Quantitative analysis of fatty-acid-based biofuels produced by wild-type and genetically engineered cyanobacteria by gas chromatography–mass spectrometry

Perfluorooctane sulfonate (PFOS) disturbs fatty acid metabolism in Caenorhabditis elegans: Evidence from chemical analysis and molecular mechanism exploration

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant that might induce disorders in fatty acid (FA) metabolism, but the underlying mechanisms remain unresolved. Caenorhabditis elegans (C. elegans) as a model organism can synthesize polyunsaturated FAs de novo via the polyunsaturated FA synthesis pathway. In this study, synchronized L1 C. elegans were exposed to 0, 0.01, 0.1, 0.5 and 1 μM PFOS for 72 h. Gas chromatography-mass spectrometry (GC-MS) was used to establish a sensitive and reliable analysis method for PFASs in exposed nematode, the instrument detection limits of nine fatty acid methyl esters examined ranged between 1.11 and 27.6 ng/mL, with satisfactory reproducibility (RSD < 10%) observed. Methyl pentadecanoate (C15:0) was used as an internal standard, the linearity of the calibration (0.1-10 μg/mL) nine FAs from the nematode were quantitatively analyzed. Comparing with the control group, PFOS exposure caused significantly decreased levels of C18:0 while significantly increased levels of C18:3n6. A decrease in the C18:3n6: C18:2n6 ratio was observed. Consistently, expression of the FA desaturation gene fat-3 was significantly down-regulated. These findings suggest that the FA disorder is associated with decrease in mRNA expression of Δ6-desaturase genes in C. elegans. Simultaneously, the disorders in FA metabolism were found to disrupt mitochondrial function with a reduction in ATP synthesis, as determined by the luciferase method. In summary, the results of the study provide insights into the adverse effects of PFOS on FA metabolism in living organisms.

Profiling and quantitative analysis of underivatized fatty acids in Chlorella vulgaris microalgae by liquid chromatography-high resolution mass spectrometry

Chlorella vulgaris is a popular microalga used for biofuel production; nevertheless, it possesses a strong cell wall that hinders the extraction of molecules, especially lipids within the cell wall. For tackling this issue, we developed an efficient and cost‐effective method for optimal lipid extraction. Microlaga cell disruption by acid hydrolysis was investigated comparing different temperatures and reaction times; after hydrolysis, lipids were extracted with n‐hexane. The best recoveries were obtained at 140°C for 90 min. The microalgae were then analyzed by an untargeted approach based on liquid chromatography with high‐resolution mass spectrometry, providing the tentative identification of 28 fatty acids. First, a relative quantification on the untargeted data was performed using peak area as a surrogate of analyte abundance. Then, a targeted quantitative method was validated for the tentatively identified fatty acids, in terms of recovery (78‐100%), intra‐ and interday relative standard deviations (<10 and <9%, respectively) and linearity (R² > 0.98). The most abundant fatty acids were palmitic, palmitoleic, oleic, linoleic, linolenic, and stearic acids.

Establishing an enzyme cascade for one-pot production of α-olefins from low-cost triglycerides and oils without exogenous H2O2 addition

BACKGROUND

Biological α-olefins can be used as both biofuels and high value-added chemical precursors to lubricants, polymers, and detergents. The prototypic CYP152 peroxygenase family member OleT_JE from Jeotgalicoccus sp. ATCC 8456 catalyzes a single-step decarboxylation of free fatty acids (FFAs) to form α-olefins using H₂O₂ as a cofactor, thus attracting much attention since its discovery. To improve the productivity of α-olefins, significant efforts on protein engineering, electron donor engineering, and metabolic engineering of OleT_JE have been made. However, little success has been achieved in obtaining α-olefin high-producer microorganisms due to multiple reasons such as the tight regulation of FFA biosynthesis, the difficulty of manipulating multi-enzyme metabolic network, and the poor catalytic performance of OleT_JE.

RESULTS

In this study, a novel enzyme cascade was developed for one-pot production of α-olefins from low-cost triacylglycerols (TAGs) and natural oils without exogenous H₂O₂ addition. This artificial biocatalytic route consists of a lipase (CRL, AOL or Lip2) for TAG hydrolysis to produce glycerol and free fatty acids (FFAs), an alditol oxidase (AldO) for H₂O₂ generation upon glycerol oxidation, and the P450 fatty acid decarboxylase OleT_JE for FFA decarboxylation using H₂O₂ generated in situ. The multi-enzyme system was systematically optimized leading to the production of α-olefins with the conversion rates ranging from 37.2 to 68.5%. Furthermore, a reaction using lyophilized CRL/OleT_JE/AldO enzymes at an optimized ratio (5 U/6 μM/30 μM) gave a promising α-olefin yield of 0.53 g/L from 1500 μM (~1 g/L) coconut oil.

