Metabolic engineering of microorganisms for the production of multifunctional non-protein amino acids: γ-aminobutyric acid and δ-aminolevulinic acid

A comprehensive review on the recent advances for 5-aminolevulinic acid production by the engineered bacteria

5-Aminolevulinic acid (5-ALA) is a non-proteinogenic amino acid essential for synthesizing tetrapyrrole compounds, including heme, chlorophyll, cytochrome, and vitamin B12. As a plant growth regulator, 5-ALA is extensively used in agriculture to enhance crop yield and quality. The complexity and low yield of chemical synthesis methods have led to significant interest in the microbial synthesis of 5-ALA. Advanced strategies, including the: enhancement of precursor and cofactor supply, compartmentalization of key enzymes, product transporters engineering, by-product formation reduction, and biosensor-based dynamic regulation, have been implemented in bacteria for 5-ALA production, significantly advancing its industrialization. This article offers a comprehensive review of recent developments in 5-ALA production using engineered bacteria and presents new insights to propel the field forward.

Developing a Bacillus licheniformis platform for de novo production of γ-aminobutyric acid and other glutamate-derived chemicals

Microbial cell factories (MCFs) have emerged as a sustainable tool for the production of value-added biochemicals. However, developing high-performance MCFs remains a major challenge to fulfill the burgeoning demands of global markets. This study aimed to establish the B. licheniformis cell factory for the cost-effective production of glutamate-derived chemicals by modular metabolic engineering. Initially, the glutamate decarboxylase from E. coli was introduced into B. licheniformis DW2 to construct the artificial γ-aminobutyric acid (GABA) pathway. By systematically optimizing the central metabolic pathway, boosting the L-Glu synthesis pathway and improving the cofactor NADPH supply, the strain G35/pHY-Pr5u12-gadBE89Q/H465A achieved a remarkable yield of 62.9 g/L of GABA in a 5-L bioreactor, representing the highest yield of 0.5 g/g glucose with a significant 49.3-fold increase. Remarkably, bioinformatics analyses and function verification identified the putative glyoxylate to glycolic acid synthesis pathway and KipR, an inhibitor of the glyoxylate cycle, as the rate-limiting steps in GABA production. Additionally, a versatile and robust platform using engineered B. licheniformis for efficient production of diverse glutamate-derived chemicals was established and the titer of 5-aminolevulinic acid, heme and indigoidine was improved by 5.3-, 4.7- and 1.9-fold, respectively. This study not only facilitates extensive application of B. licheniformis for chemical production, but also sheds light on research to improve the performance of other MCFs.

Insight into the surface discharge cold plasma efficient inactivation of Pseudomonas fluorescens in water based on exogenous reactive oxygen and nitrogen species: Synergistic mechanism and energy benefits

As well known, surface discharge cold plasma has efficient inactivation ability and a variety of RONS are main active particles for inactivation, but their synergistic mechanism is still not clear. Therefore, surface discharge cold plasma system was applied to treat Pseudomonas fluorescens to study bacterial inactivation mechanism and energy benefit. Results showed that energy efficiency was directly proportional to applied voltage and inversely proportional to initial concentration. Cold plasma treatment for 20 min was inactivated by approximately > 4-log10Pseudomonas fluorescens and application of •OH and 1O2 scavengers significantly improved survival rate. In addition, •OH and 1O2 destroyed cell membrane structure and membrane permeability, which promoted diffusion of RONS into cells and affecting energy metabolism and antioxidant capacity, leading to bacterial inactivation. Furthermore, accumulation of intracellular NO and ONOOH was related to infiltration of exogenous RNS, while accumulation of •OH, H2O2, 1O2, O2- was the result of joint action of endogenous and exogenous ROS. Transcriptome analysis revealed that different RONS of cold plasma were responsible for Pseudomonas fluorescens inactivation and related to activation of intracellular antioxidant defense system and regulation of genes expression related to amino acid metabolism and energy metabolism, which promoting cellular process, catalytic activity and other biochemical pathways.

