Cofactor regeneration in phototrophic cyanobacteria applied for asymmetric reduction of ketones

Combination of multi-enzyme expression fine-tuning and co-substrates addition improves phenyllactic acid production with an Escherichia coli whole-cell biocatalyst

The aim of this study was to develop an environmentally safe and efficient method for phenyllactic acid (PLA) production using whole-cell cascade catalysis with l-amino acid deaminase (l-AAD), lactate dehydrogenase (LDH), and formate dehydrogenase (FDH). The PPA titer was low due to relatively low expression of LDH, intermediate accumulation, and lack of cofactors. To address this issue, ribosome binding site regulation, gene duplication, and induction optimization were performed to increased the PLA titer to 43.8 g/L. Then co-substrates (glucose, yeast extract, and glycerol) were used to increase NADH concentration and cell stability, resulting that the PLA titer was increased to 54.0 g/L, which is the highest reported production by biocatalyst. Finally, glucose was replaced with wheat straw hydrolysate as co-substrate to decrease the cost. Notably, the strategies reported herein may be generally applicable to other whole-cell cascade biocatalysts.

Metabolic Redesign of Rhodobacter sphaeroides for Lycopene Production

Lycopene plays an important role as an antioxidative and anticancer agent, and is an increasingly valuable commodity in the global market. Rhodobacter sphaeroides, a carotenogenic and phototrophic bacterium, is an efficient and practical host for carotenoid production. Herein, we explored the potential of metabolically engineered Rb. sphaeroides as a novel platform to produce lycopene. The basal lycopene-producing strain was generated by introducing an exogenous crtI₄ from Rhodospirillum rubrum to replace the native crtI₃ and deleting crtC in Rb. sphaeroides. Furthermore, knocking out zwf blocked the competitive pentose phosphate pathway and improved the lycopene content by 88%. Finally, the methylerythritol phosphate pathway was reinforced by integration of dxs combined with zwf deletion, which further increased the lycopene content. The final engineered strain produced lycopene to 10.32 mg/g dry cell weight. This study describes a new lycopene producer and provides insight into a photosynthetic bacterium as a host for lycopene biosynthesis.

Production of chiral alcohols from prochiral ketones by microalgal photo-biocatalytic asymmetric reduction reaction

Microalgal photo-biocatalysis is a green technique for asymmetric synthesis. Asymmetric reduction of nonnatural prochiral ketones to produce chiral alcohols by microalgal photo-biocatalysis was studied in this work. Acetophenone (ACP) and ethyl acetoacetate (EAA) were chosen as model substrates for aromatic ketones and β-ketoesters, respectively. Two prokaryotic cyanophyta and two eukaryotic chlorophyta were selected as photo-biocatalysts. The results proved that nonnatural prochiral ketones can be reduced by microalgal photo-biocatalysis with high enantioselectivity. Illumination is indispensable to the photo-biocatalysis. For aromatic ketone, cyanophyta are eligible biocatalysts. For ACP asymmetric reduction reaction, about 45% yield and 97% e.e. can be achieved by the photo-biocatalysis reaction with Spirulina platensis as biocatalyst. On the contrary, chlorophyta are efficient biocatalysts for β-ketoester asymmetric reduction reaction among the four tested algae. For EAA asymmetric reduction reaction, about 70% yield and 90% e.e. can be achieved with Scenedesmus obliquus as biocatalyst. The microalgae used in this study outperformed other characterized biocatalysts such as microbial and plant cells.

