Bio-upgrading of ethanol to fatty acid ethyl esters by metabolic engineering of Pseudomonas putida KT2440

Microbial conversion of ethanol to high-value products: progress and challenges

Industrial biotechnology heavily relies on the microbial conversion of carbohydrate substrates derived from sugar- or starch-rich crops. This dependency poses significant challenges in the face of a rising population and food scarcity. Consequently, exploring renewable, non-competing carbon sources for sustainable bioprocessing becomes increasingly important. Ethanol, a key C2 feedstock, presents a promising alternative, especially for producing acetyl-CoA derivatives. In this review, we offer an in-depth analysis of ethanol's potential as an alternative carbon source, summarizing its distinctive characteristics when utilized by microbes, microbial ethanol metabolism pathway, and microbial responses and tolerance mechanisms to ethanol stress. We provide an update on recent progress in ethanol-based biomanufacturing and ethanol biosynthesis, discuss current challenges, and outline potential research directions to guide future advancements in this field. The insights presented here could serve as valuable theoretical support for researchers and industry professionals seeking to harness ethanol's potential for the production of high-value products.

Discovery and mechanism explanation of a novel green biocatalyst esterase Bur01 from Burkholderia ambifaria for ester synthesis under aqueous phase

Biocatalyst catalyzing the synthesis of esters under aqueous phase is an alternative with green and sustainable characteristics. Here, a biocatalyst esterase Bur01 was identified through genome sequencing and gene library construction from a Burkholderia ambifaria BJQ0010 with efficient ester synthesis property under aqueous phase for the first time. Bur01 was soluble expressed and the purified enzyme showed the highest activity at pH 4.0 and 40 °C. It had a broad substrate spectrum, especially for ethyl esters. The structure of Bur01 was categorized as a member of α/β fold hydrolase superfamily. The easier opening of lid under aqueous phase and the hydrophobicity of substrate channel contribute to easier access to the active center for substrate. Molecular docking and site-directed mutation demonstrated that the oxyanion hole Ala22, Met112 and π-bond stacking between His24 and Phe217 played essential roles in catalytic function. The mutants V149A, V149I, L159I and F137I enhanced enzyme activity to 1.42, 1.14, 1.32 and 2.19 folds due to reduced spatial resistance and increased hydrophobicity of channel and ethyl octanoate with the highest conversion ratio of 68.28 % was obtained for F137I. These results provided new ideas for developing green catalysts and catalytic basis of mechanistic studies for ester synthetase under aqueous phase.

Medium chain length polyhydroxyalkanoate produced from ethanol by Pseudomonas putida grown in liquid obtained from acidogenic digestion of organic municipal solid waste

Production of medium chain length polyhydroxyalkanoate (mcl-PHA) up to about 6 g.L^-1 was obtained by feeding ethanol to Pseudomonas putida growing in liquid obtained from acidogenic digestion of organic municipal solid waste. Washing the wet, heat-inactivated Pseudomonas cells at the end of the fermentation with ethanol obviated the need of drying the biomass and enabled the removal of contaminating lipids before solvent-mediated extraction of PHA. Using 'green' solvents, 90 to near 100% of the mcl-PHA was extracted and purities of 71-78% mcl-PHA were reached already by centrifugation and decantation without further filtration for biomass removal. The mcl-PHA produced in this way consists of 10-18% C8, 72-78% C10 and 8-12% C12 chains (entirely medium chain length), has a crystallinity and melting temperature of ∼13% and ∼49 °C, respectively, and is a stiff rubberlike, colourless material at room temperature.

Customized valorization of waste streams by Pseudomonas putida: State-of-the-art, challenges, and future trends

Preventing catastrophic climate events warrants prompt action to delay global warming, which threatens health and food security. In this context, waste management using engineered microbes has emerged as a long-term eco-friendly solution for addressing the global climate crisis and transitioning to clean energy. Notably, Pseudomonas putida can valorize industry-derived synthetic wastes including plastics, oils, food, and agricultural waste into products of interest, and it has been extensively explored for establishing a fully circular bioeconomy through the conversion of waste into bio-based products, including platform chemicals (e.g., cis,cis-muconic and adipic acid) and biopolymers (e.g., medium-chain length polyhydroxyalkanoate). However, the efficiency of waste pretreatment technologies, capability of microbial cell factories, and practicability of synthetic biology tools remain low, posing a challenge to the industrial application of P. putida. The present review discusses the state-of-the-art, challenges, and future prospects for divergent biosynthesis of versatile products from waste-derived feedstocks using P. putida.