Construction of the Rhodobacter sphaeroides strain overproducing 5-aminolevulinic acid by insertion of endogenous promoter

5-Aminolevulinic acid (ALA) is a precursor of heme and a natural amino acid synthesized in the cells of most living organisms. Currently, ALA is used as an ingredient in pharmaceuticals, supplements, cosmetics, feed, fertilizers, and other products. ALA is mainly produced by industrial fermentation by the photosynthetic bacterium Rhodobacter sphaeroides. In this study, we tried to improve the ALA productivity by R. sphaeroides using a genetic strategy to highly express ALA synthase (ALAS) genes. We inserted a constitutive promoter (PrrnB or Prsp_7571) upstream of genes encoding ALAS (hemA and/or hemT) to construct strains that constitutively express ALAS. The highest transcript levels of hemA were observed in the strain where PrrnB was inserted into the hemA promoter region and were 3.5-fold higher than those in the wild-type. The highest transcript levels of hemT were observed in the strain where PrrnB was inserted into the hemT promoter region and were 46-fold higher than those in the wild-type. The maximum ALAS activity was observed in crude cell extracts of the strain where PrrnB was inserted into the hemT promoter region under optimized growth conditions that was 2.7-fold higher than that in the wild type. This strain showed 12-fold accumulation of ALA compared to the wild-type. Thus, we improved ALA productivity without using exogenous DNA sequences. In the future, further improvement in ALA productivity may be expected by applying this approach to current industrial ALA-producing bacteria.

Efficacy and safety of fractional carbon dioxide laser followed by 5-aminolevulinic acid photodynamic therapy for keloids

BACKGROUND

Keloids are aggressive fibroproliferative disorders that cause aesthetic and functional damage. Photodynamic therapy (PDT) has shown promise as a novel treatment for keloids. However, the limited penetration of 5-aminolevulinic acid (ALA) and unsatisfactory outcomes in dense scars hinder its effectiveness as a monotherapy. The objective of this study is to assess the efficacy and safety of fractional CO2 laser followed by 5-ALA PDT for keloids.

METHODS

A total of 12 patients with keloid were included in our study. Each lesion was pretreated by fractional CO2 laser with 26-28 W to create microthermal zones. After topical application of 5-ALA solution, an irradiation of 635 nm red light with 120 J/cm2 was performed. The treatment was repeated at least every 2 weeks. Efficacy and safety were evaluated using the Vancouver Scar Scale (VSS), the Visual Analogue Scale (VAS) for keloid-related symptoms and documentation of postoperative complications. Statistical analysis was performed to compare VSS and keloid-related symptom VAS scores of the baseline and final treatment sessions.

RESULTS

The final treatment resulted in a statistically significant decrease in all parameters of VSS and VAS for pruritus and pain compared to the baseline. Except for postoperative hyperpigmentation, no infections, scar aggravation, or recurrence were observed during at least 6 months of follow-up. Overall, patients expressed a high level of satisfaction with the treatment outcome.

CONCLUSIONS

Fractional CO2 laser followed by 5-ALA PDT is a promising method for treating keloids. However, its synergetic effects need to be validated through clinical trials involving larger patient cohorts.

Enhancing the activity of disulfide-bond-containing proteins via promoting disulfide bond formation in Bacillus licheniformis

Disulfide bonds in proteins have strongly influence on the folding efficiency by constraining the conformational space. The inefficient disulfide bond formation of proteins is the main limiting factor of enzyme activity and stability. This study aimed to increase the activity of disulfide-bond-containing proteins via promoting disulfide bonds formation in Bacillus licheniformis. Initially, the glutamate decarboxylase GAD from Escherichia coli was selected as the model protein and introduced into the B. licheniformis. Then, the disulfide isomerase and oxidoreductase from different sources were excavated and overexpressed successively to improve the catalytic efficiency of GAD. The final engineered B. licheniformis showed significantly improved GAD specific activity (from 10.4 U/mg to 80.0 U/mg), which also presented perfect adaptability for other disulfide-bond-containing proteins, for instance, UDP-glucosyltransferase from Arabidopsis thaliana. Taken together, our work demonstrated that the activity of GAD in B. licheniformis was regulated by the disulfide bonds formation status and provided a promising platform for the expression of disulfide-bond-containing proteins.

Methanotrophs as a reservoir for bioactive secondary metabolites: Pitfalls, insights and promises

Methanotrophs are potent natural producers of several bioactive secondary metabolites (SMs) including isoprenoids, polymers, peptides, and vitamins. Cryptic biosynthetic gene clusters identified from these microbes via genome mining hinted at the vast and hidden SM biosynthetic potential of these microbes. Central carbon metabolism in methanotrophs offers rare pathway intermediate pools that could be further diversified using advanced synthetic biology tools to produce valuable SMs; for example, plant polyketides, rare carotenoids, and fatty acid-derived SMs. Recent advances in pathway reconstruction and production of isoprenoids, squalene, ectoine, polyhydroxyalkanoate copolymer, cadaverine, indigo, and shinorine serve as proof-of-concept. This review provides theoretical guidance for developing methanotrophs as microbial chassis for high-value SMs. We summarize the distinct secondary metabolic potentials of type I and type II methanotrophs, with specific attention to products relevant to biomedical applications. This review also includes native and non-native SMs from methanotrophs, their therapeutic potential, strategies to induce silent biosynthetic gene clusters, and challenges.

