Animal-free heme production for artificial meat in Corynebacterium glutamicum via systems metabolic and membrane engineering

Recent advances in microbial synthesis of free heme

Heme is an iron-containing porphyrin compound widely used in the fields of healthcare, food, and medicine. Compared to animal blood extraction, it is more advantageous to develop a microbial cell factory to produce heme. However, heme biosynthesis in microorganisms is tightly regulated, and its accumulation is highly cytotoxic. The current review describes the biosynthetic pathway of free heme, its fermentation production using different engineered bacteria constructed by metabolic engineering, and strategies for further improving heme synthesis. Heme synthetic pathway in Bacillus subtilis was modified utilizing genome-editing technology, resulting in significantly improved heme synthesis and secretion abilities. This technique avoided the use of multiple antibiotics and enhanced the genetic stability of strain. Hence, engineered B. subtilis could be an attractive cell factory for heme production. Further studies should be performed to enhance the expression of heme synthetic module and optimize the expression of heme exporter and fermentation processes, such as iron supply. KEY POINTS: • Strengthening the heme biosynthetic pathway can significantly increase heme production. • Heme exporter overexpression helps to promote heme secretion, thereby further promoting excessive heme synthesis. • Engineered B. subtilis is an attractive alternative for heme production.

Enhancement of Green Production of Heme by Deleting Odor-Related Genes from Bacillus amyloliquefaciens Based on CRISPR/Cas9n

Heme is a crucial component in endowing plant-based meat analogs with flavor and color. This study aimed to develop a green strategy for heme production by reducing fermentation off-odor and accelerating heme synthesis. First, an efficient CRISPR/Cas9n system was constructed in Bacillus amyloliquefaciens to construct the odor-reducing chassis cell HZC9nΔGPSU, and the odor substances including the branched-chain short fatty acids, putrescine, and ammonia were reduced by 62, 70, and 88%, respectively. Meanwhile, the hemA gene was confirmed to be the key gene for enhanced heme synthesis. Various hemA genes were compared to obtain the best gene dhemA, and the catalysis mechanism was explained by molecular docking simulation. After further expression of dhemA in HZC9nΔGPSU, the heme titer of HZC9nΔGPSU/pHY-dhemA reached 11.31 ± 0.51 mg/L, 1.70-fold higher than that of HZC9n/pHY-dhemA. The knockout of off-odor-related genes reduced the odor substances and enhanced the heme synthesis, which is promising for the green production of high-quality heme.

Multidimensional engineering of Saccharomyces cerevisiae for the efficient production of heme by exploring the cytotoxicity and tolerance of heme

Heme has attracted considerable attention due to its indispensable biological roles and applications in healthcare and artificial foods. The development and utilization of edible microorganisms instead of animals to produce heme is the most promising method to promote the large-scale industrial production and safe application of heme. However, the cytotoxicity of heme severely restricts its efficient synthesis by microorganisms, and the cytotoxic mechanism is not fully understood. In this study, the effect of heme toxicity on Saccharomyces cerevisiae was evaluated by enhancing its synthesis using metabolic engineering. The results showed that the accumulation of heme after the disruption of heme homeostasis caused serious impairments in cell growth and metabolism, as demonstrated by significantly poor growth, mitochondrial damage, cell deformations, and chapped cell surfaces, and these features which were further associated with substantially elevated reactive oxygen species (ROS) levels within the cell (mainly H2O2 and superoxide anion radicals). To improve cellular tolerance to heme, 5 rounds of laboratory evolution were performed, increasing heme production by 7.3-fold and 4.2-fold in terms of the titer (38.9 mg/L) and specific production capacity (1.4 mg/L/OD600), respectively. Based on comparative transcriptomic analyses, 32 genes were identified as candidates that can be modified to enhance heme production by more than 20% in S. cerevisiae. The combined overexpression of 5 genes (SPS22, REE1, PHO84, HEM4 and CLB2) was shown to be an optimal method to enhance heme production. Therefore, a strain with enhanced heme tolerance and ROS quenching ability (R5-M) was developed that could generate 380.5 mg/L heme with a productivity of 4.2 mg/L/h in fed-batch fermentation, with S. cerevisiae strains being the highest producers reported to date. These findings highlight the importance of improving heme tolerance for the microbial production of heme and provide a solution for efficient heme production by engineered yeasts.

