are modified for improving the performance of PAM, in particular its specificity and fidelity. Moreover, the DSBs generated by Cas9 are considered toxic to the cells, and the use of Cas9 nickase (nCas9) or catalytically deficient Cas9 (dCas9) in CRISPR has also been developed without generating DSBs. Meanwhile, different effector proteins can be fused with Cas9/dCas9/nCas9 to bring about new functions and applications in gene expression regulation, epigenome editing, and single base editing. Moreover, we introduce the current studies and applications of multiple gRNA expression strategies based on the multiplex advantages of the CRISPR/Cas9 system. In general, CRISPR/Cas9 systems have gradually become standardized and revolutionized genome editing systems for almost all possible genetic manipulations. Finally, we highlight perspectives on several applications of the versatile CRISPR/Cas9 toolbox as a genome editing tool, and discuss the safety and risk control issues when it is used in gene therapy.

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    Mining, engineering and functional expansion of CRISPR/Cas systems
    LIU Ke, LIN Guihong, LIU Kun, ZHOU Wei, WANG Fengqing, WEI Dongzhi
    Synthetic Biology Journal    2023, 4 (1): 47-66.   DOI: 10.12211/2096-8280.2021-022
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    The clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated proteins (Cas) were derived from an acquired immune system in microbes. Since their functions on gene editing have been reported, they have been rapidly used to enhance our ability to edit, regulate, annotate, detect, and image DNA and RNA fragments of various organisms, which consequently have faciliated fundamental research in life science, medicine, bioengineering and so on, and drived the development of synthetic biology and other disciplines. However, CRISPR/Cas systems also have some inherent drawbacks, such as off-target effect, constraint of protospacer-adjacent motif (PAM) on the target, and controllability of the gene editing activity, which substantially compromise their applications in highly precise and controllable gene editing. In order to overcome these challenges, two important strategies have been employed to develop enhanced CRISPR/Cas systems and expand the CRISPR toolbox, including modifying the Cas proteins by protein engineering and mining novel CRISPR/Cas systems with bioinformatics. In the review, focusing on the most widely used the type II CRISPR/Cas systems, we mainly introduce the basic structures and functions of three representative systems, including CRISPR/Cas9, CRISPR/Cas12a and CRISPR/Cas13a, as well as recent progress in their structural modifications and functional expansion. Thereinto, engineering strategies for CRISPR/Cas systems have been systematically commented, which include modification methods for Cas proteins and the way to expand the function of CRISPR/Cas systems by coupling specfic proteins with Cas. In addition, we also review some novel CRISPR/Cas systems with important characteristics and potential applications that have been discovered in recently years, such as CRISPR/CasФ and CRISPR/Cas12k. These engineering modifications and mining work have greatly addressed the inherent problems of the CRISPR/Cas systems, and effectively expanded their functions and applicability, which will further promote the applications of the CRISPR/Cas systems in many fields.

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    Advances with applications of Raman spectroscopy in single-cell phenotype sorting and analysis
    WANG Xixian, SUN Qing, DIAO Zhidian, XU Jian, MA Bo
    Synthetic Biology Journal    2023, 4 (1): 204-224.   DOI: 10.12211/2096-8280.2022-043
    Abstract3080)   HTML125)    PDF(pc) (2643KB)(3603)       Save

    In synthetic biology, methodological innovations in sequencing, editing and synthesis of genes and whole genomes have resulted in unprecedented development in "design and manufacturing of genotypes". On the other hand, "testing of cellular phenotypes and functions" has increasingly become one of major bottlenecks. Single-cell technologies have tremendous impacts and potentials in rapid testing of cellular phenotypes and functions. However, such single-cell methods should allow non-invasive live-cell probing, be label-free, provide landscape-like phenotype sorting, distinguish complex functions, operate with high speed, sufficient throughput and low-cost, and finally, be able to integrate with downstream omics analysis. Raman spectroscopy has all the above features, and can provide information on the chemical composition and molecular structure of single cells, making it an efficient single-cell phenotyping technology. In this review, we first introduce the concept of Ramanome and Ramanome-based phenotyping technologies, including detecting and quantifying products, measuring profiles of substrates and metabolites, discriminating cell types or states, and characterizing stress response and modeling environmental changes. We then summarize the development of existing Raman-activated cell sorting (RACS) platforms in phenotyping and sorting of cell factories such as including spontaneous Raman, resonance Raman, and coherent Raman, the modes for acquiring Raman signals including static modes on dry slice and in liquid as well, flow modes by trap-free and trap-and-release manners, and principles for target cells sorting including Ejection by pulsed laser, dragging by optical tweezer, and sorting by microfluidics operation and droplets. We also highlight the applications of different RACS platforms, including the sorting of carotenoid-producing yeast and cyanobacteria cells, astaxanthin (AXT)-hyperproducing microalgae cells, triacylglycerol (TAG)-producing yeast cells, etc. Finally, challenges with single cell Raman spectroscopy (SCRS) in the phenotyping and sorting of synthetic cells and their perspectives are outlined and discussed. We propose that SCRS will bridge phenotypes and genotypes in science and technologies through coupling with downstream high-throughput cell sorting and omics profiling. This bridge will lead to novel and creative solutions to high-throughput, landscape-like testing and screening of synthetic cells. Moreover, it will fulfill the promise of Raman spectroscopy-enabled single-cell "phenome-genome" as a new type of biological big-data, and accelerate the pace of "data-driven" synthetic biology.

