Metabolic engineering of microorganisms for the production of structurally diverse esters

Towards biotechnological production of bio-based low molecular weight esters: a patent review

Low molecular weight (LMW) esters, like ethyl acetate, methyl formate or butyl acetate, are widespread bulk chemicals in many industries. Each of them is currently produced in huge amounts (millions of tons per year scale) starting from fossil-based feedstock and they are used mainly because of their low toxicity and complete biodegradability. Energy transition is just half of the story on the path of fighting climate change: 45% of the global greenhouse gas emissions are caused by the production and use of all the products, materials and food necessary for modern human life. If the world is to reach its climate goals, there is the need to leave underground a significant proportion of the fossil feedstock and minimize environmental impacts of chemical manufacturing. This is the reason why a lot of efforts have been made to find novel routes for LMW esters production starting from renewable raw materials (e.g. biomasses or off-gases) and exploiting low-impact manufacturing, such as microbial fermentation or enzymatic reactions. This review reports the most significant patents, in the field of white biotechnology, that will hopefully lead to the commercialization of bio-based LMW esters as well as novel strategies, current problems to be solved, newer technologies, and some patent applications aiming at possible future developments.

Alcohol acyltransferases for the biosynthesis of esters

Esters are widely used in food, energy, spices, chemical industry, etc., becoming an indispensable part of life. However, their production heavily relies on the fossil energy industry, which presents significant challenges associated with energy shortages and environmental pollution. Consequently, there is an urgent need to identify alternative green methods for ester production. One promising solution is biosynthesis, which offers sustainable and environmentally friendly processes. In ester biosynthesis, alcohol acyltransferases (AATs) catalyze the condensation of acyl-CoAs and alcohols to form esters, enabling the biosynthesis of nearly 100 different kinds of esters, such as ethyl acetate, hexyl acetate, ethyl crotonate, isoamyl acetate, and butyl butyrate. However, low catalytic efficiency and low selectivity of AATs represent the major bottlenecks for the biosynthesis of certain specific esters, which should be addressed with protein molecular engineering approaches before practical biotechnological applications. This review provides an overview of AAT enzymes, including their sequences, structures, active sites, catalytic mechanisms, and metabolic engineering applications. Furthermore, considering the critical role of AATs in determining the final ester products, the current research progresses of AAT modification using protein molecular engineering are also discussed. This review summarized the major challenges and prospects of AAT enzymes in ester biosynthesis.

The Studies in Constructing Yeast Cell Factories for the Production of Fatty Acid Alkyl Esters

Fatty acid alkyl esters have broad applications in biofuels, lubricant formulas, paints, coatings, and cosmetics. Traditionally, these esters are mostly produced through unsustainable and energy-intensive processes. In contrast, microbial production of esters from renewable and sustainable feedstocks may provide a promising alternative and has attracted widespread attention in recent years. At present, yeasts are used as ideal hosts for producing such esters, due to their availability for high-density fermentation, resistance to phage infection, and tolerance against toxic inhibitors. Here, we summarize recent development on the biosynthesis of alkyl esters, including fatty acid ethyl esters (FAEEs), fatty acid short-branched chain alkyl esters (FASBEs), and wax esters (WEs) by various yeast cell factories. We focus mainly on the enzyme engineering strategies of critical wax ester synthases, and the pathway engineering strategies employed for the biosynthesis of various ester products. The bottlenecks that limit productivity and their potential solutions are also discussed in this review.

Microbial production of butyl butyrate: from single strain to cognate consortium

Butyl butyrate (BB) is an important chemical with versatile applications in beverage, food and cosmetics industries. Since chemical synthesis of BB may cause adverse impacts on the environment, biotechnology is an emerging alternative approach for microbial esters biosynthesis. BB can be synthesized by using a single Clostridium strain natively producing butanol or butyrate, with exogenously supplemented butyrate or butanol, in the presence of lipase. Recently, E. coli strains have been engineered to produce BB, but the titer and yield remained very low. This review highlighted a new trend of developing cognate microbial consortium for BB production and associated challenges, and end up with new prospects for further improvement for microbial BB biosynthesis.