High-yield porphyrin production through metabolic engineering and biocatalysis

Porphyrins and their derivatives find extensive applications in medicine, food, energy and materials. In this study, we produced porphyrin compounds by combining Rhodobacter sphaeroides as an efficient cell factory with enzymatic catalysis. Genome-wide CRISPRi-based screening in R. sphaeroides identifies hemN as a target for improved coproporphyrin III (CPIII) production, and exploiting phosphorylation of PrrA further improves the production of bioactive CPIII to 16.5 g L-1 by fed-batch fermentation. Subsequent screening and engineering high-activity metal chelatases and coproheme decarboxylase results in the synthesis of various metalloporphyrins, including heme and the anti-tumor agent zincphyrin. After pilot-scale fermentation (200 L) and setting up the purification process for CPIII (purity >95%), we scaled up the production of heme and zincphyrin through enzymatic catalysis in a 5-L bioreactor, with CPIII achieving respective enzyme conversion rates of 63% and 98% and yielding 10.8 g L-1 and 21.3 g L-1, respectively. Our strategy offers a solution for high-yield bioproduction of heme and other valuable porphyrins with substantial industrial and medical applications.

Multi-modular metabolic engineering of heme synthesis in Corynebacterium glutamicum

Heme, an iron-containing porphyrin derivative, holds great promise in fields like medicine, food production and chemicals. Here, we developed an engineered Corynebacterium glutamicum strain for efficient heme production by combining modular engineering and RBS engineering. The whole heme biosynthetic pathway was methodically divided into 5-ALA synthetic module, uroporphyrinogen III (UPG III) synthetic module and heme synthetic module for further construction and optimization. Three heme synthetic modules were compared and the siroheme-dependent (SHD) pathway was identified to be optimal in C. glutamicum for the first time. To further improve heme production, the expression of genes in UPG III synthetic module and heme synthetic module was coordinated optimized through RBS engineering, respectively. Subsequently, heme oxygenase was knocked out to reduce heme degradation. The engineered strain HS12 showed a maximum iron-containing porphyrin derivatives titer of 1592 mg/L with the extracellular secretion rate of 45.5% in fed-batch fermentation. Our study constructed a C. glutamicum chassis strain for efficient heme accumulation, which was beneficial for the advancement of efficient heme and other porphyrins production.

Enhancing Phycocyanobilin Production Efficiency in Engineered Corynebacterium glutamicum: Strategies and Potential Application

Phycocyanobilin, an algae-originated light-harvesting pigment known for its antioxidant properties, has gained attention as it plays important roles in the food and medication industries and has surged in demand owing to its low-yield extraction from natural resources. In this study, engineered Corynebacterium glutamicum was developed to achieve high PCB production, and three strategies were proposed: reinforcement of the heme biosynthesis pathway with the introduction of two PCB-related enzymes, strengthening of the pentose phosphate pathway to generate an efficient cycle of NADPH, and fed-batch fermentation to maximize PCB production. Each approach increased PCB synthesis, and the final engineered strain successfully produced 78.19 mg/L in a flask and 259.63 mg/L in a 5 L bioreactor, representing the highest bacterial production of PCB reported to date, to our knowledge. The strategies applied in this study will be useful for the synthesis of PCB derivatives and can be applied in the food and pharmaceutical industries.

Development of a Normal Porcine Cell Line Growing in a Heme-Supplemented, Serum-Free Condition for Cultured Meat

A key element for the cost-effective development of cultured meat is a cell line culturable in serum-free conditions to reduce production costs. Heme supplementation in cultured meat mimics the original meat flavor and color. This study introduced a bacterial extract generated from Corynebacterium that was selected for high-heme expression by directed evolution. A normal porcine cell line, PK15, was used to apply the bacterial heme extract as a supplement. Consistent with prior research, we observed the cytotoxicity of PK15 to the heme extract at 10 mM or higher. However, after long-term exposure, PK15 adapted to tolerate up to 40 mM of heme. An RNA-seq analysis of these heme-adapted PK15 cells (PK15H) revealed a set of altered genes, mainly involved in cell proliferation, metabolism, and inflammation. We found that cytochrome P450, family 1, subfamily A, polypeptide 1 (CYP1A1), lactoperoxidase (LPO), and glutathione peroxidase 5 (GPX5) were upregulated in the PK15H heme dose dependently. When we reduced serum serially from 2% to serum free, we derived the PK15H subpopulation that was transiently maintained with 5-10 mM heme extract. Altogether, our study reports a porcine cell culturable in high-heme media that can be maintained in serum-free conditions and proposes a marker gene that plays a critical role in this adaptation process.

