Abstract
Worldwide, ethylene is the most produced organic compound. It serves as a building block for a wide variety of plastics, textiles, and chemicals, and a process has been developed for its conversion into liquid transportation fuels. Currently, commercial ethylene production involves steam cracking of fossil fuels, and is the highest CO2-emitting process in the chemical industry. Therefore, there is great interest in developing technology for ethylene production from renewable resources including CO2 and biomass. Ethylene is produced naturally by plants and some microbes that live with plants. One of the metabolic pathways used by microbes is via an ethylene-forming enzyme (EFE), which uses α-ketoglutarate and arginine as substrates. EFE is a promising biotechnology target because the expression of a single gene is sufficient for ethylene production in the absence of toxic intermediates. Here we present the first comprehensive review and analysis of EFE, including its discovery, sequence diversity, reaction mechanism, predicted involvement in diverse metabolic modes, heterologous expression, and requirements for harvesting of bioethylene. A number of knowledge gaps and factors that limit ethylene productivity are identified, as well as strategies that could guide future research directions.
Highlights
The rising global demand for petroleum, its restricted supply base, and its deleterious effects on the environment has prompted the development of infrastructurecompatible renewable fuels and chemicals
This review aims to provide a summary of the existing literature and to present our own analysis of enzymes and pathways, which together outline a strategy for future research and development of bioethylene production
We found 17 amino acids conserved between Kudzu ethylene-forming enzyme (EFE), ACCOs, and 2OG-Fe(II) oxygenases (Figure 2, highlighted in yellow), suggesting that these residues may play important roles in enzyme structure/function
Summary
The rising global demand for petroleum, its restricted supply base, and its deleterious effects on the environment has prompted the development of infrastructurecompatible renewable fuels and chemicals. Pirkov et al observed that in S. cerevisiae, ethylene production nearly tripled when the nitrogen source in minimal media (1.0% glucose) was changed from ammonium to glutamate in batch cultures when the Kudzu efe gene was expressed by a strong, constitutive tpi promoter on a multicopy 2 μ plasmid (Table 1) [17], in agreement with an in silico production model (see previous section) [36] This model revealed that experimentally measured ethylene yields were consistent with the yields predicted under limited respiration (Table 1) [36], suggesting that O2 availability is necessary for maximal ethylene production. The authors postulated that addition of arginine may result in a “push” towards the succinateforming sub-reaction proposed by the dual-circuit mechanism [41] Together, these studies highlight that beyond strategies to improve EFE stability, further analysis of substrate enrichment and increased O2 availability are necessary to maximize ethylene production. For scaled-up production, inexpensive bioreactors must be developed with enhanced O2 mass transfer for non-photosynthetic systems, light delivery for photosynthetic systems, and associated harvesting systems tailored to a biologically derived gas stream
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