Metabolic Profiling of VOCs Emitted by Bacteria Isolated from Pressure Ulcers and Treated with Different Concentrations of Bio-AgNPs

Considering the advent of antibiotic resistance, the study of bacterial metabolic behavior stimulated by novel antimicrobial agents becomes a relevant tool to elucidate involved adaptive pathways. Profiling of volatile metabolites was performed to monitor alterations of bacterial metabolism induced by biosynthesized silver nanoparticles (bio-AgNPs). Escherichia coli, Enterococcus faecalis, Klebsiella pneumoniae and Proteus mirabilis were isolated from pressure ulcers, and their cultures were prepared in the presence/absence of bio-AgNPs at 12.5, 25 and 50 µg mL-1. Headspace solid phase microextraction associated to gas chromatography-mass spectrometry was the employed analytical platform. At the lower concentration level, the agent promoted positive modulation of products of fermentation routes and bioactive volatiles, indicating an attempt of bacteria to adapt to an ongoing suppression of cellular respiration. Augmented response of aldehydes and other possible products of lipid oxidative cleavage was noticed for increasing levels of bio-AgNPs. The greatest concentration of agent caused a reduction of 44 to 80% in the variety of compounds found in the control samples. Pathway analysis indicated overall inhibition of amino acids and fatty acids routes. The present assessment may provide a deeper understanding of molecular mechanisms of bio-AgNPs and how the metabolic response of bacteria is untangled.

Modification of PapA5 acyltransferase substrate selectivity for optimization of short-chain alcohol-derived multimethyl-branched ester production in Escherichia coli

Plant waxes are interesting substitutes of fossil-derived compounds; however, their limited sources and narrow structural diversity prompted the development of microbial platforms to produce esters with novel chemical structures and properties. One successful strategy was the heterologous expression of the mycocerosic polyketide synthase-based biosynthetic pathway (MAS-PKS, PapA5 and FadD28 enzymes) from Mycobacterium tuberculosis in Escherichia coli. This recombinant strain has the ability to produce a broad spectrum of multimethyl-branched long-chain esters (MBE) with novel chemical structures and high oxidation stability. However, one limitation of this microbial platform was the low yields obtained for MBE derived of short-chain alcohols. In an attempt to improve the titers of the short-chain alcohol-derived MBE, we focused on the PapA5 acyltransferase-enzyme that catalyzes the ester formation reaction. Specific amino acid residues located in the two-substrate recognition channels of this enzyme were identified, rationally mutated, and the corresponding mutants characterized both in vivo and in vitro. The phenylalanine located at 331 position in PapA5 (F331) was found to be a key residue that when substituted by other bulky and aromatic or bulky and polar amino acid residues (F331W, F331Y or F331H), gave rise to PapA5 mutants with improved bioconversion efficiency; showing in average, 2.5 higher yields of short-chain alcohol-derived MBE compared with the wild-type enzyme. Furthermore, two alternative pathways for synthetizing ethanol were engineered into the MBE producer microorganism, allowing de novo production of ethanol-derived MBE at levels comparable with those obtained by the external supply of this alcohol. KEY POINTS: • Mutation in channel 2 changes PapA5 acyltransferase bioconversion efficiency. • Improved production of short-chain alcohol derived multimethyl-branched esters. • Establishing ethanologenic pathways for de novo production of ethanol derived MBE. • Characterization of a novel phenylethanol-derived MBE.

Synthesis of 4-Hydroxybenzoic Acid Derivatives in Escherichia coli

Hydroxybenzoic acids (HBAs) such as 4-hydroxybenzoic acid (4-HBA) and 3,4-dihydroxybenzoic acid (DHB; protocatechuic acid) and its ester with methanol (methylparaben [MP]) are known to have various functional biological properties, including antibacterial, anticancer, antidiabetic, antiaging, antiviral, and anti-inflammatory activities. Since these compounds are widely used in cosmetic, food, and pharmaceutical industries, the use of renewable feedstocks for the production of HBAs is an area of growing interest. In this study, we used Escherichia coli to synthesize these three hydroxybenzoic acid derivatives (4-HBA, DHB, and MP). We overexpressed ubiC in E. coli to synthesize 4-HBA from chorismate, a substrate that is produced by the shikimate pathway in E. coli. For the synthesis of DHB, an additional gene (pobA) was introduced, while hbad and EHT1 were co-expressed to synthesize MP. To supply more chorismate, we introduced the shikimate gene module construct and selected the best construct for increased yields. Using this approach, 723.5 mg/L 4-HBA, 942.0 mg/L DHB, and 347.7 mg/L MP were synthesized. Our study showed that the shikimate gene module constructs can be applicable to increase the yields of HBA derivatives in HBA-tolerant microorganisms.