Bioproduction of porphyrins, phycobilins, and their proteins using microbial cell factories: engineering, metabolic regulations, challenges, and perspectives

Porphyrins, phycobilins, and their proteins have abundant π-electrons and strongly absorb visible light, some of which bind a metal ion in the center. Because of the structural and optical properties, they not only play critical roles as an essential component in natural systems but also have attracted much attention as a high value specialty chemical in various fields, including renewable energy, cosmetics, medicines, and foods. However, their commercial application seems to be still limited because the market price of porphyrins and phycobilins is generally expensive to apply them easily. Furthermore, their petroleum-based chemical synthesis is energy-intensive and emits a pollutant. Recently, to replace petroleum-based production, many studies on the bioproduction of metalloporphyrins, including Zn-porphyrin, Co-porphyrin, and heme, porphyrin derivatives including chlorophyll, biliverdin, and phycobilins, and their proteins including hemoproteins, phycobiliproteins, and phytochromes from renewable carbon sources using microbial cell factories have been reported. This review outlines recent advances in the bioproduction of porphyrins, phycobilins, and their proteins using microbial cell factories developed by various microbial biotechnology techniques, provides well-organized information on metabolic regulations of the porphyrin metabolism, and then critically discusses challenges and future perspectives. Through these, it is expected to be able to achieve possible solutions and insights and to develop an outstanding platform to be applied to the industry in future research.

Modularized Engineering of Shewanella oneidensis MR-1 for Efficient and Directional Synthesis of 5-Aminolevulinic Acid

Shewanella oneidensis MR-1 has found widespread applications in pollutant transformation and bioenergy production, closely tied to its outstanding heme synthesis capabilities. However, this significant biosynthetic potential is still unexploited so far. Here, we turned this bacterium into a highly-efficient bio-factory for green synthesis of 5-Aminolevulinic Acid (5-ALA), an important chemical for broad applications in agriculture, medicine, and the food industries. The native C5 pathway genes of S. oneidensis was employed, together with the introduction of foreign anti-oxidation module, to establish the 5-ALA production module, resulting 87-fold higher 5-ALA yield and drastically enhanced tolerance than the wild type. Furthermore, the metabolic flux was regulated by using CRISPR interference and base editing techniques to suppress the competitive pathways to further improve the 5-ALA titer. The engineered strain exhibited 123-fold higher 5-ALA production capability than the wild type. This study not only provides an appealing new route for 5-ALA biosynthesis, but also presents a multi-dimensional modularized engineering strategy to broaden the application scope of S. oneidensis.

Development strategy of non-GMO organism for increased hemoproteins in Corynebacterium glutamicum: a growth-acceleration-targeted evolution

Heme, found in hemoproteins, is a valuable source of iron, an essential mineral. The need for an alternative hemoprotein source has emerged due to the inherent risks of large-scale livestock farming and animal proteins. Corynebacterium glutamicum, regarded for Qualified Presumption of Safety or Generally Recognized as Safe, can biosynthesize hemoproteins. C. glutamicum single-cell protein (SCP) can be a valuable alternative hemoprotein for supplying heme iron without adversely affecting blood fat levels. We constructed the chemostat culture system to increase hemoprotein content in C. glutamicum SCP. Through adaptive evolution, hemoprotein levels could be naturally increased to address oxidative stress resulting from enhanced growth rate. In addition, we used several specific plasmids containing growth-accelerating genes and the hemA promoter to expedite the evolutionary process. Following chemostat culture for 15 days, the plasmid in selected descendants was cured. The evolved strains showed improved specific growth rates from 0.59 h-1 to 0.62 h-1, 20% enhanced resistance to oxidative stress, and increased heme concentration from 12.95 µg/g-DCW to 14.22-15.24 µg/g-DCW. Notably, the putative peptidyl-tRNA hydrolase-based evolved strain manifested the most significant increase (30%) of hemoproteins. This is the first report presenting the potential of a growth-acceleration-targeted evolution (GATE) strategy for developing non-GMO industrial strains with increased bio-product productivity.

Biosynthesis, acquisition, regulation, and upcycling of heme: recent advances

Heme, an iron-containing tetrapyrrole in hemoproteins, including: hemoglobin, myoglobin, catalase, cytochrome c, and cytochrome P450, plays critical physiological roles in different organisms. Heme-derived chemicals, such as biliverdin, bilirubin, and phycocyanobilin, are known for their antioxidant and anti-inflammatory properties and have shown great potential in fighting viruses and diseases. Therefore, more and more attention has been paid to the biosynthesis of hemoproteins and heme derivatives, which depends on the adequate heme supply in various microbial cell factories. The enhancement of endogenous biosynthesis and exogenous uptake can improve the intracellular heme supply, but the excess free heme is toxic to the cells. Therefore, based on the heme-responsive regulators, several sensitive biosensors were developed to fine-tune the intracellular levels of heme. In this review, recent advances in the: biosynthesis, acquisition, regulation, and upcycling of heme were summarized to provide a solid foundation for the efficient production and application of high-value-added hemoproteins and heme derivatives.

