De Novo Biosynthesis of Anisyl Alcohol and Anisyl Acetate in Engineered Escherichia coli

The decarboxylative and dehydrogenative coupling of formate: from anisyl formate to a high value-added diamine

The deoxygenative carbon-carbon bond formation from alcohols remains a formidable challenge toward cleaner coupling reactions. In this work, we first unlocked a novel homocoupling upgrading mode of benzyl alcohol by virtue of the two-side roles of the pre-introduced formyl group, which serves as both impetus for easier C-O bond dissociation and an endogenous hydrogen source. As a proof-of-concept, the decarboxylative and dehydrogenative homocoupling of anisyl formate toward 1,2-bis(4-methoxyphenyl)ethane was realized using a Ru/MoS2 catalyst with an optimized yield of 66%. Structural characterizations and control experiments indicated the synergy effect between Ru and Mo. A tailored nitration-reduction route was further designed for upgrading the coupling product into a high value-added aromatic diamine, the overall yield of which reached 56% based on anisyl formate. This method opens a new avenue for catalytic transformations of alcohols through ester intermediates.

Characterization and Mechanism Study of a Novel Ethanol Acetyltransferase from Hanseniaspora uvarum (EatH) with Good Thermostability, pH Stability, and Broad Alcohol Substrate Specificity

Ethyl acetate, one of the most essential industrial compounds, has a broad range of applications, including flavors, fragrances, pharmaceuticals, cosmetics, and green solvents. Eat1 is accountable for bulk ethyl acetate production in yeasts, yet its properties and molecular mechanism are not well characterized. In this study, an eat1 gene from Hanseniaspora uvarum was obtained through gene mining. EatH showed the highest activity at pH 7.5 and 35 °C and preferred short-chain acyl substrates but had a broad alcohol substrate spectrum from short-chain primary alcohols to aromatic alcohols. Its Km and kcat/Km values toward pNPA were measured to be 1.16 mM and 29.03 L·mmol-1·s-1, respectively. The structure of EatH was composed of a lid domain and a core catalytic domain, with the catalytic triad of Ser124, Asp148, and His296. Additionally, crucial residues and their mechanism were analyzed through molecular docking, site-directed mutagenesis, and molecular dynamics simulation. The mutants N149A, N149K, and N149S showed enhanced enzyme activity toward pNP-hexanoate to 5.0-, 6.6-, and 3.6-fold, and Y204S enhanced enzyme activity for pNP-butyrate by 2.6 times via creating a wider substrate binding pocket and enhancing hydrophobicity. Collectively, this work provided a theoretical basis for the further rational design of EatH and enriched the understanding of the Eat family.

Ultrasound-assisted enzymatic synthesis of cinnamyl acetate by immobilized lipase on ordered mesoporous silicon with CFD simulation and molecular docking analysis

Flavor esters are used in food and cosmetic industries, but sustainable production remains a big challenging. This study proposed the ultrasound-assisted biosynthesis of cinnamyl acetate using self-made immobilized lipase CSL@OMS-C8, with computational fluid dynamics (CFD) and molecular simulations revealing the hydrodynamic properties and lipase-catalytic mechanisms. The results demonstrate that ultrasonication-intensified enzymatic reaction facilitated 96.6 % conversion of cinnamic alcohol, due to the ultrasound-assisted catalytic efficiency of 13.7 mmol/g·min. Molecular docking analysis identifies the lowest binding energy of -3.7 kcal·mol-1 between lipase and vinyl acetate, contributing to the highest conversion rates compared to acetic acid, ethyl acetate. CFD simulation indicates that ultrasonic energy waves promote substrate diffusion and mixing with lower shear stress. The catalytic stability of CSL@OMS-C8 was confirmed by 60.1 % of relative activity after 10-time reuse. This paper theoretically and experimentally studied the ultrasonic-assisted enzymatic synthesis of cinnamyl acetate, showcasing huge potential for sustainable production of flavor esters.

Selective Multiphase-Assisted Oxidation of Bio-Sourced Primary Alcohols over Ru- and Mo- Carbon Supported Catalysts

The oxidation of representative bio-based benzyl-type alcohols has been successfully carried out in a multiphase (MP) system comprised of three mutually immiscible liquid components as water, isooctane, and a hydrophobic ionic liquid as methyltrioctylammonium chloride ([CH3(CH2)6CH2]3N(Cl)CH3), a heterogeneous catalyst (either ad-hoc synthesized carbon-supported Mo or a commercial 5 % Ru/C), and air as an oxidant. The MP-reaction proceeded as an interfacial process with Mo/C or Ru/C perfectly segregated in the ionic liquid phase and the reactant(s)/products(s) dissolved in the aqueous solution. This environment proved excellent to convert quantitatively benzyl alcohols into the corresponding aldehydes with a selectivity up to 99 %, without overoxidation to carboxylic acids. The nature of the catalyst, however, affected the operating conditions with Ru/C active at a lower T and t (130 °C, 4-6 h) compared to Mo/C (150 °C, 24 h). The phase confinement was advantageous also to facilitate the products isolation and the recycle of the catalyst. Notably, in the Mo/C-catalyzed oxidation of benzyl alcohol, benzaldehyde was achieved with unaltered selectivity (>99 %) at complete conversion, for five subsequent reactions through a semicontinuous procedure in which the catalyst was reused in-situ, without ever removing it from the reactor or treating it in any way.

Recent developments in enzymatic and microbial biosynthesis of flavor and fragrance molecules

Flavors and fragrances are an important class of specialty chemicals for which interest in biomanufacturing has risen during recent years. These naturally occurring compounds are often amenable to biosynthesis using purified enzyme catalysts or metabolically engineered microbial cells in fermentation processes. In this review, we provide a brief overview of the categories of molecules that have received the greatest interest, both academically and industrially, by examining scholarly publications as well as patent literature. Overall, we seek to highlight innovations in the key reaction steps and microbial hosts used in flavor and fragrance manufacturing.

De Novo Biosynthesis of 4-Vinylanisole in Engineered Escherichia coli

4-Vinylanisole is an aggregation pheromone of the locust. Both gregarious and solitary locusts exhibit a strong attraction toward 4-vinylanisole, irrespective of gender or age. Therefore, 4-vinylanisole can be used for trapping and monitoring locusts. In this study, the construction of a de novo 4-vinylanisole pathway in Escherichia coli has been demonstrated for the first time. Subsequently, by increasing the supply of precursor substrates, we further improved the biosynthesis of 4-vinylanisole. Finally, a two-phase organic overlay culture was used to increase the titer to 206 mg/L. It presents a sustainable and ecofriendly alternative for the synthesis of 4-vinylanisole.

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.