CONCLUSIONS

The one-pot enzyme cascade was successfully established and applied to prepare high value-added α-olefins from low-cost and renewable TAGs/natural oils. This system is independent of exogenous addition of H₂O₂, thus not only circumventing the detrimental effect of H₂O₂ on the stability and activity of involved enzymes, but also lower the overall costs on the TAG-to-olefin transformation. It is anticipated that this biotransformation system will become industrially relevant in the future upon more engineering efforts based on this proof-of-concept work.

Exploration of green integrated approach for effluent treatment through mass culture and biofuel production from unicellular alga, Acutodesmus obliquus RDS01

This study deals with the open pond (OP) pilot scale treatment of cassava effluent and enhancement of Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) enzyme through CO₂ utilization by the microalga, Acutodesmus obliquus RDS01. The cassava effluent treatment (ET) revealed maximum reduction of ammonia (96.8%), calcium (94.6%), chloride (95.2%), chlorine (98.5%), inorganic phosphate (94.6%), magnesium (96.8%), nitrate (96.89%), organic carbon (95.9%), organic phosphorus (96.3%), potassium (97.9%), sodium (97.1%), and sulfate (95.4%) on 15^th day using A. obliquus. The microalga produced highest RuBisCO enzyme activity (90%), CO₂ utilization efficiency (95%), biomass (8.9 gL^-1), lipid (176.65 mg mL^-1), carbohydrate (96.78 mg mL^-1), biodiesel (4.1 mL g^-1), and bioethanol (3.7 mL g^-1) during OP treatment. The isolated RuBisCO gene (rbcL) was used to construct the protein model by homology modeling. The microalgal-lipid content was analyzed through thin layer chromatography, the biodiesel produced was analyzed using Fourier-transform infrared spectroscopy and gas chromatography mass spectrometry (GCMS). The bioethanol production was confirmed by high performance liquid chromatography and GCMS analyses. A. obliquus produced of 98.75% biodiesel and 96.83% bioethanol in the OP pilot scale treatment A. obliquus. Overall, the microalga A. obliquus could act as an effective CO₂ capturing and bioremediation agent in the cassava ET, and also for the biodiesel and bioethanol can be produced.

Leptolyngbya fragilis ISC 108 is the most effective strain for dodecane biodegradation in contaminated soils

One of the major environmental problems nowadays is petroleum hydrocarbons contamination. Bioremediation is widely used for cleaning ecosystems contaminated with petroleum hydrocarbons. This study was carried out to investigate the response of five microalgae strains isolated from different regions in Iran for 1% n-dodecane (DOD) degradation. The results revealed that Leptolyngbya fragilis ISC 108 is the most effective strain to utilize n-DOD as growth substrate under a mixotrophic condition. Currently, there is little information about mechanisms involved in microalgae response against DOD. The activity of antioxidant enzymes and total lipid and carbohydrate contents were observed to be greater in DOD-treated L. fragilis ISC 108. Lower values of lipid peroxidation and H₂O₂ along with an increase of dry weight and specific growth rate in L. fragilis ISC 108 under DOD treatment shows that at the cellular level this strain is better equipped with an efficient oxygen radical scavenging system. In conclusion, this study proposes that L. fragilis ISC 108 can be considered an ideal candidate for use in bioremediation of DOD contaminated sites.

Biochemical characterization of three new α-olefin-producing P450 fatty acid decarboxylases with a halophilic property

BACKGROUND

The CYP152 family member OleT_JE from Jeotgalicoccus sp. ATCC 8456 has been well-known to catalyze the unusual one-step decarboxylation of free fatty acids towards the formation of terminal alkenes. Efforts to tune up its decarboxylation activity for better production of biological alkenes have been extensively explored via approaches such as site-directed mutagenesis and electron source engineering, but with limited success. To gain more insights into the decarboxylation mechanism and reaction bifurcation (decarboxylation versus hydroxylation), we turned to an alternative approach to explore the natural CYP152 resources for a better variety of enzyme candidates.