High-level expression of leghemoglobin in Kluyveromyces marxianus by remodeling the heme metabolism pathway

Soy leghemoglobin, when bound to heme, imparts a meat-like color and flavor and can serve as a substitute for animal-derived proteins. Enhancing cellular heme synthesis improves the recombinant expression of leghemoglobin in yeast. To achieve high-level expression of leghemoglobin A (LBA) in Kluyveromyces marxianus, a food-safe yeast, large-scale heme synthesis modules were transferred into K. marxianus using yeast artificial chromosomes (KmYACs). These modules contained up to 8 native and heterologous genes to promote the supply of heme precursors and downstream synthesis. Next, eight genes inhibiting heme or LBA synthesis were individually or combinatorially deleted, with the lsc1Δssn3Δ mutant yielding the best results. Subsequently, heme synthesis modules were combined with the lsc1Δssn3Δ mutant. In the resulting strains, the module genes were all actively expressed. Among these module genes, heterologous S. cerevisiae genes in the downstream heme synthesis pathway significantly enhanced the expression of their counterparts in K. marxianus, resulting in high heme content and LBA yield. After optimizing the medium recipe by adjusting the concentrations of glucose, glycine, and FeSO4·7H2O, a heme content of 66.32 mg/L and an intracellular LBA titer of 7.27 g/L were achieved in the engineered strain in a 5 L fermentor. This represents the highest intracellular expression of leghemoglobin in microorganisms to date. The leghemoglobin produced by K. marxianus can be utilized as a safe ingredient for plant-based protein products.

Metabolic engineering and synthetic biology strategies for producing high-value natural pigments in Microalgae

Microalgae are microorganisms capable of producing bioactive compounds using photosynthesis. Microalgae contain a variety of high value-added natural pigments such as carotenoids, phycobilins, and chlorophylls. These pigments play an important role in many areas such as food, pharmaceuticals, and cosmetics. Natural pigments have a health value that is unmatched by synthetic pigments. However, the current commercial production of natural pigments from microalgae is not able to meet the growing market demand. The use of metabolic engineering and synthetic biological strategies to improve the production performance of microalgal cell factories is essential to promote the large-scale production of high-value pigments from microalgae. This paper reviews the health and economic values, the applications, and the synthesis pathways of microalgal pigments. Overall, this review aims to highlight the latest research progress in metabolic engineering and synthetic biology in constructing engineered strains of microalgae with high-value pigments and the application of CRISPR technology and multi-omics in this context. Finally, we conclude with a discussion on the bottlenecks and challenges of microalgal pigment production and their future development prospects.

Development of a growth-coupled selection platform for directed evolution of heme biosynthetic enzymes in Corynebacterium glutamicum

Heme is an important tetrapyrrole compound, and has been widely applied in food and medicine industries. Although microbial production of heme has been developed with metabolic engineering strategies during the past 20 years, the production levels are relatively low due to the multistep enzymatic processes and complicated regulatory mechanisms of microbes. Previous studies mainly adopted the strategies of strengthening precursor supply and product transportation to engineer microbes for improving heme biosynthesis. Few studies focused on the engineering and screening of efficient enzymes involved in heme biosynthesis. Herein, a growth-coupled, high-throughput selection platform based on the detoxification of Zinc-protoporphyrin IX (an analogue of heme) was developed and applied to directed evolution of coproporphyrin ferrochelatase, catalyzing the insertion of metal ions into porphyrin ring to generate heme or other tetrapyrrole compounds. A mutant with 3.03-fold increase in k cat/K M was selected. Finally, growth-coupled directed evolution of another three key enzymes involved in heme biosynthesis was tested by using this selection platform. The growth-coupled selection platform developed here can be a simple and effective strategy for directed evolution of the enzymes involved in the biosynthesis of heme or other tetrapyrrole compounds.