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    Prediction of protein complex structure: methods and progress
    HUANG He, WU Tong, WANG Wenda, LI Jiashan, SUN Daiwen, YE Qiwei, GONG Xinqi
    Synthetic Biology Journal    2023, 4 (3): 507-523.   DOI: 10.12211/2096-8280.2022-079
    Abstract3016)   HTML168)    PDF(pc) (1732KB)(5054)       Save

    Protein complexes carry out a variety of biological functions, and obtaining the three-dimensional structure of protein complexes is critical for understanding their functions. In many cases, not only can two proteins interact to form a protein dimer, but also multiple proteins interact to form a protein multimer. It is difficult and time-consuming to resolve the structure of protein complexes by experiments. Recently, there have been some attempts and methods to predict the structure of multimers based on the structure prediction for the monomers. Several groups in the CASP14 competition submitted the prediction of protein complex targets, which mainly included template -based methods or protein docking. Later, on the basis of AlphaFold2, researchers developed some end-to-end structure prediction methods for complexes, which accelerates the study of protein complex structure prediction. However, compared with the prediction of monomeric protein structure, the accuracy of prediction for protein complex structure is still lower. This review surveys updated methods and advances in protein complex prediction, including inter-chain residue contact prediction, protein docking, and end-to-end protein complex structure prediction. Firstly, AI algorithms for protein structure prediction are briefly introduced, including coevolutionary analysis and protein contact prediction, deep learning method and protein structure prediction, pretraining model, and protein representation learning. Secondly, basic methods for predicting interactions between protein complexes are systematically summarized, from the construction of multiple sequence alignments of the complexes to the prediction of the inter-residue contact between chains of homologous or heterologous complexes. Finally, basic methods and ideas for protein complex structure prediction are explored from the viewpoint of interaction sites guiding complex structure prediction, protein molecular docking algorithm, end-to-end complex structure prediction methods, etc. In order to better predict the structure of protein complexes, we need to devote our effort to following aspects: 1) constructing protein complexes datasets for training and evaluation of prediction methods for the structure of multimers, 2) developing efficient algorithms to improve the prediction accuracy such as MSA paring algorithm and building templates for multi-chain protein complex, and 3) enlarging databases for protein sequences and structures for better modeling protein complex with pretraining and self-supervised learning methods. In all, predicting protein complex structure still remains a challenge, and new methods to improve accuracy will be helpful for analyzing protein functions, designing proteins and drug discovery.

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    Synthetic biology drives the sustainable production of terpenoid fragrances and flavors
    ZHANG Mengyao, CAI Peng, ZHOU Yongjin
    Synthetic Biology Journal    2025, 6 (2): 334-356.   DOI: 10.12211/2096-8280.2024-057
    Abstract2979)   HTML204)    PDF(pc) (3063KB)(2084)       Save

    The demand for personal care products has been increasing steadily. Consumers are now seeking for products that offer enhanced functionality, natural ingredients, and superior feeling experiences. Fragrances and flavors are key components in personal care formulations. Terpenes and their derivatives dominate natural fragrances due to their diverse structures and scents, widespread availability from plants and animals, stable function, and high safety profile. The terpene fragrance market is projected to grow at an annual growth rate of 6.4%, reaching $1.01 billion by 2028, indicating a high market revenue and promising future. Currently, the acquisition of natural terpene fragrances is constrained by the long growth cycle of plants, low terpene content, and high extraction cost. Thus, there is an urgent need for developing new technology, such as synthetic biology, to achieve large-scale production of diverse fragrance compounds at an environment-friendly manner. This review explores the application and development of synthetic biology in the sustainable production of terpene fragrances, highlighting how data-driven synthetic biology and biotechnological innovations empower terpene fragrance production. It also compares classical and alternative terpenoid biosynthesis pathways, elucidating their differences and advantages, which can offer comprehensive insights for chassis design toward terpenoid efficient biosynthesis. Additionally, this review explores recent advances in terpene synthase discovery and engineering as well as cell factory construction. Furthermore, we comprehensively summarizes challenges encountered in the construction of three major types of terpene fragrance cell factories: monoterpenes, sesquiterpenes, and nor-isoprenoids, and discusses metabolic engineering strategies that can be employed to address these issues, including enzyme optimization, pathway reconstruction, and cellular detoxification. At the end, we comment the current landscape of patents and industrial competition, offering insights into future challenges and opportunities, including the hurdles of biosynthesis technology, the discovery and design of new products, as well as the market regulation and safety concerns.