RESULTS

We biochemically characterized three new P450 fatty acid decarboxylases including OleT_JH, OleT_SQ and OleT_SA, with respect to their substrate specificity, steady-state kinetics, and salt effects. These enzymes all act as an OleT_JE-like fatty acid decarboxylase being able to decarboxylate a range of straight-chain saturated fatty acids (C₈-C₂₀) to various degrees. Site-directed mutagenesis analysis to the lower activity P450 enzyme OleT_SA revealed a number of key amino acid residues within the substrate-binding pocket (T47F, I177L, V319A and L405I) that are important for delicate substrate positioning of different chain-length fatty acids and thus the decarboxylation versus hydroxylation chemoselectivity, in particular for the mid-chain fatty acids (C₈-C₁₂). In addition, the three new decarboxylases exhibited optimal catalytic activity and stability at a salt concentration of 0.5 M, and were thus classified as moderate halophilic enzymes.

CONCLUSION

The P450 fatty acid decarboxylases OleT_JE, OleT_JH, OleT_SQ and OleT_SA belong to a novel group of moderate halophilic P450 enzymes. OleT_JH from Jeotgalicoccus halophilus shows the decarboxylation activity, kinetic parameters, as well as salt tolerance and stability that are comparable to OleT_JE. Site-directed mutagenesis of several key amino acid residues near substrate-binding pocket provides important guidance for further engineering of these P450 fatty acid decarboxylases that hold promising application potential for production of α-olefin biohydrocarbons.

Terminal Olefin Profiles and Phylogenetic Analyses of Olefin Synthases of Diverse Cyanobacterial Species

Cyanobacteria can synthesize alkanes and alkenes, which are considered to be infrastructure-compatible biofuels. In terms of physiological function, cyanobacterial hydrocarbons are thought to be essential for membrane flexibility for cell division, size, and growth. The genetic basis for the biosynthesis of terminal olefins (1-alkenes) is a modular type I polyketide synthase (PKS) termed olefin synthase (Ols). The modular architectures of Ols and structural characteristics of alkenes have been investigated only in a few species of the small percentage (approximately 10%) of cyanobacteria that harbor putative Ols pathways. In this study, investigations of the domains, modular architectures, and phylogenies of Ols in 28 cyanobacterial strains suggested distinctive pathway evolution. Structural feature analyses revealed 1-alkenes with three carbon chain lengths (C₁₅, C₁₇, and C₁₉). In addition, the total cellular fatty acid profile revealed the diversity of the carbon chain lengths, while the fatty acid feeding assay indicated substrate carbon chain length specificity of cyanobacterial Ols enzymes. Finally, in silico analyses suggested that the N terminus of the modular Ols enzyme exhibited characteristics typical of a fatty acyl-adenylate ligase (FAAL), suggesting a mechanism of fatty acid activation via the formation of acyl-adenylates. Our results shed new light on the diversity of cyanobacterial terminal olefins and a mechanism for substrate activation in the biosynthesis of these olefins.IMPORTANCE Cyanobacterial terminal olefins are hydrocarbons with promising applications as advanced biofuels. Despite the basic understanding of the genetic basis of olefin biosynthesis, the structural diversity and phylogeny of the key modular olefin synthase (Ols) have been poorly explored. An overview of the chemical structural traits of terminal olefins in cyanobacteria is provided in this study. In addition, we demonstrated by in vivo fatty acid feeding assays that cyanobacterial Ols enzymes might exhibit substrate carbon chain length specificity. Furthermore, by performing bioinformatic analyses, we observed that the substrate activation domain of Ols exhibited features typical of a fatty acyl-adenylate ligase (FAAL), which activates fatty acids by converting them to fatty acyl-adenylates. Our results provide further insight into the chemical structures of terminal olefins and further elucidate the mechanism of substrate activation for terminal olefin biosynthesis in cyanobacteria.

Bioenergy application of Dunaliella salina SA 134 grown at various salinity levels for lipid production

The biofuels are receiving considerable attention as a substitute for petro diesel. For microalgae, the cell density or biomass and lipid contents are key components for biodiesel production. This study was conducted to develop favorable culture conditions for Dunaliella salina to maximize its biomass and lipid accumulation. The effect of salinity (0.5 to 2.5 M NaCl) on the cell population, biochemical composition, and lipid output of Dunaliella salina was examined under a controlled environment for 21 days. Maximum growth (6.57 × 107 to 7.17 × 107cells mL-1) potentials were observed at 1.5 to 2 M NaCl. The photosynthetic pigments and carbohydrates also showed trends similar to growth. The maximum carotenoid level (5.16 mg L-1) was recorded at 2 M NaCl. Almost all physicochemical parameters increased with increases in salinity, biomass (1231.66 ± 1.26 mg L-1) and lipid content (248.33 mg L-1), as recorded at 2 M NaCl. Based on fluorescence intensity, the highest values (11.84 × 107cells/ml) of neutral lipids and total lipids (22.28%) were recorded at optimum salinity levels. The present study suggests that a high biomass and lipid accumulation of Dunaliella salina SA 134 could be obtained at the 2 M NaCl level.