Promoting Heme and Phycocyanin Biosynthesis in Synechocystis sp. PCC 6803 by Overexpression of Porphyrin Pathway Genes with Genetic Engineering

Due to their unique biochemical and spectroscopic properties, both heme and phycocyanobilin are widely applied in the medical and food industries. Synechocystis sp. PCC 6803 contains both heme and phycocyanin, and is capable of synthesizing phycocyanin using heme as a precursor. The aim of this study was to uncover viable metabolic targets in the porphyrin pathway from Synechocystis sp. PCC 6803 to promote the accumulation of heme and phycocyanin in the recombinant strains of microalgae. A total of 10 genes related to heme synthesis pathway derived from Synechococcus elongatus PCC 7942 and 12 genes related to endogenous heme synthesis were individually overexpressed in strain PCC 6803. The growth rate and pigment content (heme, phycocyanin, chlorophyll a and carotenoids) of 22 recombinant algal strains were characterized. Quantitative real-time PCR technology was used to investigate the molecular mechanisms underlying the changes in physiological indicators in the recombinant algal strains. Among the 22 mutant strains, the mutant overexpressing the haemoglobin gene (glbN) of strain PCC 6803 had the highest heme content, which was 2.5 times higher than the wild type; the mutant overexpressing the gene of strain PCC 7942 (hemF) had the highest phycocyanin content, which was 4.57 times higher than the wild type. Overall, the results suggest that genes in the porphyrin pathway could significantly affect the heme and phycocyanin content in strain PCC 6803. Our study provides novel crucial targets for promoting the accumulation of heme and phycocyanin in cyanobacteria.

Enhancement of phycocyanobilin biosynthesis in Escherichia coli by strengthening the supply of precursor and artificially self-assembly complex

Phycocyanobilin (PCB) is widely used in healthcare, food processing, and cosmetics. Escherichia coli is the common engineered bacterium used to produce PCB. However, it still suffers from low production level, precursor deficiency, and low catalytic efficiency. In this study, a highly efficient PCB-producing strain was created. First, chassis strains and enzyme sources were screened, and copy numbers were optimized, affording a PCB titer of 9.1 mg/L. Most importantly, the rate-limiting steps of the PCB biosynthetic pathway were determined, and the supply of precursors necessary for PCB synthesis was increased from endogenous sources, affording a titer of 21.4 mg/L. Then, the key enzymes for PCB synthesis, HO1 and PcyA, were assembled into a multi-enzyme complex using the short peptide tag RIAD-RIDD, and 23.5 mg/L of PCB was obtained. Finally, the basic conditions for PCB fermentation were initially determined in 250 mL shake flasks and a 5-L bioreactor to obtain higher titers of PCB. The final titer of PCB reached 147.0 mg/L, which is the highest reported titer of PCB so far. This research provided the foundation for the industrial production of PCB and its derivatives.

Sustainable and high-level microbial production of plant hemoglobin in Corynebacterium glutamicum

BACKGROUND

Plant hemoglobin shows great potential as a food additive to circumvent the controversy of using animal materials. Microbial fermentation with engineered microorganisms is considered as a promising strategy for sustainable production of hemoglobin. As an endotoxin-free and GRAS (generally regarded as safe) bacterium, Corynebacterium glutamicum is an attractive host for hemoglobin biosynthesis.

RESULTS

Herein, C. glutamicum was engineered to efficiently produce plant hemoglobin. Hemoglobin genes from different sources including soybean and maize were selected and subjected to codon optimization. Interestingly, some candidates optimized for the codon usage bias of Escherichia coli outperformed those for C. glutamicum regarding the heterologous expression in C. glutamicum. Then, saturated synonymous mutation of the N-terminal coding sequences of hemoglobin genes and fluorescence-based high-throughput screening produced variants with 1.66- to 3.45-fold increase in hemoglobin expression level. To avoid the use of toxic inducers, such as isopropyl-β-D-thiogalactopyranoside, two native inducible expression systems based on food additives propionate and gluconate were developed. Promoter engineering improved the hemoglobin expression level by 2.2- to 12.2-fold. Combination of these strategies and plasmid copy number modification allowed intracellular production of hemoglobin up to approximately 20% of total protein. Transcriptome and proteome analyses of the hemoglobin-producing strain revealed the cellular response to excess hemoglobin accumulation. Several genes were identified as potential targets for further enhancing hemoglobin production.

CONCLUSIONS

In this study, production of plant hemoglobin in C. glutamicum was systematically engineered by combining codon optimization, promoter engineering, plasmid copy number modification, and multi-omics-guided novel target discovery. This study offers useful design principles to genetically engineer C. glutamicum for the production of hemoglobin and other recombinant proteins.

Applications of the Whole-Cell System in the Efficient Biosynthesis of Heme

Heme has a variety of functions, from electronic reactions to binding gases, which makes it useful in medical treatments, dietary supplements, and food processing. In recent years, whole-cell system-based heme biosynthesis methods have been continuously explored and optimized as an alternative to the low-yield, lasting, and adverse ecological environment of chemical synthesis methods. This method relies on two biosynthetic pathways of microbial precursor 5-aminolevulinic acid (C4, C5) and three known downstream biosynthetic pathways of heme. This paper reviews the genetic and metabolic engineering strategies for heme production in recent years by optimizing culture conditions and techniques from different microorganisms. Specifically, we summarized and analyzed the possibility of using biosensors to explore new strategies for the biosynthesis of heme from the perspective of synthetic biology, providing a new direction for future exploration.