Leveraging substrate flexibility and product selectivity of acetogens in two-stage systems for chemical production

Carbon dioxide (CO2 ) stands out as sustainable feedstock for developing a circular carbon economy whose energy supply could be obtained by boosting the production of clean hydrogen from renewable electricity. H2 -dependent CO2 gas fermentation using acetogenic microorganisms offers a viable solution of increasingly demonstrated value. While gas fermentation advances to achieve commercial process scalability, which is currently limited to a few products such as acetate and ethanol, it is worth taking the best of the current state-of-the-art technology by its integration within innovative bioconversion schemes. This review presents multiple scenarios where gas fermentation by acetogens integrate into double-stage biotechnological production processes that use CO2 as sole carbon feedstock and H2 as energy carrier for products' synthesis. In the integration schemes here reviewed, the first stage can be biotic or abiotic while the second stage is biotic. When the first stage is biotic, acetogens act as a biological platform to generate chemical intermediates such as acetate, formate and ethanol that become substrates for a second fermentation stage. This approach holds the potential to enhance process titre/rate/yield metrics and products' spectrum. Alternatively, when the first stage is abiotic, the integrated two-stage scheme foresees, in the first stage, the catalytic transformation of CO2 into C1 products that, in the second stage, can be metabolized by acetogens. This latter scheme leverages the metabolic flexibility of acetogens in efficient utilization of the products of CO2 abiotic hydrogenation, namely formate and methanol, to synthesize multicarbon compounds but also to act as flexible catalysts for hydrogen storage or production.

Engineering Pseudomonas putida KT2440 for chain length tailored free fatty acid and oleochemical production

Despite advances in understanding the metabolism of Pseudomonas putida KT2440, a promising bacterial host for producing valuable chemicals from plant-derived feedstocks, a strain capable of producing free fatty acid-derived chemicals has not been developed. Guided by functional genomics, we engineered P. putida to produce medium- and long-chain free fatty acids (FFAs) to titers of up to 670 mg/L. Additionally, by taking advantage of the varying substrate preferences of paralogous native fatty acyl-CoA ligases, we employed a strategy to control FFA chain length that resulted in a P. putida strain specialized in producing medium-chain FFAs. Finally, we demonstrate the production of oleochemicals in these strains by synthesizing medium-chain fatty acid methyl esters, compounds useful as biodiesel blending agents, in various media including sorghum hydrolysate at titers greater than 300 mg/L. This work paves the road to produce high-value oleochemicals and biofuels from cheap feedstocks, such as plant biomass, using this host.

Synthetic biology promotes the capture of CO2 to produce fatty acid derivatives in microbial cell factories

Environmental problems such as greenhouse effect, the consumption of fossil energy, and the increase of human demand for energy are becoming more and more serious, which force researcher to turn their attention to the reduction of CO2 and the development of renewable energy. Unsafety, easy to lead to secondary environmental pollution, cost inefficiency, and other problems limit the development of conventional CO2 capture technology. In recent years, many microorganisms have attracted much attention to capture CO2 and synthesize valuable products directly. Fatty acid derivatives (e.g., fatty acid esters, fatty alcohols, and aliphatic hydrocarbons), which can be used as a kind of environmentally friendly and renewable biofuels, are sustainable substitutes for fossil energy. In this review, conventional CO2 capture techniques pathways, microbial CO2 concentration mechanisms and fixation pathways were introduced. Then, the metabolic pathway and progress of direct production of fatty acid derivatives from CO2 in microbial cell factories were discussed. The synthetic biology means used to design engineering microorganisms and optimize their metabolic pathways were depicted, with final discussion on the potential of optoelectronic-microbial integrated capture and production systems.

Reconstructing ethanol oxidation pathway in Pseudomonas putida KT2440 for bio-upgrading of ethanol to biodegradable polyhydroxybutanoates

Ethanol has recently been demonstrated as a suitable carbon source for acetyl-CoA-derived products with high theoretical yield. Herein, the short-chain-length polyhydroxyalkanoates production pathway was constructed in an industrial platform P. putida KT2440, allowing the engineered strain to produce 674.97 ± 22.3 mg/L of Polyhydroxybutyrate (PHB) from ethanol as sole carbon source. Furthermore, the ethanol catabolic pathway was reconstructed to enhance the acetyl-coA pool by expressing the novel Aldehyde dehydrogenases from Klebsiella pneumonia and Dickeya zeae, resulting in a titer of 1385.34 ± 16.5 mg/L and 9300 ± 0.56 mg/L of PHB in shake flask and fermenter, respectively. Furthermore, transcriptome analysis was conducted to provide insights into the central metabolic pathways and different expression patterns in response to changes in substrate. Additionally, the production of co-polymer poly(3-hydroxybutyrate-co-3-hydroxypropionate) was shown using glycerol and ethanol as co-substrates from recombinant P. putida KT2440. This work demonstrates the potential of P. putida KT2440 as a promising industrial platform for short-chain-length PHAs production from structurally unrelated carbon sources.