Mutagenesis and redox partners analysis of the P450 fatty acid decarboxylase OleTJE

The cytochrome P450 enzyme OleT_JE from Jeotgalicoccus sp. ATCC 8456 is capable of converting free long-chain fatty acids into α-alkenes via one-step oxidative decarboxylation in presence of H₂O₂ as cofactor or using redox partner systems. This enzyme has attracted much attention due to its intriguing but unclear catalytic mechanism and potential application in biofuel production. Here, we investigated the functionality of a select group of residues (Arg245, Cys365, His85, and Ile170) in the active site of OleT_JE through extensive mutagenesis analysis. The key roles of these residues for catalytic activity and reaction type selectivity were identified. In addition, a range of heterologous redox partners were found to be able to efficiently support the decarboxylation activity of OleT_JE. The best combination turned out to be SeFdx-6 (ferredoxin) from Synechococcus elongatus PCC 7942 and CgFdR-2 (ferredoxin reductase) from Corynebacterium glutamicum ATCC 13032, which gave the highest myristic acid conversion rate of 94.4%. Moreover, Michaelis-Menton kinetic parameters of OleT_JE towards myristic acid were determined.

In vitro oxidative decarboxylation of free fatty acids to terminal alkenes by two new P450 peroxygenases

BACKGROUND

P450 fatty acid decarboxylases represented by the unusual CYP152 peroxygenase family member OleT_JE have been receiving great attention recently since these P450 enzymes are able to catalyze the simple and direct production of 1-alkenes for potential applications in biofuels and biomaterials. To gain more mechanistic insights, broader substrate spectra, and improved decarboxylative activities, it is demanded to discover and investigate more P450 fatty acid decarboxylases.

RESULTS

Here, we describe for the first time the expression, purification, and in vitro biochemical characterization of two new CYP152 peroxygenases, CYP-Aa162 and CYP-Sm46Δ29, that are capable of decarboxylating straight-chain saturated fatty acids. Both enzymes were found to catalyze the decarboxylation and hydroxylation of a broad range of free fatty acids (C₁₀-C₂₀) with overlapping substrate specificity, yet distinct chemoselectivity. CYP-Sm46Δ29 works primarily as a fatty (lauric) acid decarboxylase (66.1 ± 3.9% 1-undecene production) while CYP-Aa162 more as a fatty (lauric) acid hydroxylase (72.2 ± 0.9% hydroxy lauric acid production). Notably, the optical spectroscopic analysis of functional CYP-Sm46Δ29 revealed no characteristic P450 band, suggesting a unique heme coordination environment. Active-site mutagenesis analysis showed that substitution with the proposed key decarboxylation-modulating residues, His85 and Ile170, enhanced the decarboxylation activity of CYP-Aa162 and P450_BSβ, emphasizing the importance of these residues in directing the decarboxylation pathway. Furthermore, the steady-state kinetic analysis of CYP-Aa162 and CYP-Sm46Δ29 revealed both cooperative and substrate inhibition behaviors which are substrate carbon chain length dependent.

CONCLUSIONS

Our data identify CYP-Sm46Δ29 as an efficient OleT_JE-like fatty acid decarboxylase. Oxidative decarboxylation chemoselectivity of the CYP152 decarboxylases is largely dependent upon the carbon chain length of fatty acid substrates and their precise positioning in the enzyme active site. Finally, the kinetic mode analysis of the enzymes could provide important guidance for future process design.

Hydrogen peroxide-independent production of α-alkenes by OleTJE P450 fatty acid decarboxylase

BACKGROUND

Cytochrome P450 OleTJE from Jeotgalicoccus sp. ATCC 8456, a new member of the CYP152 peroxygenase family, was recently found to catalyze the unusual decarboxylation of long-chain fatty acids to form α-alkenes using H2O2 as the sole electron and oxygen donor. Because aliphatic α-alkenes are important chemicals that can be used as biofuels to replace fossil fuels, or for making lubricants, polymers and detergents, studies on OleTJE fatty acid decarboxylase are significant and may lead to commercial production of biogenic α-alkenes in the future, which are renewable and more environmentally friendly than petroleum-derived equivalents.

