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Production of Ethylene Glycol from Glycerol Using an in Vitro Enzymatic Cascade

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Production of Ethylene Glycol from Glycerol Using an in Vitro Enzymatic Cascade catalysts Communication Production of Ethylene Glycol from Glycerol Using an In Vitro Enzymatic Cascade Kai Li, Weikang Sun, Wensi Meng, Jinxin Yan, Yipeng Zhang, Shiting Guo, Chuanjuan Lü *, Cuiqing Ma and Chao Gao State Key Laboratory of Microbial Technology, Shandong University, Qingdao 266237, China; [email protected] (K.L.); [email protected] (W.S.); [email protected] (W.M.); [email protected] (J.Y.); [email protected] (Y.Z.); [email protected] (S.G.); [email protected] (C.M.); [email protected] (C.G.) * Correspondence: [email protected]; Tel.: +86-(532)-58631561 Abstract: Glycerol is a readily available and inexpensive substance that is mostly generated during biofuel production processes. In order to ensure the viability of the biofuel industry, it is essential to develop complementing technologies for the resource utilization of glycerol. Ethylene glycol is a two-carbon organic chemical with multiple applications and a huge market. In this study, an artificial enzymatic cascade comprised alditol oxidase, catalase, glyoxylate/hydroxypyruvate reductase, pyruvate decarboxylase and lactaldehyde:propanediol oxidoreductase was developed for the production of ethylene glycol from glycerol. The reduced nicotinamide adenine dinucleotide (NADH) generated during the dehydrogenation of the glycerol oxidation product D-glycerate can be as the reductant to support the ethylene glycol production. Using this in vitro synthetic system with self-sufficient NADH recycling, 7.64 ± 0.15 mM ethylene glycol was produced from 10 mM glycerol in 10 h, with a high yield of 0.515 ± 0.1 g/g. The in vitro enzymatic cascade is not only a promising Citation: Li, K.; Sun, W.; Meng, W.; alternative for the generation of ethylene glycol but also a successful example of the value-added Yan, J.; Zhang, Y.; Guo, S.; Lü, C.; Ma, utilization of glycerol. C.; Gao, C. Production of Ethylene Glycol from Glycerol Using an In Keywords: glycerol; ethylene glycol; biocatalysis; in vitro enzymatic cascade Vitro Enzymatic Cascade. Catalysts 2021, 11, 214. https://doi.org/ 10.3390/catal11020214 1. Introduction Academic Editors: Peter Adewale The manufacturing of biofuels from reproducible feedstocks is increasing throughout and Michele C. Loewen the world [1–3]. The development of the biofuel industry has generated a large amount Received: 18 January 2021 of glycerol as an inevitable byproduct [4]. An excess of glycerol production in the biofuel Accepted: 2 February 2021 industry leads to a dramatic decrease of the price of glycerol, which has a negative impact Published: 5 February 2021 on the development of the biofuels industry [5]. Thus, the conversion of glycerol into value-added compounds through chemical or biotechnological routes is now urgently Publisher’s Note: MDPI stays neutral needed for the improvement of the viability of the biofuel industry. with regard to jurisdictional claims in published maps and institutional affil- Ethylene glycol is a large-volume commodity that is widely used in many industrial iations. processes, such as glycolic acid production and polyethylene terephthalate synthesis [6]. It is predominantly produced from ethylene oxide through carbonation and subsequent hydrolyzation, or by the hydration of ethylene oxide [7,8]. Direct ethylene glycol production from biomass-derived carbohydrates through chemical catalysis has also been reported [9]. Importantly, ethylene glycol can be produced from glycerol through liquid-phase catalytic Copyright: © 2021 by the authors. hydrogenolysis [10,11]. However, this process usually needs high pressures (4.0–8.0 MPa), Licensee MDPI, Basel, Switzerland. high temperatures (453–473 K) and expensive noble metal catalysts (Pt, Ni, Ru, etc.). Several This article is an open access article distributed under the terms and metabolic pathways have been constructed for the biotechnological production of ethylene conditions of the Creative Commons glycol from renewable substrates such as D-glucose, D-xylose and L-arabinose [12–14]. Attribution (CC BY) license (https:// Nevertheless, the production of ethylene glycol from glycerol via a biotechnological route creativecommons.org/licenses/by/ has never been reported, until now. 4.0/). Catalysts 2021, 11, 214. https://doi.org/10.3390/catal11020214 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 9 Catalysts 2021, 11, 214 2 of 9 An in vitro synthetic system is emerging as a promising technique for the synthesis of highAn-invalue vitro productssynthetic from system cheap is emergingand renewable as a promising substrates technique [15,16]. This for thebiomanufactur- synthesis of high-valueing platform products leaves fromout