Purification, characterization, and molecular cloning of organic-solvent-tolerant cholesterol esterase from cyclohexane-tolerant Burkholderia cepacia strain ST-200

Development and validation of reliable astaxanthin quantification from natural sources

Astaxanthin derived from natural sources occurs in the form of various esters and stereomers, which complicates its quantitative and qualitative analysis. To simplify and standardize astaxanthin measurement with high precision, an enzymolysis-based astaxanthin quantification method was developed to hydrolyze astaxanthin esters and determine free astaxanthin in all its diastereomeric forms. Astaxanthin standards and differently processed Haematococcus pluvialis biomass were investigated. Linear correlation of standards of all-E-astaxanthin was observed in a measurement range between extract concentrations of 1.0 μg/mL and 11.2 μg/mL with a coefficient of variation below 5%. The diastereomers 9Z-, and 13Z-astaxanthin, and two di-Z-forms were detected. In contrast to the measurement of standards, the observed measurement range was extended to 30 μg/mL in extracts from H. pluvialis. The nature of the sample had to be taken into account for measurement, as cell, respectively, sample composition altered the optimal concentration for astaxanthin determination. The measurement precision of all-E-astaxanthin quantification in dried H. pluvialis biomass (1.2-1.8 mg dried biomass per sample) was calculated with a coefficient of variation of maximum 1.1%, whereas it was below 10% regarding the diastereomers. Complete enzymolysis was performed with 1.0 to 2.0 units of cholesterol esterase in the presence of various solvents with up to 2.0 mg biomass (dry weight). The method was compared with other astaxanthin determination approaches in which astaxanthin is converted to acetone in a further step before measurement. The developed method resulted in a higher total astaxanthin recovery but lower selectivity of the diastereomers. The reliability of photometric astaxanthin estimations was assessed by comparing them with the developed chromatographic method. At later stages in the cell cycle of H. pluvialis, all methods yielded similar results (down to 0.1% deviation), but photometry lost precision at earlier stages (up to 31.5% deviation). To optimize sample storage, the shelf life of astaxanthin-containing samples was investigated. Temperatures below -20°C, excluding oxygen, and storing intact H. pluvialis cells instead of dried or disrupted biomass reduced astaxanthin degradation.

Bacterial triacylglycerol lipase is a potential cholesterol esterase: Identification of a key determinant for sterol-binding specificity

Cholesterol esterase (Che) from Burkholderia stabilis (BsChe) is a homolog of well-characterized and industrially relevant bacterial triacylglycerol lipases (Lips). BsChe is a rare bacterial Lip enzyme that exhibits practical Che activity and is currently used in clinical applications to determine total serum cholesterol levels. To investigate the sterol specificity of BsChe, we determined the X-ray structure of BsChe. We discovered a local structural change in the active-site cleft, which might be related to substrate binding and product release. We also performed molecular docking studies by using the X-ray models of BsChe and cholesterol linoleate (CLL), the most favorable substrate for BsChe. The results showed that the sterol moieties of reasonable CLL docking poses localized to a specific active-site cleft surface formed by Leu266 and Ile287, which are unconserved among Burkholderia Lip homologs. Site-directed mutagenesis identified these residues as essential for the Che activity of BsChe, and Leu or Ile substitution conferred marked Che activity to Burkholderia Lips. In particular, Burkholderia cepacia and Burkholderia ubonensis Lips with the V266L/L287I double mutation exhibited ~50-fold and 500-fold higher Che activities than those of the wild-type enzymes, respectively. These results provide new insights into the substrate-binding mechanisms and selectivities of bacterial Lips.

Production of recombinant extracellular cholesterol esterase using consistently active promoters in Burkholderia stabilis

Burkholderia stabilis FERMP-21014 produces highly active cholesterol esterase in the presence of fatty acids. To develop an overexpression system for cholesterol esterase production, we carried out RNA sequencing analyses to screen strongly active promoters in FERMP-21014. Based on gene expression consistency analysis, we selected nine genes that were consistently expressed at high levels, following which we constructed expression vectors using their promoter sequences and achieved overproduction of extracellular cholesterol esterase under fatty acid-free conditions. Of the tested promoters, the promoter of BSFP_0720, which encodes the alkyl hydroperoxide reductase subunit AhpC, resulted in the highest cholesterol esterase activity (24.3 U mL−1). This activity level was 243-fold higher than that of the wild-type strain under fatty acid-free conditions. We confirmed that cholesterol esterase was secreted without excessive accumulation within the cells. The gene expression consistency analysis will be useful to screen promoters applicable to the overexpression of other industrially important enzymes.

