Construction of a synthetic metabolic pathway for biosynthesis of 2,4-dihydroxybutyric acid from ethylene glycol

A mycofactocin-associated dehydrogenase is essential for ethylene glycol metabolism by Rhodococcus jostii RHA1

Ethylene glycol is an industrially important diol in many manufacturing processes and a building block of polymers, such as poly(ethylene terephthalate). In this study, we found that a mycolic acid-containing bacterium Rhodococcus jostii RHA1 can grow with ethylene glycol as a sole source of carbon and energy. Deletion of a putative glycolate dehydrogenase gene (RHA1_ro03227) abolished growth with ethylene glycol, indicating that ethylene glycol is assimilated via glycolate in R. jostii RHA1. Transcriptome sequencing and gene deletion analyses revealed that a gene homologous to mycofactocin (MFT)-associated dehydrogenase (RHA1_ro06057), hereafter referred to as EgaA, is essential for ethylene glycol assimilation. Furthermore, egaA deletion also negatively affected the utilization of ethanol, 1-propanol, propylene glycol, and 1-butanol, suggesting that EgaA is involved in the utilization of various alcohols in R. jostii RHA1. Deletion of MFT biosynthetic genes abolished growth with ethylene glycol, indicating that MFT is the physiological electron acceptor of EgaA. Further genetic studies revealed that a putative aldehyde dehydrogenase (RHA1_ro06081) is a major aldehyde dehydrogenase in ethylene glycol metabolism by R. jostii RHA1. KEY POINTS: • Rhodococcus jostii RHA1 can assimilate ethylene glycol via glycolate • A mycofactocin-associated dehydrogenase is involved in the oxidation of ethylene glycol • An aldehyde dehydrogenase gene is important for the ethylene glycol assimilation.

Novel aspects of ethylene glycol catabolism

Ethylene glycol (EG) is an industrially important two-carbon diol used as a solvent, antifreeze agent, and building block of polymers such as poly(ethylene terephthalate) (PET). Recently, the use of EG as a starting material for the production of bio-fuels or bio-chemicals is gaining attention as a sustainable process since EG can be derived from materials not competing with human food stocks including CO2, syngas, lignocellulolytic biomass, and PET waste. In order to design and construct microbial process for the conversion of EG to value-added chemicals, microbes capable of catabolizing EG such as Escherichia coli, Pseudomonas putida, Rhodococcus jostii, Ideonella sakaiensis, Paracoccus denitrificans, and Acetobacterium woodii are candidates of chassis for the construction of synthetic pathways. In this mini-review, we describe EG catabolic pathways and catabolic enzymes in these microbes, and further review recent advances in microbial conversion of EG to value-added chemicals by means of metabolic engineering. KEY POINTS: • Ethylene glycol is a potential next-generation feedstock for sustainable industry. • Microbial conversion of ethylene glycol to value-added chemicals is gaining attention. • Ethylene glycol-utilizing microbes are useful as chassis for synthetic pathways.

Towards carbon neutrality: Sustainable recycling and upcycling strategies and mechanisms for polyethylene terephthalate via biotic/abiotic pathways

Polyethylene terephthalate (PET), one of the most ubiquitous engineering plastics, presents both environmental challenges and opportunities for carbon neutrality and a circular economy. This review comprehensively addressed the latest developments in biotic and abiotic approaches for PET recycling/upcycling. Biotically, microbial depolymerization of PET, along with the biosynthesis of reclaimed monomers [terephthalic acid (TPA), ethylene glycol (EG)] to value-added products, presents an alternative for managing PET waste and enables CO2 reduction. Abiotically, thermal treatments (i.e., hydrolysis, glycolysis, methanolysis, etc.) and photo/electrocatalysis, enabled by catalysis advances, can depolymerize or convert PET/PET monomers in a more flexible, simple, fast, and controllable manner. Tandem abiotic/biotic catalysis offers great potential for PET upcycling to generate commodity chemicals and alternative materials, ideally at lower energy inputs, greenhouse gas emissions, and costs, compared to virgin polymer fabrication. Remarkably, over 25 types of upgraded PET products (e.g., adipic acid, muconic acid, catechol, vanillin, and glycolic acid, etc.) have been identified, underscoring the potential of PET upcycling in diverse applications. Efforts can be made to develop chemo-catalytic depolymerization of PET, improve microbial depolymerization of PET (e.g., hydrolysis efficiency, enzymatic activity, thermal and pH level stability, etc.), as well as identify new microorganisms or hydrolases capable of degrading PET through computational and machine learning algorithms. Consequently, this review provides a roadmap for advancing PET recycling and upcycling technologies, which hold the potential to shape the future of PET waste management and contribute to the preservation of our ecosystems.

Next-generation feedstocks methanol and ethylene glycol and their potential in industrial biotechnology

Microbial fermentation processes are expected to play an important role in reducing dependence on fossil-based raw materials for the production of everyday chemicals. In order to meet the growing demand for biotechnological products in the future, alternative carbon sources that do not compete with human nutrition must be exploited. The chemical conversion of the industrially emitted greenhouse gas CO2 into microbially utilizable platform chemicals such as methanol represents a sustainable strategy for the utilization of an abundant carbon source and has attracted enormous scientific interest in recent years. A relatively new approach is the microbial synthesis of products from the C2-compound ethylene glycol, which can also be synthesized from CO2 and non-edible biomass and, in addition, can be recovered from plastic waste. Here we summarize the main chemical routes for the synthesis of methanol and ethylene glycol from sustainable resources and give an overview of recent metabolic engineering work for establishing natural and synthetic microbial assimilation pathways. The different metabolic routes for C1 and C2 alcohol-dependent bioconversions were compared in terms of their theoretical maximum yields and their oxygen requirements for a wide range of value-added products. Assessment of the process engineering challenges for methanol and ethylene glycol-based fermentations underscores the theoretical advantages of new synthetic metabolic routes and advocates greater consideration of ethylene glycol, a C2 substrate that has received comparatively little attention to date.

Progress advances in the production of bio-sourced methionine and its hydroxyl analogues

The essential sulphur-containing amino acid, methionine, is becoming a mass-commodity product with an annual production that exceeded 1,500,000 tons in 2018. This amino acid is today almost exclusively produced by chemical process from fossil resources. The environmental problems caused by this industrial process, and the expected scarcity of oil resources in the coming years, have recently accelerated the development of bioprocesses for producing methionine from renewable carbon feedstock. After a brief description of the chemical process and the techno-economic context that still justify the production of methionine by petrochemical processes, this review will present the current state of the art of biobased alternatives aiming at a sustainable production of this amino acid and its hydroxyl analogues from renewable carbon feedstock. In particular, this review will focus on three bio-based processes, namely a purely fermentative process based on the metabolic engineering of the natural methionine pathway, a mixed process combining the production of the O-acetyl/O-succinyl homoserine intermediate of this pathway by fermentation followed by an enzyme-based conversion of this intermediate into L-methionine and lately, a hybrid process in which the non-natural chemical synthon, 2,4-dihydroxybutyric acid, obtained by fermentation of sugars is converted by chemo-catalysis into hydroxyl methionine analogues. The industrial potential of these three bioprocesses, as well as the major technical and economic obstacles that remain to be overcome to reach industrial maturity are discussed. This review concludes by bringing up the assets of these bioprocesses to meet the challenge of the "green transition", with the accomplishment of the objective "zero carbon" by 2050 and how they can be part of a model of Bioeconomy enhancing local resources.