the cheap use andof cells renewable, and can substrates thus exclude [15,16 cell]. This-associated biomanufacturing process bar- platform leaves out the use of cells, and can thus exclude cell-associated process barriers, riers, such as the transmembrane transport of substances, the toxicity of products, and such as the transmembrane transport of substances, the toxicity of products, and unde- undesired metabolism pathways. In addition, an in vitro synthetic system exclusively em- sired metabolism pathways. In addition, an in vitro synthetic system exclusively employs ploys purified enzymes, and makes the purification of the target products rather simple purified enzymes, and makes the purification of the target products rather simple to con- to conduct. Some in vitro synthetic systems have been constructed for the generation of duct. Some in vitro synthetic systems have been constructed for the generation of valuable valuable chemicals from renewable substrates, such as D-glucose, D-xylose and glycerol chemicals from renewable substrates, such as D-glucose, D-xylose and glycerol [17–19]. [17–19]. In this study, we have designed an artificial enzymic cascade for the conversion In this study, we have designed an artificial enzymic cascade for the conversion of glycerol of glycerol to ethylene glycol that only requires five enzymes. The artificial pathway is to ethylene glycol that only requires five enzymes. The artificial pathway is also com- also completely redox balanced through self-sufficient reduced nicotinamide adenine di- pletely redox balanced through self-sufficient reduced nicotinamide adenine dinucleotide nucleotide (NADH) recycling. (NADH) recycling. 2.2. ResultsResults andand DiscussionDiscussion 2.1.2.1. DesignDesign ofof thethe InInVitro Vitro Biosystem Biosystem for for Ethylene Ethylene Glycol Glycol Production Production Here,Here, anan artificialartificial enzymaticenzymatic reactionreaction cascadecascade waswas designeddesigned forfor thetheconversion conversion ofof glycerolglycerol intointo ethyleneethylene glycolglycol (Figure(Figure1 ).1). Glycerol Glycerol is is first first oxidized oxidized to to DD-glycerate-glycerate viavia DD-- glyceraldehydeglyceraldehyde byby alditol alditol oxidase oxidase (Aldo) (Aldo) catalyzed catalyzed two-step two-step oxidation. oxidation. The TheD D-Glycerate-Glycerate producedproduced fromfrom glycerolglycerol isis thenthen transformedtransformed into 3-hydroxypyruvate3-hydroxypyruvate by by oxidized oxidized nicotin- nicoti- namideamide adenine dinucleotidedinucleotide (NAD(NAD++)-dependent)-dependent glyoxylate/hydroxypyruvateglyoxylate/hydroxypyruvate reductasereductase (GRHPR).(GRHPR). Then,Then, 3-hydroxypyruvate3-hydroxypyruvate is is decarboxylated decarboxylated to to glycolaldehyde glycolaldehyde by by pyruvate pyruvate de- decarboxylasecarboxylase (P (PDC).DC). Finally, Finally, the the glycolaldehyde glycolaldehyde is isreduced reduced into into ethylene ethylene glycol glycol by by lactal- lac- taldehyde:propanedioldehyde:propanediol oxidoreductase oxidoreductase (FucO), (FucO), using using the the NADH NADH produced produced during during the the D- Dglycerate-glycerate dehydrogenation dehydrogenation as as the the cofactor. cofactor. The The H H2O2O2 generated2 generated during during the the oxidation oxidation of ofthe the glycerol glycerol is isdecomposed decomposed by by catalase catalase in inorder order to toavoid avoid the the spontaneous spontaneous oxidative oxidative of the of theintermediate intermediate products products [20]. [20 As]. As a aresult, result, one one mole mole of of glycerol glycerol can generate oneone molemole of of ethyleneethylene glycol. glycol. FigureFigure 1. 1.Scheme Scheme of of ethylene ethylene glycol glycol production production from from glycerol glycerol based based on theon artificialthe artificialin vitro in vitroenzymatic enzy- cascade.matic cascade. The enzymes The enzymes are alditol are alditol oxidase oxidase (AldO) (AldO) from Streptomycesfrom Streptomyces coelicolor coelicolorA3(2), A3(2), catalase catalase from from Aspergillus niger, glyoxylate/hydroxypyruvate reductase (GRHPR) from Pyrococcus furiosus, Aspergillus niger, glyoxylate/hydroxypyruvate reductase (GRHPR) from Pyrococcus furiosus, pyruvate pyruvate decarboxylase (PDC) from Zymomonas mobile, and lactaldehyde:propanediol oxidoreduc- decarboxylase (PDC) from Zymomonas mobile, and lactaldehyde:propanediol oxidoreductase (FucO) tase (FucO) from Escherichia coli. from Escherichia coli. 2.2. Substrate Specificity of AldOmt 2.2. Substrate Specificity of AldOmt AldOAldO isis a a soluble soluble monomeric monomeric flavoprotein flavoprotein containing containing covalently-boundcovalently-bound FADFAD as as its its cofactorcofactor [ 21[21].]. ThisThis enzymeenzyme is is first first isolated isolated from fromStreptomyces
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