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    Advances in optogenetics for biomedical research
    YU Yuanhuan, ZHOU Yang, WANG Xinyi, KONG Deqiang, YE Haifeng
    Synthetic Biology Journal    2023, 4 (1): 102-140.   DOI: 10.12211/2096-8280.2022-030
    Abstract2922)   HTML223)    PDF(pc) (5942KB)(6248)       Save

    Synthetic biology enables rational design of regulatory molecules and circuits to reprogram cellular behaviors, and its applications to human cells could lead to powerful gene- and cell-based therapies, which are well recognized as central pillars of next-generation medicines. However, the safety of these therapies remains to be assessed, and controllability is a critical issue affecting their safety and limiting their clinical applications. In recent years, optogenetic technologies have been widely used in biomedical applications, which provides new insights for treating intractable diseases due to their distinguishing features of non-invasiveness, reversibility, and spatiotemporal resolution. Light is an ideal inducer to control gene expression, enabling precise and spatiotemporal manipulation of gene expression and cell behaviors by illuminating with light of appropriate intensity and wavelength as a triggering signal to achieve pinpoint spatiotemporal control of cellular activities. With the development of optogenetic toolkits, optogenetics has recently been developed for therapeutic applications. In this review, we summarize various optogenetic tools responsive to different wavelengths and their applications for precise treatment of neurological diseases, tumors, cardiovascular diseases, diabetes, enteric diseases as well as for the optogenetic control of gene transcription, gene editing, gene recombination and organelle movement. At the same time, we introduce recent research progress in portable bioelectronic medicine and artificial intelligence-assisted diagnosis and treatment systems, which are based on optogenetic techniques and the intelligent electronic devices. The rapid development of optogenetics has enormously extended the scope of traditional bioelectronic medicine, and the remote-controllability, reversibility, and negligible toxicity of optical control systems provide a solid foundation for the application of optogenetics in biomedicine. The success of these approaches would have an impact on precision medicine in the future practice. Finally, we also discuss the shortcomings of existing optogenetic tools and the challenges that would be faced in the future clinical applications as well as the prospects of their development.

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    Application of deep learning in protein function prediction
    SONG Yidong, YUAN Qianmu, YANG Yuedong
    Synthetic Biology Journal    2023, 4 (3): 488-506.   DOI: 10.12211/2096-8280.2022-078
    Abstract2876)   HTML209)    PDF(pc) (1457KB)(4823)       Save

    Protein function prediction is essential for bioinformatics analysis, which benefits a wide range of biological studies such as understanding the functions of metagenomes, uncovering mechanism underlying diseases, and finding new drug targets. With the rapid development of high-throughput sequencing technology, protein sequence data have been increased quickly, but functions of most proteins have not yet been identified. Since traditional biochemical experiments to determine protein functions are usually expensive, time-consuming, and less efficient, developing more efficient and effective computational methods for protein function prediction is of great significance. Deep learning technology has made breakthroughs in many fields, including image recognition, natural language processing, genomic analysis and drug discovery. In this review, we address applications of deep learning in protein function prediction, which can be divided into residue-level binding site prediction and protein-level gene ontology (GO) prediction. Protein binding sites are regions that bind to specific ligands, which play an important role in signal transduction, metabolism, revealing molecular mechanisms underlying diseases, and designing new drugs. Gene ontology is a standard function classification system for genes, which provides a set of annotations to comprehensively describe the properties of genes and gene products. Firstly, we introduce commonly used large-scale protein structure and function databases. Secondly, discriminative protein sequence and structure features are described. Thirdly, we summarize the latest protein function prediction methods: in terms of the prediction of binding sites, we introduce the latest methods based on the ligand type, including protein, peptide, nucleic acid and small molecule as well as ion ligand, and in the aspect of GO prediction, we highlight the latest sequence-based, structure-based, and protein interaction network-based methods developed with protein information. Finally, we comment the advantages and disadvantages of the current protein function prediction methods, and discuss the future development in this field.

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