Microbial Synthesis of Heme b: Biosynthetic Pathways, Current Strategies, Detection, and Future Prospects

Heme b, which is characterized by a ferrous ion and a porphyrin macrocycle, acts as a prosthetic group for many enzymes and contributes to various physiological processes. Consequently, it has wide applications in medicine, food, chemical production, and other burgeoning fields. Due to the shortcomings of chemical syntheses and bio-extraction techniques, alternative biotechnological methods have drawn increasing attention. In this review, we provide the first systematic summary of the progress in the microbial synthesis of heme b. Three different pathways are described in detail, and the metabolic engineering strategies for the biosynthesis of heme b via the protoporphyrin-dependent and coproporphyrin-dependent pathways are highlighted. The UV spectrophotometric detection of heme b is gradually being replaced by newly developed detection methods, such as HPLC and biosensors, and for the first time, this review summarizes the methods used in recent years. Finally, we discuss the future prospects, with an emphasis on the potential strategies for improving the biosynthesis of heme b and understanding the regulatory mechanisms for building efficient microbial cell factories.

Heme biosensor-guided in vivo pathway optimization and directed evolution for efficient biosynthesis of heme

BACKGROUND

Heme has attracted much attention because of its wide applications in medicine and food. The products of genes hemBCDEFY convert 5-aminolevulinic acid to protoporphyrin IX (PPIX; the immediate precursor of heme); protoporphyrin ferrochelatase (FECH) inserts Fe²⁺ into PPIX to generate heme. Biosynthesis of heme is limited by the need for optimized expression levels of multiple genes, complex regulatory mechanisms, and low enzymatic activity; these problems need to be overcome in metabolic engineering to improve heme synthesis.

RESULTS

We report a heme biosensor-guided screening strategy using the heme-responsive protein HrtR to regulate tcR expression in Escherichia coli, providing a quantifiable link between the intracellular heme concentration and cell survival in selective conditions (i.e., the presence of tetracycline). This system was used for rapid enrichment screening of heme-producing strains from a library with random ribosome binding site (RBS) variants and from a FECH mutant library. Through up to four rounds of iterative evolution, strains with optimal RBS intensities for the combination of hemBCDEFY were screened; we obtained a PPIX titer of 160.8 mg/L, the highest yield yet reported in shaken-flask fermentation. A high-activity FECH variant was obtained from the saturation mutagenesis library. Fed-batch fermentation of strain SH20C, harboring the optimized hemBCDEFY and the FECH mutant, produced 127.6 mg/L of heme.

CONCLUSION

We sequentially improved the multigene biosynthesis pathway of PPIX and performed in vivo directed evolution of FECH, based on a heme biosensor, which demonstrated the effectiveness of the heme biosensor-based pathway optimization strategy and broadens our understanding of the mechanism of heme synthesis.

Whole-Cell P450 Biocatalysis Using Engineered Escherichia coli with Fine-Tuned Heme Biosynthesis

By exploiting versatile P450 enzymes, whole-cell biocatalysis can be performed to synthesize valuable compounds in Escherichia coli. However, the insufficient supply of heme limits the whole-cell P450 biocatalytic activity. Here a strategy for improving intracellular heme biosynthesis to enhance the catalytic efficiencies of P450s is reported. After comparing the effects of improving heme transport and biosynthesis on P450 activities, intracellular heme biosynthesis is optimized through the integrated expression of necessary synthetic genes at proper ratios and the assembly of rate-limiting enzymes using DNA-guided scaffolds. The intracellular heme level is fine-tuned by the combined use of mutated heme-sensitive biosensors and small regulatory RNA systems. The catalytic efficiencies of three different P450s, BM3, sca-2, and CYP105D7, are enhanced through fine-tuning heme biosynthesis for the synthesis of hydroquinone, pravastatin, and 7,3',4'-trihydroxyisoflavone as example products of chemical intermediate, drug, and natural product, respectively. This strategy of fine-tuned heme biosynthesis will be generally useful for developing whole-cell biocatalysts involving hemoproteins.