A Non-functional γ-Aminobutyric Acid Shunt Pathway in Cyanobacterium Synechocystis sp. PCC 6803 Enhances δ-Aminolevulinic Acid Accumulation under Modified Nutrient Conditions

To redirect carbon flux from the γ-aminobutyric acid (GABA) shunt to the δ-aminolevulinic acid (ALA) biosynthetic pathway, we disrupted the GABA shunt route of the model cyanobacterium Synechocystis sp. PCC 6803 by inactivating Gdc, the gene-encoding glutamate decarboxylase. The generated ΔGdc strain exhibited lower intracellular GABA and higher ALA levels than the wild-type (WT) one. The ΔGdc strain’s ALA levels were ~2.8 times higher than those of the WT one when grown with levulinic acid (LA), a competitive inhibitor of porphobilinogen synthase. Abiotic stress conditions including salinity induced by 10 mM NaCl and cold at 4 °C increased the ALA levels in ΔGdc up to ~2.5 and 5 ng g−1 cell DW, respectively. The highest ALA production in the ΔGdc cyanobacteria grown in BG11 medium was triggered by glucose induction, followed by glutamate supplementation with 60 mM of LA, thereby resulting in ~360 ng g−1 cell DW of ALA, that is >300-fold higher ALA accumulation than that observed in ΔGdc cyanobacteria grown in normal medium. Increased levels of the gdhA (involved in the interconversion of α-ketoglutarate to glutamate) and the hemA (a major regulatory target of the ALA biosynthetic pathway) transcripts occurred in ΔGdc cyanobacteria grown under modified growth conditions. Our study provides critical insight into the facilitation of ALA production in cyanobacteria.

Evolutionary Trend Analysis of Research on 5-ALA Delivery and Theranostic Applications Based on a Scientometrics Study

5-aminolevulinic acid (5-ALA) has been extensively studied for its sustainability and broad-spectrum applications in medical research and theranostics, as well as other areas. It’s a precursor of protoporphyrin IX (PpIX), a sustainable endogenous and naturally-existing photosensitizer. However, to the best of our knowledge, a scientometrics study based on the scientific knowledge assay of the overall situation on 5-ALA research has not been reported so far, which would be of major importance to the relevant researchers. In this study, we collected all the research articles published in the last two decades from the Web of Science Core Collection database and employed bibliometric methods to comprehensively analyze the dataset from different perspectives using CiteSpace. A total of 1595 articles were identified. The analysis results showed that China published the largest number of articles, and SBI Pharmaceuticals Co., Ltd. was the most productive institution that sponsored several of the most productive authors. The cluster analysis and burst detections indicated that the improvement of photodynamic efficacy theranostics is the up-to-date key direction in 5-ALA research. Furthermore, we emphatically studied nanotechnology involvement in 5-ALA delivery and theranostics research. We envision that our results will be beneficial for researchers to have a panorama of and deep insights into this area, thus inspiring further exploitations, especially of the nanomaterial-based systems for 5-ALA delivery and theranostic applications.

Root Exposure to 5-Aminolevulinic Acid (ALA) Affects Leaf Element Accumulation, Isoprene Emission, Phytohormonal Balance, and Photosynthesis of Salt-Stressed Arundo donax

Arundo donax has been recognized as a promising crop for biomass production on marginal lands due to its superior productivity and stress tolerance. However, salt stress negatively impacts A. donax growth and photosynthesis. In this study, we tested whether the tolerance of A. donax to salinity stress can be enhanced by the addition of 5-aminolevulinic acid (ALA), a known promoter of plant growth and abiotic stress tolerance. Our results indicated that root exposure to ALA increased the ALA levels in leaves along the A. donax plant profile. ALA enhanced Na+ accumulation in the roots of salt-stressed plants and, at the same time, lowered Na+ concentration in leaves, while a reduced callose amount was found in the root tissue. ALA also improved the photosynthetic performance of salt-stressed apical leaves by stimulating stomatal opening and preventing an increase in the ratio between abscisic acid (ABA) and indol-3-acetic acid (IAA), without affecting leaf methanol emission and plant growth. Supply of ALA to the roots reduced isoprene fluxes from leaves of non-stressed plants, while it sustained isoprene fluxes along the profile of salt-stressed A. donax. Thus, ALA likely interacted with the methylerythritol 4-phosphate (MEP) pathway and modulate the synthesis of either ABA or isoprene under stressful conditions. Overall, our study highlights the effectiveness of ALA supply through soil fertirrigation in preserving the young apical developing leaves from the detrimental effects of salt stress, thus helping of A. donax to cope with salinity and favoring the recovery of the whole plant once the stress is removed.