Reduction of exogenous ketones depends upon NADPH generated photosynthetically in cells of the cyanobacterium Synechococcus PCC 7942

Contribution of the alternative pathway on spring rejuvenation of alfalfa

Alfalfa often suffers from low temperature during spring rejuvenation, so it is important to improve the cold tolerance of alfalfa leaves for its smooth rejuvenation, and the alternative pathway (AP) could effectively improve the plant's tolerance. In this study, the contribution of AP on spring rejuvenation of alfalfa was investigated in Xinmu No.4 and Gannong No.5 with different fall dormancy levels. Though the protein and AP capacity were decreased during the rejuvenation, the ratio of AP/TP were increased in two alfalfa varieties, compared to those in alfalfa before overwintering. This indicated that AP had positive response to alfalfa rejuvenation. The limitation of AP significantly affected the leaf length, leaf width and growth rate of greening alfalfa, showing that AP played an important role in alfalfa rejuvenation. Inhibition of AP resulted in a significant decrease in Pn, Ci, Gs and stomatal structure deformity, suggestion that AP affected photosynthesis by influencing stomatal development during rejuvenation. AP reduces oxidative damage to PSII core protein repair in alfalfa leaves and optimizes photosynthesis by up-regulating NADP-MDH activity, decreasing the accumulation of excess reducing power in the chloroplasts, and by increasing SOD and POD activities and decreasing the accumulation of hydrogen peroxide. The higher proportion of AP keeps it more tolerant to low temperature for rejuvenation in Xinmu No.4 with a lower fall dormancy level.

Cyanobacteria-Mediated Light-Driven Biotransformation: The Current Status and Perspectives

Most chemicals are manufactured by traditional chemical processes but at the expense of toxic catalyst use, high energy consumption, and waste generation. Biotransformation is a green, sustainable, and cost-effective process. As cyanobacteria can use light as the energy source to power the synthesis of NADPH and ATP, using cyanobacteria as the chassis organisms to design and develop light-driven biotransformation platforms for chemical synthesis has been gaining attention, since it can provide a theoretical and practical basis for the sustainable and green production of chemicals. Meanwhile, metabolic engineering and genome editing techniques have tremendous prospects for further engineering and optimizing chassis cells to achieve efficient light-driven systems for synthesizing various chemicals. Here, we display the potential of cyanobacteria as a promising light-driven biotransformation platform for the efficient synthesis of green chemicals and current achievements of light-driven biotransformation processes in wild-type or genetically modified cyanobacteria. Meanwhile, future perspectives of one-pot enzymatic cascade biotransformation from biobased materials in cyanobacteria have been proposed, which could provide additional research insights for green biotransformation and accelerate the advancement of biomanufacturing industries.

Internal Illumination to Overcome the Cell Density Limitation in the Scale-up of Whole-Cell Photobiocatalysis

Cyanobacteria have the capacity to use photosynthesis to fuel their metabolism, which makes them highly promising production systems for the sustainable production of chemicals. Yet, their dependency on visible light limits the cell-density, which is a challenge for the scale-up. Here, it was shown with the example of a light-dependent biotransformation that internal illumination in a bubble column reactor equipped with wireless light emitters (WLEs) could overcome this limitation. Cells of the cyanobacterium Synechocystis sp. PCC 6803 expressing the gene of the ene-reductase YqjM were used for the reduction of 2-methylmaleimide to (R)-2-methylsuccinimide with high optical purity (>99 % ee). Compared to external source of light, illumination by floating wireless light emitters allowed a more than two-fold rate increase. Under optimized conditions, product formation rates up to 3.7 mm h-1 and specific activities of up to 65.5 U gDCW -1 were obtained, allowing the reduction of 40 mm 2-methylmaleimide with 650 mg isolated enantiopure product (73 % yield). The results demonstrate the principle of internal illumination as a means to overcome the intrinsic cell density limitation of cyanobacterial biotransformations, obtaining high reaction rates in a scalable photobioreactor.

Reductive capabilities of different cyanobacterial strains towards acetophenone as a model substrate - Prospect of applications for chiral building blocks synthesis

Bioreductive capabilities of four morphologically different strains of cyanobacteria have been assessed in this work. Arthrospira maxima, Leptolyngbya foveolarum, Nodularia sphaerocarpa and Synechococcus bigranulatus were applied as catalysts for the reduction of acetophenone to the corresponding chiral phenylethyl alcohol. The process was modified regarding substrate concentration, duration of pre-cultivation period, duration of biotransformation, light regime and glucose addition to the culture media. Obtained results clearly showed that cyanobacteria were active towards acetophenone what resulted in the substrate reduction to (S)-1-phenylethanol with high enantiomeric excess. The reaction efficiency increased with the biotransformation time, but the higher concentration of substrate limited the process yield. Also, all tested strains performed reaction with the highest efficacy under continuous light regime. The most active strains - N. sphaerocarpa and S. bigranulatus carried out the conversion of 1 mM acetophenone with high efficiency of respectively 97.6% and 96.2% after 13 days of biotransformation. A. maxima reached 45.8% of conversion after 13 days of biotransformation whereas L. foveolarum did not exceed 20%. The enantiomeric excesses were respectively 98.8%- A. maxima, 91.7%- L. foveolarum, 72.6%- S. bigranulatus and N. sphaerocarpa 16.2%.