Metabolic Engineering: Methodologies and Applications

Metabolic engineering aims to improve the production of economically valuable molecules through the genetic manipulation of microbial metabolism. While the discipline is a little over 30 years old, advancements in metabolic engineering have given way to industrial-level molecule production benefitting multiple industries such as chemical, agriculture, food, pharmaceutical, and energy industries. This review describes the design, build, test, and learn steps necessary for leading a successful metabolic engineering campaign. Moreover, we highlight major applications of metabolic engineering, including synthesizing chemicals and fuels, broadening substrate utilization, and improving host robustness with a focus on specific case studies. Finally, we conclude with a discussion on perspectives and future challenges related to metabolic engineering.

Efficient De Novo Biosynthesis of Heme by Membrane Engineering in Escherichia coli

Heme is of great significance in food nutrition and food coloring, and the successful launch of artificial meat has greatly improved the application of heme in meat products. The precursor of heme, 5-aminolevulinic acid (ALA), has a wide range of applications in the agricultural and medical fields, including in the treatment of corona virus disease 2019 (COVID-19). In this study, E. coli recombinants capable of heme production were developed by metabolic engineering and membrane engineering. Firstly, by optimizing the key genes of the heme synthesis pathway and the screening of hosts and plasmids, the recombinant strain EJM-pCD-AL produced 4.34 ± 0.02 mg/L heme. Then, the transport genes of heme precursors CysG, hemX and CyoE were knocked out, and the extracellular transport pathways of heme Dpp and Ccm were strengthened, obtaining the strain EJM-ΔCyoE-pCD-AL that produced 9.43 ± 0.03 mg/L heme. Finally, fed-batch fermentation was performed in a 3-L fermenter and reached 28.20 ± 0.77 mg/L heme and 303 ± 1.21 mg/L ALA. This study indicates that E. coli recombinant strains show a promising future in the field of heme and ALA production.

High-level secretory production of leghemoglobin in Pichia pastoris through enhanced globin expression and heme biosynthesis

Soy leghemoglobin is a key food additive that imparts meaty flavor and color to meat analogs. Here, a Pichia pastoris strain capable of high-yield secretory production of functional leghemoglobin was developed through gene dosage optimization and heme pathway consolidation. First, multi-copy integration of LegH expression cassette was achieved via both post-transformational vector amplification and CRISPR/Cas9 mediated genome editing methods. A combination of inducible expression and constitutive expression resulted in the highest production of leghemoglobin. Then, heme biosynthetic pathway was engineered to address challenges in heme depletion and leghemoglobin secretion. Finally, the disruption of ku70 was complemented in engineered P. pastoris strain to enable high-density fermentation in a 10-L bioreactor. These engineering strategies increased the secretion of leghemoglobin by more than 83-fold, whose maximal leghemoglobin titer and heme binding ratio reached as high as 3.5 g/L and 93 %, respectively. This represents the highest secretory production of heme-containing proteins ever reported.

A pseudokinase version of the histidine kinase ChrS promotes high heme tolerance of Corynebacterium glutamicum

Heme is an essential cofactor for almost all living cells by acting as prosthetic group for various proteins or serving as alternative iron source. However, elevated levels are highly toxic for cells. Several corynebacterial species employ two paralogous, heme-responsive two-component systems (TCS), ChrSA and HrrSA, to cope with heme stress and to maintain intracellular heme homeostasis. Significant cross-talk at the level of phosphorylation between these systems was previously demonstrated. In this study, we have performed a laboratory evolution experiment to adapt Corynebacterium glutamicum to increasing heme levels. Isolated strains showed a highly increased tolerance to heme growing at concentrations of up to 100 μM. The strain featuring the highest heme tolerance harbored a frameshift mutation in the catalytical and ATPase-domain (CA-domain) of the chrS gene, converting it into a catalytically-inactive pseudokinase (ChrS_CA-fs). Reintroduction of the respective mutation in the parental C. glutamicum strain confirmed high heme tolerance and showed a drastic upregulation of hrtBA encoding a heme export system, conserved in Firmicutes and Actinobacteria. The strain encoding the ChrS pseudokinase variant showed significantly higher heme tolerance than a strain lacking chrS. Mutational analysis revealed that induction of hrtBA in the evolved strain is solely mediated via the cross-phosphorylation of the response regulator (RR) ChrA by the kinase HrrS and BACTH assays revealed the formation of heterodimers between HrrS and ChrS. Overall, our results emphasize an important role of the ChrS pseudokinase in high heme tolerance of the evolved C. glutamicum and demonstrate the promiscuity in heme-dependent signaling of the paralogous two-component systems facilitating fast adaptation to changing environmental conditions.

