Mechanisms of production and control of acetate esters in yeasts

Acetate esters, such as isoamyl acetate and ethyl acetate, are major aroma components of alcoholic beverages. They are produced through synthesis from acetyl CoA and the corresponding alcohol by alcohol acetyltransferase (AATase) with specific control of reaction factors, including unsaturated fatty acids and precursors, the percentage of nitrogen, and oxygen. However, the mechanisms by which these specific reaction factors affect acetate ester production remain largely unknown. The cellular mechanisms underlying the effects of these factors on acetate ester production were examined by purifying AATase from yeast, characterizing it, and cloning the ATF gene encoding AATase from sake yeast and bottom-fermenting yeast. Genetic and biochemical studies suggested that the decrease in acetate production with the addition of oxygen and unsaturated fatty acids was due to a decrease in enzyme synthesis resulting from transcriptional repression of the ATF1 gene, which is responsible for most of the AATase activity. Furthermore, these results suggest that expression of the ATF1 gene is intricately regulated by a number of transcriptional regulatory genes such as ROX1 and RAP1. Based on these results, the mechanism of ester regulation by oxygen, unsaturated fatty acids and precursors, and ratio of nitrogen source are becoming clearer from a molecular biological point of view. The physiological significance of ester production by yeast is then discussed. In this review, we summarize the studies on AATase, ATF gene, regulation of ester production, and physiological significance of acetate ester.

Metabolic engineering of Pseudomonas putida KT2440 for medium-chain-length fatty alcohol and ester production from fatty acids

Medium-chain-length fatty alcohols have broad applications in the surfactant, lubricant, and cosmetic industries. Their acetate esters are widely used as flavoring and fragrance substances. Pseudomonas putida KT2440 is a promising chassis for fatty alcohol and ester production at the industrial scale due to its robustness, versatility, and high oxidative capacity. However, P. putida has also numerous native alcohol dehydrogenases, which lead to the degradation of these alcohols and thereby hinder its use as an effective biocatalyst. Therefore, to harness its capacity as a producer, we constructed two engineered strains (WTΔpedFΔadhP, GN346ΔadhP) incapable of growing on mcl-fatty alcohols by deleting either a cytochrome c oxidase PedF and a short-chain alcohol dehydrogenase AdhP in P. putida or AdhP in P. putida GN346. Carboxylic acid reductase, phosphopantetheinyl transferase, and alcohol acetyltransferase were expressed in the engineered P. putida strains to produce hexyl acetate. Overexpression of transporters further increased 1-hexanol and hexyl acetate production. The optimal strain G23E-MPAscTP produced 93.8 mg/L 1-hexanol and 160.5 mg/L hexyl acetate, with a yield of 63.1%. The engineered strain is applicable for C₆-C₁₀ fatty alcohols and their acetate ester production. This study lays a foundation for P. putida being used as a microbial cell factory for sustainable synthesis of a broad range of products based on medium-chain-length fatty alcohols.

Metabolic engineering of Escherichia coli for efficient biosynthesis of butyl acetate

Background

Butyl acetate is a versatile compound that is widely used in the chemical and food industry. The conventional butyl acetate synthesis via Fischer esterification of butanol and acetic acid using catalytic strong acids under high temperature is not environmentally benign. Alternative lipase-catalyzed ester formation requires a significant amount of organic solvent which also presents another environmental challenge. Therefore, a microbial cell factory capable of producing butyl acetate through fermentation of renewable resources would provide a greener approach to butyl acetate production.

Result

Here, we developed a metabolically engineered strain of Escherichia coli that efficiently converts glucose to butyl acetate. A modified Clostridium CoA-dependent butanol production pathway was used to synthesize butanol which was then condensed with acetyl-CoA through an alcohol acetyltransferase. Optimization of alcohol acetyltransferase expression and redox balance with auto-inducible fermentative controlled gene expression led to an effective titer of 22.8 ± 1.8 g/L butyl acetate produced in a bench-top bioreactor.

Conclusion

Building on the well-developed Clostridium CoA-dependent butanol biosynthetic pathway, expression of an alcohol acetyltransferase converts the butanol produced into butyl acetate. The results from this study provided a strain of E. coli capable of directly producing butyl acetate from renewable resources at ambient conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01755-y.