Biochemical profiles of two thermostable and organic solvent-tolerant esterases derived from a compost metagenome

Owing to the functional versatility and potential applications in industry, interest in lipolytic enzymes tolerant to organic solvents is increasing. In this study, functional screening of a compost soil metagenome resulted in identification of two lipolytic genes, est1 and est2, encoding 270 and 389 amino acids, respectively. The two genes were heterologously expressed and characterized. Est1 and Est2 are thermostable enzymes with optimal enzyme activities at 80 and 70 °C, respectively. A second-order rotatable design, which allows establishing the relationship between multiple variables with the obtained responses, was used to explore the combined effects of temperature and pH on esterase stability. The response curve indicated that Est1, and particularly Est2, retained high stability within a broad range of temperature and pH values. Furthermore, the effects of organic solvents on Est1 and Est2 activities and stabilities were assessed. Notably, Est2 activity was significantly enhanced (two- to tenfold) in the presence of ethanol, methanol, isopropanol, and 1-propanol over a concentration range between 6 and 30% (v/v). For the short-term stability (2 h of incubation), Est2 exhibited high tolerance against 60% (v/v) of ethanol, methanol, isopropanol, DMSO, and acetone, while Est1 activity resisted these solvents only at lower concentrations (below 30%, v/v). Est2 also displayed high stability towards some water-immiscible organic solvents, such as ethyl acetate, diethyl ether, and toluene. With respect to long-term stability, Est2 retained most of its activity after 26 days of incubation in the presence of 30% (v/v) ethanol, methanol, isopropanol, DMSO, or acetone. All of these features indicate that Est1 and Est2 possess application potential.

Complete Genome Sequence of Burkholderia stabilis FERMP-21014

Cholesterol esterase (EC 3.1.1.13) was identified in a bacterium, Burkholderia stabilis strain FERMP-21014. Here, we report the complete genome sequence of B. stabilis FERMP-21014, which has been used in the commercial production of cholesterol esterase. The genome sequence information may be useful for improving production levels of cholesterol esterase.

Lipase catalysis in organic solvents: advantages and applications

Lipases are industrial biocatalysts, which are involved in several novel reactions, occurring in aqueous medium as well as non-aqueous medium. Furthermore, they are well-known for their remarkable ability to carry out a wide variety of chemo-, regio- and enantio-selective transformations. Lipases have been gained attention worldwide by organic chemists due to their general ease of handling, broad substrate tolerance, high stability towards temperatures and solvents and convenient commercial availability. Most of the synthetic reactions on industrial scale are carried out in organic solvents because of the easy solubility of non-polar compounds. The effect of organic system on their stability and activity may determine the biocatalysis pace. Because of worldwide use of lipases, there is a need to understand the mechanisms behind the lipase-catalyzed reactions in organic solvents. The unique interfacial activation of lipases has always fascinated enzymologists and recently, biophysicists and crystallographers have made progress in understanding the structure-function relationships of these enzymes. The present review describes the advantages of lipase-catalyzed reactions in organic solvents and various effects of organic solvents on their activity.

Enzyme stability, thermodynamics and secondary structures of α-amylase as probed by the CD spectroscopy

An amylase of a thermophilic bacterium, Bacillus sp. TSSC-3 (GenBank Number, EU710557) isolated from the Tulsi Shyam hot spring reservoir (Gujarat, India) was purified to the homogeneity in a single step on phenyl sepharose 6FF. The molecular weight of the enzyme was 25kD, while the temperature and pH optima for the enzyme catalysis were 80°C and 7, respectively. The purified enzyme was highly thermostable with broad pH stability and displayed remarkable resistance against surfactants, chelators, urea, guanidine HCl and various solvents as well. The stability and changes in the secondary structure of the enzyme under various extreme conditions were determined by the circular dichroism (CD) spectroscopy. The stability trends and the changes in the α-helices and β-sheets were analyzed by Mean Residual Ellipticity (MRE) and K2D3. The CD data confirmed the structural stability of the enzyme under various harsh conditions, yet it indicated reduced α-helix content and increased β-sheets upon denaturation. The thermodynamic parameters; deactivation rate constant, half-life, changes in entropy, enthalpy, activation energy and Gibb's free energy indicated that the enzyme-substrate reactions were highly stable. The overall profile of the enzyme: high thermostability, alkalitolerance, calcium independent nature, dextrose equivalent values and resistance against chemical denaturants, solvents and surfactants suggest its commercial applications.