RESULTS

We report the H2O2-independent activity of OleTJE for the first time. In the presence of NADPH and O2, this P450 enzyme efficiently decarboxylates long-chain fatty acids (C12 to C20) in vitro when partnering with either the fused P450 reductase domain RhFRED from Rhodococcus sp. or the separate flavodoxin/flavodoxin reductase from Escherichia coli. In vivo, expression of OleTJE or OleTJE-RhFRED in different E. coli strains overproducing free fatty acids resulted in production of variant levels of multiple α-alkenes, with a highest total hydrocarbon titer of 97.6 mg·l-1.

CONCLUSIONS

The discovery of the H2O2-independent activity of OleTJE not only raises a number of fundamental questions on the monooxygenase-like mechanism of this peroxygenase, but also will direct the future metabolic engineering work toward improvement of O2/redox partner(s)/NADPH for overproduction of α-alkenes by OleTJE.

Engineering fatty acid biosynthesis in microalgae for sustainable biodiesel

Microalgae are a promising feedstock for biodiesel and other liquid fuels due to their fast growth rate, high lipid yields, and ability to grow in a broad range of environments. However, many microalgae achieve maximal lipid yields only under stress conditions hindering growth and providing compositions not ideal for biofuel applications. Metabolic engineering of algal fatty acid biosynthesis promises to create strains capable of economically producing fungible and sustainable biofuels. The algal fatty acid biosynthetic pathway has been deduced by homology to bacterial and plant systems, and much of our understanding is gleaned from basic studies in these systems. However, successful engineering of lipid metabolism in algae will necessitate a thorough characterization of the algal fatty acid synthase (FAS) including protein-protein interactions and regulation. This review describes recent efforts to engineer fatty acid biosynthesis toward optimizing microalgae as a biodiesel feedstock.

Enzymatic and physiological characterization of fatty acid activation in Synechocystis sp. PCC6803

Free fatty acids are typically activated by thioesterification processes and catalyzed by the fatty acyl-CoA synthetase or fatty acyl-ACP synthetase. However, the routes for fatty acid activation in cyanobacteria are not well understood. In this investigation, the slr1609 gene, which encodes the fatty acid activation enzyme, was cloned from Synechocystis sp. PCC6803. This gene was identified by heterologous expression and in vitro enzymatic activity analyses. Different from previous reports stating that free fatty acids are only activated through the fatty acyl-ACP synthetases encoded by these genes in cyanobacteria, this gene was also proven to possess a fatty acyl-CoA synthetase activity, by in vitro enzymatic activity analyses and in vivo complementation experiments. The protein Slr1609 is located in both the cell membrane and the cytosol of Synechocystis sp. PCC6803. The differences in the transcriptional profiles between the wild type and the slr1609 deletion mutant strain were evaluated using microarray analyses. These analyses indicated that 299 differentially expressed genes are involved in fatty acid metabolism, photosynthesis, carbon fixation, stress tolerance and other metabolic processes. Our experiments demonstrate the observed compositional changes in the unsaturated fatty acids from the membrane lipids of the slr1609 deletion mutant when shifted from 30 to 24 °C.

Recent applications of metabolomics toward cyanobacteria

Our knowledge on cyanobacterial molecular biology increased tremendously by the application of the "omics" techniques. Only recently, metabolomics was applied systematically to model cyanobacteria. Metabolomics, the quantitative estimation of ideally the complete set of cellular metabolites, is particularly well suited to mirror cellular metabolism and its flexibility under diverse conditions. Traditionally, small sets of metabolites are quantified in targeted metabolome approaches. The development of separation technologies coupled to mass-spectroscopy- or nuclear-magnetic-resonance-based identification of low molecular mass molecules presently allows the profiling of hundreds of metabolites of diverse chemical nature. Metabolome analysis was applied to characterize changes in the cyanobacterial primary metabolism under diverse environmental conditions or in defined mutants. The resulting lists of metabolites and their steady state concentrations in combination with transcriptomics can be used in system biology approaches. The application of stable isotopes in fluxomics, i.e. the quantitative estimation of carbon and nitrogen fluxes through the biochemical network, has only rarely been applied to cyanobacteria, but particularly this technique will allow the making of kinetic models of cyanobacterial systems. The further application of metabolomics in the concert of other "omics" technologies will not only broaden our knowledge, but will also certainly strengthen the base for the biotechnological application of cyanobacteria.