The effect of CO2 in enhancing photosynthetic cofactor recycling for alcohol dehydrogenase mediated chiral synthesis in cyanobacteria

The light harvesting photosystem in cyanobacteria offers a potential pathway for the regeneration of the nicotinamide cofactor NADPH, thereby facilitating the application of cyanobacteria as excellent whole cell biocatalysts in oxidoreductase-mediated biotransformation. The use of cyanobacterial metabolism for cofactor recycling improves the atom economy of the process compared to the commonly employed enzyme-coupled cofactor recycling using enzymes such as glucose dehydrogenase. Here we report the asymmetric conversion of acetophenone to chiral 1-phenylethanol by recombinant Synechococcus elongatus PCC 7942 whole cell biocatalyst that expresses the NADPH dependent L. kefir alcohol dehydrogenase. Besides light, it was observed that carbon dioxide levels play a critical role in improving the bioconversion efficiency possibly due to the enhanced growth rate and improved cofactor availability at elevated CO₂ levels. Complete reduction of acetophenone to optically pure (R)-1-phenylethanol at 99% enantiomeric excess was achieved within 6 h with a relatively low cell density of 0.66 g/l by coupling optimum light and CO₂ levels and without the need for a co-substrate.

Enhanced bioconversion rate and released substrate inhibition in (R)-phenylephrine whole-cell bioconversion via partial acetone treatment

An approach was developed to enhance the efficiency for the bioconversion of 1-(3-hydroxyphenyl)-2-(methyamino)-ethanone to (R)-phenylephrine. The strain Serratia marcescens N10612, giving the benefit of 99% enantiomeric excess in (R)-PE conversion, was used. The fermentation was devised to harvest cells with high hydrophobic prodigiosin content inside the cells. Then, the partial acetone extraction was applied to remove prodigiosin from the cells. The treatment was found to increase the cells conversion rate without loss of the cells NADPH redox system. When using 50% (v/v) acetone for 5min, the processed cells can give a specific conversion rate of 16.03μmol/h/g-cells. As compared the treated cells with cells under the basal medium, the maximum reaction rate (Vmax) increased from 6.69 to 10.27 (μmol/h/g-cells), the dissociation constant (Km) decreased from 0.236 to 0.167mM and the substrate inhibition constant (KSi) increased from 0.073 to 1.521mM. The 20-fold increase in substrate inhibition constant referred to a great release from the substrate inhibition for the use of S. marcescens N10612 in the bioconversion, which would greatly benefit the bioconversion to be industrialized.

Asymmetric reduction of 4-hydroxy-2-butanone to (R)-1,3-butanediol with absolute stereochemical selectivity by a newly isolated strain of Pichia jadinii

In this study, a novel strain of Pichia jadinii, HBY61, capable of the biocatalysis of 4-hydroxy-2-butanone (4H2B) to (R)-1,3-BD was isolated. HBY61 produced (R)-1,3-BD with high activity and absolute stereochemical selectivity (100 % e.e). Glucose and beef extract were found to be the key factors governing the fermentation, and their optimal concentrations were determined to be 84.2 and 43.7 g/L, respectively. The optimal bioconversion conditions of 4H2B catalyzed by HBY61 were pH 7.4, 30 °C, and 250 rpm with 6 % (v/v) glucose as the co-substrate. Accordingly, when 45 g/L of 4H2B was divided into three equal parts and added successively into the system at set time intervals, the maximum (R)-1,3-BD concentration reached 38.3 g/L with high yield (85.1 %) and strict 100 % enantioselectivity. Compared with previously reported yields for the biocatalytic production of (R)-1,3-BD, the use of strain HBY61 provided a high yield with excellent stereoselectivity.

Mechanisms by which the infection of Sclerotinia sclerotiorum (Lib.) de Bary affects the photosynthetic performance in tobacco leaves

BACKGROUND

Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic fungal pathogen which causes disease in a wide range of plants. An observed decrease in photosynthetic performance is the primary reason for the reduction of crop yield induced by S. sclerotiorum. The H2C2O4 is the main pathogenic material secreted by S. sclerotiorum, but the effects of H2C2O4 acidity and the C2O4 2- ion on photosynthetic performance remain unknown.

RESULTS

S. sclerotiorum infection significantly decreased photosynthetic O2 evolution and the maximum quantum yield of photosystem II (Fv/Fm) in tobacco leaves under high-light. H2C2O4 (the main pathogenic material secreted by S. sclerotiorum) with pH 4.0 also significantly decreased photosynthetic performance. However, treatment with H3PO4 and HCl at the same pH as H2C2O4 caused much less decrease in photosynthetic performance than H2C2O4 did. These results verify that the acidity of the H2C2O4 secreted by S. sclerotiorum was only partially responsible for the observed decreases in photosynthesis. Treatment with 40 mM K2C2O4 decreased Fv/Fm by about 70% of the levels observed under 40 mM H2C2O4, which further demonstrates that C2O4(2-) was the primary factor that impaired photosynthetic performance during S. sclerotiorum infection. K2C2O4 treatment did not further decrease photosynthetic performance when D1 protein synthesis was fully inhibited, indicating that C2O4(2-) inhibited PSII by repressing D1 protein synthesis. It was observed that K2C2O4 treatment inhibited the rate of RuBP regeneration and carboxylation efficiency. In the presence of a carbon assimilation inhibitor, K2C2O4 2 treatment did not further decrease photosynthetic performance, which infers that C2O4(2-) inhibited PSII activity partly by repressing the carbon assimilation. In addition, it was showed that C2O4(2-) treatment inhibited the PSII activity but not the PSI activity.

CONCLUSIONS

This study demonstrated that the damage to the photosynthetic apparatus induced by S. sclerotiorum is not only caused by the acidity of H2C2O4, but also by C2O4(2-) which plays a much more important role in damaging the photosynthetic apparatus. C2O4(2-) inhibits PSII activity, as well as the rate of RuBP regeneration and carboxylation efficiency, leading to the over production of reactive oxygen species (ROS). By inhibiting the synthesis of D1, ROS may further accelerate PSII photoinhibition.