Hyper-activation of foldase-dependent lipase with lipase-specific foldase

The LST-03 lipase from Pseudomonas aeruginosa LST-03 requires lipase-specific foldase for activation. Abundant expression of the active lipase was successfully accomplished with individual expression of the lipase and foldase in a heterologous host and subsequent in vitro activation. Although the activity of the native lipase from culture supernatant of P. aeruginosa LST-03 was 110 kI.U./g, that after in vitro activation using individually expressed lipase and foldase was 228 kI.U./g. Furthermore, the activity after in vitro activation with afterwards adding calcium ions was 359 kI.U./g. However, the incubation of the lipase with the foldase in the presence of calcium ions resulted in a small conformational transition and low activation levels of the lipase by the foldase. The lipase showed high affinity for the foldase in the presence of calcium ions. The results indicate that in a cellular environment that contains calcium ions, the lipase would not become a hyperactive form by the foldase.

Overexpression and characterization of a new organic solvent-tolerant esterase derived from soil metagenomic DNA

In this study, an esterase, designated EstC23, was isolated from a soil metagenomic library. The protein was amenable to overexpression in Escherichia coli under control of the T7 promoter, resulting in expression of the active, soluble protein that constituted 30% of the total cell protein content. This enzyme showed optimal activity at 40 °C and retained about 50% maximal activity at 5-10 °C. EstC23 showed remarkable stability in up to 50% (v/v) benzene and alkanes (high logP solvents). When incubated for 7 days in the presence of 50% benzene or alkanes, the enzyme maintained its 2-3 fold elevated activity. The purified enzyme also cleaved sterically hindered esters of tertiary alcohols. These results indicate that EstC23 has potential for use in industrial processes.

Enzymes from solvent-tolerant microbes: useful biocatalysts for non-aqueous enzymology

Solvent-tolerant microbes are a newly emerging class that possesses the unique ability to thrive in the presence of organic solvents. Their enzymes adapted to mediate cellular and metabolic processes in a solvent-rich environment and are logically stable in the presence of organic solvents. Enzyme catalysis in non-aqueous/low-water media is finding increasing applications for the synthesis of industrially important products, namely peptides, esters, and other trans-esterification products. Solvent stability, however, remains a prerequisite for employing enzymes in non-aqueous systems. Enzymes, in general, get inactivated or give very low rates of reaction in non-aqueous media. Thus, early efforts, and even some recent ones, have aimed at stabilization of enzymes in organic media by immobilization, surface modifications, mutagenesis, and protein engineering. Enzymes from solvent-tolerant microbes appear to be the choicest source for studying solvent-stable enzymes because of their unique ability to survive in the presence of a range of organic solvents. These bacteria circumvent the solvent's toxic effects by virtue of various adaptations, e.g. at the level of the cytoplasmic membrane, by degradation and transformation of solvents, and by active excretion of solvents. The recent screening of these exotic microbes has generated some naturally solvent-stable proteases, lipases, cholesterol oxidase, cholesterol esterase, cyclodextrin glucanotransferase, and other important enzymes. The unique properties of these novel biocatalysts have great potential for applications in non-aqueous enzymology for a range of industrial processes.

Mining enzymes from extreme environments

Current advances in metagenomics have revolutionized the research in fields of microbial ecology and biotechnology, enabling not only a glimpse into the uncultured microbial population and mechanistic understanding of possible biogeochemical cycles and lifestyles of extreme organisms but also the high-throughput discovery of new enzymes for industrial bioconversions. Nowadays, the genetic and enzymatic differences across the gradients from 'neutral and pristine' to 'extreme and polluted' environments are well documented. Yet, extremophilic organisms are possibly the least well understood because our ability to study and understand their metabolic potential has been hampered by our inability to isolate pure cultures. There are at least two obstacles for reaping the fruit of the microbial diversity of extremophiles: first, in spite of the recent progress in development of new culturing techniques most extremophiles cannot be cultured using traditional culturing technologies; and second, the problem of the very low biomass densities often occurs under the conditions hostile for life, which often do not yield enough DNA and reduces the effectiveness of cloning.