Improved production of heme using metabolically engineered Escherichia coli

Heme has recently attracted much attention due to its promising applications in food and healthcare industries. However, the current titers and productivities of heme produced by recombinant microorganisms are not high enough for a wide range of applications. In this study, the process for the fermentation of the metabolically engineered E. coli HAEM7 strain was optimized for the high-level production of heme. To improve the production of heme, different carbon sources, iron concentration in the medium, pH control strategies, induction points, and iron content in feeding solution were examined. Moreover, strategies of increasing cell density, regular iron supplementation, and supply of excess feeding solution were developed to further improve the production of heme. In the optimized fermentation process, the HAEM7 strain produced 1.03 g/L heme with a productivity of 21.5 mg/L/h. The fermentation process and strategies reported here will expedite establishing industry-level production of heme. This article is protected by copyright. All rights reserved.

Bio-isopropanol production in Corynebacterium glutamicum: Metabolic redesign of synthetic bypasses and two-stage fermentation with gas stripping

Isopropanol is a commodity chemical widely used as a biofuel, fuel additive, rubbing alcohol and intermediate in various fields. Here, an engineered Corynebacterium glutamicum overproducing isopropanol was developed. To our knowledge, despite a representative industrial host to produce valuable chemicals, the high-level production of isopropanol in C. glutamicum has never been reported. First, the problem of the inability to produce isopropanol was solved by finding a key factor in its metabolism. The consolidation and modular optimization of synthetic bypasses including succinate and mevalonate bypasses enhanced isopropanol production. Flux redistribution of central metabolism significantly directed the carbon flux toward isopropanol biosynthesis. The final engineered strain produced 10.25 ± 1.12 g/L isopropanol in two-stage fed-batch fermentation with an optimized gas stripping, which is the highest titer, yield and productivity in C. glutamicum. These strategies could be useful for the high-level production of isopropanol in C. glutamicum.

The diversity of heme sensor systems - heme-responsive transcriptional regulation mediated by transient heme protein interactions

Heme is a versatile molecule that is vital for nearly all cellular life by serving as prosthetic group for various enzymes or as nutritional iron source for diverse microbial species. However, elevated levels of heme molecule are toxic to cells. The complexity of this stimulus has shaped the evolution of diverse heme sensor systems, which are involved in heme-dependent transcriptional regulation in eukaryotes and prokaryotes. The functions of these systems are manifold - ranging from the specific control of heme detoxification or uptake systems to the global integration of heme and iron homeostasis. This review focuses on heme sensor systems, regulating heme homeostasis by transient heme protein interaction. We provide an overview of known heme-binding motifs in prokaryotic and eukaryotic transcription factors. Besides the central ligands, the surrounding amino acid environment was shown to play a pivotal role in heme binding. The diversity of heme-regulatory systems therefore illustrates that prediction based on pure sequence information is hardly possible and requires careful experimental validation. Comprehensive understanding of heme-regulated processes is not only important for our understanding of cellular physiology, but also provides a basis for the development of novel antibacterial drugs and metabolic engineering strategies.

Recent advances in microbial production of high-value compounds in the tetrapyrrole biosynthesis pathway

Tetrapyrroles are essential metabolic components produced by almost all organisms, and they participate in various fundamental biological processes. Tetrapyrroles are used as pharmaceuticals, food additives, and nutraceuticals, as well as in agricultural applications. However, their production is limited by their low extraction yields from natural resources and by the complex reaction steps involved in their chemical synthesis. Through advances in metabolic engineering and synthetic biology strategies, microbial cell factories were developed as an alternative method for tetrapyrrole production. Herein, we review recent developments in metabolic engineering and synthetic biology strategies that promote the microbial production of high-value compounds in the tetrapyrrole biosynthesis pathway (e.g., 5-aminolevulinic acid, heme, bilins, chlorophyll, and vitamin B₁₂). Furthermore, outstanding challenges to the microbial production of tetrapyrrole compounds, as well as their possible solutions, are discussed.

Next-Generation Genetic and Fermentation Technologies for Safe and Sustainable Production of Food Ingredients: Colors and Flavorings

A growing human population is a significant issue in food security owing to the limited land and resources available for agricultural food production. To solve these problems, sustainable food manufacturing processes and the development of alternative foods and ingredients are needed. Metabolic engineering and synthetic biology can help solve the food security issue and satisfy the demand for alternative food production. Bioproduction of food ingredients by microbial fermentation is a promising method to replace current manufacturing processes, such as extraction from natural materials and chemical synthesis, with more ecofriendly and sustainable operations. This review highlights successful examples of bioproduction for food additives by engineered microorganisms, with an emphasis on colorants and flavors that are extensively used in the food industry. Recent strain engineering developments and fermentation strategies for producing selected food colorants and flavors are introduced with discussions on the current status and future perspectives.

Expected final online publication date for the Annual Review of Food Science and Technology, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.