Biodiesel production from microalgal isolates of southern Pakistan and quantification of FAMEs by GC-MS/MS analysis

Background

Microalgae have attracted major interest as a sustainable source for biodiesel production on commercial scale. This paper describes the screening of six microalgal species, Scenedesmus quadricauda, Scenedesmus acuminatus, Nannochloropsis sp., Anabaena sp., Chlorella sp. and Oscillatoria sp., isolated from fresh and marine water resources of southern Pakistan for biodiesel production and the GC-MS/MS analysis of their fatty acid methyl esters (FAMEs).

Results

Growth rate, biomass productivity and oil content of each algal species have been investigated under autotrophic condition. Biodiesel was produced from algal oil by acid catalyzed transesterification reaction and resulting fatty acid methyl esters (FAMEs) content was analyzed by GC/MS. Fatty acid profiling of the biodiesel, obtained from various microalgal oils showed high content of C-16:0, C-18:0, cis-Δ⁹C-18:1, cis-Δ¹¹C-18:1 (except Scenedesmus quadricauda) and 10-hydroxyoctadecanoic (except Scenedesmus acuminatus). Absolute amount of C-14:0, C-16:0 and C-18:0 by a validated GC-MS/MS method were found to be 1.5-1.7, 15.0-42.5 and 4.2-18.4 mg/g, respectively, in biodiesel obtained from various microalgal oils. Biodiesel was also characterized in terms of cetane number, kinematic viscosity, density and higher heating value and compared with the standard values.

Conclusion

Six microalgae of local origin were screened for biodiesel production. A method for absolute quantification of three important saturated fatty acid methyl esters (C-14, C-16 and C-18) by gas chromatography-tandem mass spectrometry (GC-MS/MS), using multiple reactions monitoring (MRM) mode, was employed for the identification and quantification of biodiesels obtained from various microalgal oils. The results suggested that locally found microalgae can be sustainably harvested for the production of biodiesel. This offers the tremendous economic opportunity for an energy-deficient nation.

Boosting the free fatty acid synthesis of Escherichia coli by expression of a cytosolic Acinetobacter baylyi thioesterase

BACKGROUND

Thioesterases remove the fatty acyl moiety from the fatty acyl-acyl carrier proteins (ACPs), releasing them as free fatty acids (FFAs), which can be further used to produce a variety of fatty acid-based biofuels, such as biodiesel, fatty alcohols and alkanes. Thioesterases play a key role in the regulation of the fatty acid synthesis in Escherichia coli. Therefore, exploring more promising thioesterases will contribute to the development of industrial microbial lipids production.

RESULTS

We cloned and expressed a cytosolic Acinetobacter baylyi thioesterase ('AcTesA) in E. coli by deleting its leader sequence. Protein sequence alignment, structure modeling and site-directed mutagenesis demonstrated that Ser10, Gly48, Asn77, Asp158 and His161 residues composed the active centre of 'AcTesA. The engineered strain that overexpressed 'AcTesA achieved a FFAs titer of up to 501.2 mg/L in shake flask, in contrast to only 20.5 mg/L obtained in wild-type E. coli, demonstrating that the expression of 'AcTesA indeed boosted the synthesis of FFAs. The 'AcTesA exhibited a substrate preference towards the C8-C16 acyl groups, with C14:0, C16:1, C12:0 and C8:0 FFAs being the top four components. Optimization of expression level of 'AcTesA made the FFAs production increase to 551.3 mg/L. The FFAs production further increased to 716.1 mg/L by optimization of the culture medium. Fed-batch fermentation was also carried out to evaluate the FFAs production in a scaleable process. Finally, 3.6 g/L FFAs were accumulated within 48 h, and a maximal FFAs yield of 6.1% was achieved in 12-16 h post induction.

CONCLUSIONS

For the first time, an A. baylyi thioesterase was cloned and solubly expressed in the cytosol of E. coli. This leaderless thioesterase ('AcTesA) was found to be capable of enhancing the FFAs production of E. coli. Without detailed optimization of the strain and fermentation, the finally achieved 3.6 g/L FFAs is encouraging. In addition, 'AcTesA exhibited different substrate specificity from other thioesterases previously reported, and can be used to supply the fatty acid-based biofuels with high quality of FFAs. Altogether, this study provides a promising thioesterase for FFAs production, and is of great importance in enriching the library of useful thioesterases.