Study of the Enzymatic Capacity of Kluyveromyces marxianus for the Synthesis of Esters

Recently, biotechnological opportunities have been found in non-Saccharomyces yeasts because they possess metabolic characteristics that lead to the production of compounds of interest. It has been observed that Kluyveromyces marxianus has a great potential in the production of esters, which are aromatic compounds of industrial importance. The genetic bases that govern the synthesis of esters include a large group of enzymes, among which the most important are alcohol acetyl transferases (AATases) and esterases (AEATases), and it is known that some are present in K. marxianus, because it has genetic characteristics like S. cerevisiae. It also has a physiology suitable for biotechnological use since it is the eukaryotic microorganism with the fastest growth rate and has a wide range of thermotolerance with respect to other yeasts. In this work, the enzymatic background of K. marxianus involved in the synthesis of esters is analyzed, based on the sequences reported in the NCBI database.

Engineering Escherichia coli for the production of butyl octanoate from endogenous octanoyl-CoA

Medium chain esters produced from fruits and flowering plants have a number of commercial applications including use as flavour and fragrance ingredients, biofuels, and in pharmaceutical formulations. These esters are typically made via the activity of an alcohol acyl transferase (AAT) enzyme which catalyses the condensation of an alcohol and an acyl-CoA. Developing a microbial platform for medium chain ester production using AAT activity presents several obstacles, including the low product specificity of these enzymes for the desired ester and/or low endogenous substrate availability. In this study, we engineered Escherichia coli for the production of butyl octanoate from endogenously produced octanoyl-CoA. This was achieved through rational protein engineering of an AAT enzyme from Actinidia chinensis for improved octanoyl-CoA substrate specificity and metabolic engineering of E. coli fatty acid metabolism for increased endogenous octanoyl-CoA availability. This resulted in accumulation of 3.3 + 0.1 mg/L butyl octanoate as the sole product from E. coli after 48 h. This study represents a preliminary examination of the feasibility of developing E. coli platforms for the synthesis single medium chain esters from endogenous fatty acids.

Microbial production of butyl butyrate, a flavor and fragrance compound

Butyl butyrate (BB) has been widely used as a flavor and fragrance compound in the beverage, food, perfume, and cosmetic industries. Currently, BB is produced through two-step processes; butanol and butyrate are first produced and are used as precursors for the esterification reactions to yield BB in the next step. Recently, an alternative process to the current process has been developed by using microorganisms for the one-pot BB production. In the one-pot BB process, alcohol acyl transferases (AATs) and lipases play roles in the esterification of butanol together with their co-substrates butyryl-CoA and butyrate, respectively. In this paper, we review the characteristics of two enzymes including AAT and lipase in the esterification reaction. Also, we review the one-pot processes for BB production by employing the wild-type and engineered Clostridium species and the engineered Escherichia coli strains, with the combination of AATs and lipases.

Recent advances on conversion and co-production of acetone-butanol-ethanol into high value-added bioproducts

Butanol is an important bulk chemical and has been regarded as an advanced biofuel. Large-scale production of butanol has been applied for more than 100 years, but its production through acetone-butanol-ethanol (ABE) fermentation process by solventogenic Clostridium species is still not economically viable due to the low butanol titer and yield caused by the toxicity of butanol and a by-product, such as acetone. Renewed interest in biobutanol as a biofuel has spurred technological advances to strain modification and fermentation process design. Especially, with the development of interdisciplinary processes, the sole product or even the mixture of ABE produced through ABE fermentation process can be further used as platform chemicals for high value added product production through enzymatic or chemical catalysis. This review aims to comprehensively summarize the most recent advances on the conversion of acetone, butanol and ABE mixture into various products, such as isopropanol, butyl-butyrate and higher-molecular mass alkanes. Additionally, co-production of other value added products with ABE was also discussed.