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

Effective utilization of photosynthetic microorganisms as potential biocatalysts is favorable for the production of useful biomaterials and the reduction of atmospheric CO2. For example, biocatalytic transformations are used in the synthesis of optically active alcohols. We previously found that ketone reduction in cells of the cyanobacterium Synechococcus PCC 7942 is highly enantioselective and remarkably enhanced under light illumination. In this study, the mechanism of light-enhanced ketone reduction was investigated in detail using several inhibitors of photosynthetic electron transport and of enzymes of the Calvin cycle. It is demonstrated that light intensity and photosynthesis inhibitors significantly affect the ketone reduction activity in Synechococcus. This indicates that the reduction correlates well with photosynthetic activity. Moreover, ketone reduction in Synechococcus specifically depends upon NADPH and not NADH. These results also suggest that cyanobacteria have the potential to be utilized as biocatalytic systems for direct usage of light energy in various applications such as syntheses of useful compounds and remediation of environmental pollutants.

Enantioselective reduction of prochiral ketones by engineered bifunctional fusion proteins

NADPH-dependent oxidoreductases are useful catalysts for the production of chiral synthons. However, preparative applications of oxidoreductases require efficient methods for in situ regeneration of the expensive nicotinamide cofactors. An advantageous method for cofactor regeneration is the construction of bifunctional fusion proteins composed of two enzymes, one catalysing the reduction reaction and the other one mediating the recycling of cofactors. Herein, we describe the in-frame fusion between an NADP+-accepting mutant of FDH (formate dehydrogenase) from Mycobacterium vaccae N10 and KR [3-ketoacyl-(acyl-carrier-protein) reductase] from Synechococcus sp. strain PCC 7942. The generation of linker insertion mutants led to a fusion protein exhibiting 100 and 80% of the enzymatic activities of native KR and FDH respectively. Escherichia coli cells expressing the fusion protein showed an approx. 2-fold higher initial reaction rate in the production of chiral alcohols than cells expressing the enzymes separately. The application of the engineered fusion protein in whole-cell bioreduction of pentafluoroacetophenone resulted in a substrate conversion of 99.97% with an excellent enantiomeric excess of 99.9% (S)-1-(pentafluorophenyl)ethanol.

Identification, cloning, and characterization of a novel ketoreductase from the cyanobacterium Synechococcus sp. strain PCC 7942

A new ketoreductase useful for asymmetric synthesis of chiral alcohols was identified in the cyanobacterium Synechococcus sp. strain PCC 7942. Mass spectrometry of trypsin-digested peptides identified the protein as 3-ketoacyl-[acyl-carrier-protein] reductase (KR) (EC 1.1.1.100). The gene, referred to as fabG, was cloned, functionally expressed in Escherichia coli, and subsequently purified to homogeneity. The enzyme displayed a temperature optimum at 44 degrees C and a broad pH optimum between pH 7 and pH 9. The NADPH-dependent KR was able to asymmetrically reduce a variety of prochiral ketones with good to excellent enantioselectivities (>99.8%). The KR showed particular high specific activity for asymmetric reduction of ethyl 4-chloroacetoacetate (38.29 +/- 2.15 U mg(-1)) and 2',3',4',5',6'-pentafluoroacetophenone (8.57 +/- 0.49 U mg(-1)) to the corresponding (S)-alcohols. In comparison with an established industrial enzyme like the alcohol dehydrogenase from Lactobacillus brevis, the KR showed seven-times-higher activity toward 2',3',4',5',6'-pentafluoroacetophenone, with a remarkably higher enantiomeric excess (>99.8% [S] versus 43.3% [S]).

A parallel bubble column system for the cultivation of phototrophic microorganisms

An incubator with up to 16 parallel bubble columns was equipped with artificial light sources assuring a light supply with a homogenous light spectrum directly above the bioreactors. Cylindrical light reflecting tubes were positioned around every single bubble column to avoid light scattering effects and to redirect the light from the top onto the cylindrical outer glass surface of each bubble column. The light reflecting tubes were equipped with light intensity filters to control the total light intensity for every single photo-bioreactor. Parallel cultivations of the unicellular obligate phototrophic cyanobacterium, Synechococcus PCC7942, were studied under different constant light intensities ranging from 20 to 102 microE m(-2)s(-1) at a constant humidified air flow rate supplemented with CO(2).