Single mutation at a highly conserved region of chloramphenicol acetyltransferase enables isobutyl acetate production directly from cellulose by Clostridium thermocellum at elevated temperatures

BACKGROUND

Esters are versatile chemicals and potential drop-in biofuels. To develop a sustainable production platform, microbial ester biosynthesis using alcohol acetyltransferases (AATs) has been studied for decades. Volatility of esters endows high-temperature fermentation with advantageous downstream product separation. However, due to the limited thermostability of AATs known, the ester biosynthesis has largely relied on use of mesophilic microbes. Therefore, developing thermostable AATs is important for ester production directly from lignocellulosic biomass by the thermophilic consolidated bioprocessing (CBP) microbes, e.g., Clostridium thermocellum.

RESULTS

In this study, we engineered a thermostable chloramphenicol acetyltransferase from Staphylococcus aureus (CAT_Sa) for enhanced isobutyl acetate production at elevated temperatures. We first analyzed the broad alcohol substrate range of CAT_Sa. Then, we targeted a highly conserved region in the binding pocket of CAT_Sa for mutagenesis. The mutagenesis revealed that F97W significantly increased conversion of isobutanol to isobutyl acetate. Using CAT_Sa F97W, we demonstrated direct conversion of cellulose into isobutyl acetate by an engineered C. thermocellum at elevated temperatures.

CONCLUSIONS

This study highlights that CAT is a potential thermostable AAT that can be harnessed to develop the thermophilic CBP microbial platform for biosynthesis of designer bioesters directly from lignocellulosic biomass.

Biosynthesis of advanced biofuel farnesyl acetate using engineered Escherichia coli

Diminishing petroleum reserves and the rapid accumulation of greenhouse gases lead to increasing interest in microbial biofuels. In this study, a heterologous farnesyl acetate biosynthesis pathway was constructed in Escherichia coli for the first time. Firstly, the AtoB, ERG13, tHMG1, ERG12, ERG8, MVD1, Idi, IspA and PgpB were expressed to accumulate farnesol in the E. coli cells. Then the alcohol acetyltransferase (ATF1) was heterologous overexpressed for the subsequent esterification farnesol to farnesyl acetate. The engineered strain DG 106 accumulated 128 ± 10.5 mg/L of farnesyl acetate. Finally, the isopentenyl-diphosphate isomerase was further overexpressed, and the recombinant strain DG107 produced 201 ± 11.7 mg/L of farnesyl acetate. This study shows the novel method for the biosynthesis of the advanced biofuel farnesyl acetate directly from glucose and highlight the enormous designing strategies for metabolic engineering of bioproducts.

Concealed ester formation and amino acid metabolism to volatile compounds in table grape (Vitis vinifera L.) berries

Volatile esters contribute to the aroma and flavor of many fruits but are normally absent in grape berries (Vitis vinifera L.). To examine the biosynthetic potential of grape berries to form volatile esters, berry sections were incubated with exogenous L-Phe, L-Leu or L-Met. In general, amino-acid incubation caused the accumulation of the respective aldehydes and alcohols. Moreover, L-Leu incubation resulted in the accumulation of 3-methylbutyl acetate and L-Phe incubation resulted in the accumulation 2-phenylethyl acetate in 'Muscat Hamburg' but not in the other grape accessions. Exogenous L-Met administration did not result in volatile esters accumulation but the accumulation of sulfur volatile compounds such as methional and dimethyl disulfide was prominent. Berry-derived cell-free extracts displayed differential alcohol acetyltransferase activities and supported the formation of 3-methylbutyl acetate and benzyl acetate. 2-Phenylethyl acetate was produced only in 'Muscat Hamburg' cell-free extracts. VvAAT2, a newly characterized gene, was preferentially expressed in 'Muscat Hamburg' berries and functionally expressed in E. coli. VvAAT2 possesses alcohol acetyltransferase activity utilizing benzyl alcohol, 2-phenylethanol, hexanol or 3-methylbutanol as substrates. Our study demonstrates that grape berries have a concealed potential to accumulate volatile esters and this process is limited by substrate availability.