Two-liquid phase biocatalysis: epoxidation of 1,7-octadiene by Pseudomonas putida

Deterioration of Tolerance to Hydrophobic Organic Solvents in a Toluene-Tolerant Strain of Pseudomonas putida under the Conditions Lowering Aerobic Respiration

The growth curve (increase in the number of viable cells) of a toluene-tolerant strain Pseudomonas putida Px51T was not reproducible in the presence of harmful organic solvents, such as p-xylene and toluene. The survival often fluctuated the during late exponential phase of growth. The repetitive growth was obtained by maintaining pO2 20-40% (v/v) in the culture flask. However, even under these aerobic conditions, the cells starved for a carbon source were killed by exposure to harmful solvents. The tolerance to organic solvents was lowered greatly by treatment with a proton conductor, carbonyl cyanide m-chlorophenylhydrazone (CCCP), or an electron transport chain inhibitor, sodium azide. Px51T treated with CCCP lost tolerance to a wide variety of organic solvents with log P ow of 2.6-4.2, which the organism usually tolerates. These results indicate that the solvent tolerance of Px51T depends upon on energy produced by aerobic respiration.

In situ product removal (ISPR) in whole cell biotechnology during the last twenty years

This review sums up the activity in the field of in situ product removal in whole cell bioprocesses over the last 20 years. It gives a complete summary of ISPR operations with microbial cells and cites a series of interesting ISPR applications in plant and animal cell technology. All the ISPR projects with microbial cells are categorized according to their products, their ISPR techniques, and their applied configurations of the ISPR set-up. Research on ISPR application has primarily increased in the field of microbial production of aromas and organic acids such lactic acid over the last ten years. Apart from the field of de novo formation of bioproducts, ISPR is increasingly applied to microbial bioconversion processes. However, despite of the large number of microbial whole cell ISPR projects (approximately 250), very few processes have been transferred to an industrial scale. The proposed processes have mostly been too complex and consequently not cost effective. Therefore, this review emphasizes that the planning of a successful whole cell ISPR process should not only consider the choice of ISPR technique according to the physicochemical properties of the product, but also the potential configuration of the whole process set-up. Furthermore, additional process aspects, biological and legal constraint need to be considered from the very beginning for the design of an ISPR project. Finally, future trends of new, modified or improved ISPR techniques are given.

Integrated bioproduction and extraction of 3-methylcatechol

Pseudomonas putida MC2 is a solvent-tolerant strain that accumulates 3-methylcatechol. In aqueous media, 10 mM of 3-methylcatechol was produced and production was limited by 3-methylcatechol toxicity to the biocatalyst. Production levels increased by introduction of a second, organic phase that provides the substrate toluene and extracts the product from the culture medium. Octanol was shown to be an appropriate second phase with respect to tolerance of the strain for this solvent and with respect to partitioning of both substrate and product. Per unit of overall reactor volume (octanol and water), best results were obtained with 50% (v/v) of octanol: an overall 3-methylcatechol concentration of 25 mM was reached with 96% of the product present in the octanol phase. These product concentrations are much higher than in aqueous media without organic solvent, indicating that biocatalysis in an organic/aqueous two-phase system is an improved set-up for high production levels of 3-methylcatechol.

Bioconversion of hydrophobic compounds in a continuous closed-gas-loop bioreactor: feasibility assessment and epoxide production

Microorganisms can be used as catalysts to produce organic compounds in a highly chemo-, regio- and enantioselective manner, and whole cells do not require the costly addition of cofactors for redox reactions. However, bioconversions are slow compared to alternative chemical reactions, and the biocatalyst works at its best in an aqueous medium, while the transformations of interest frequently involve compounds with a low-aqueous solubility and that are toxic to microorganisms. This results in low-volumetric productivity in classical bioreactors. The Continuous Closed-Gas-Loop Bioreactor is described here-a reactor system with high productivity, but without the problems associated with two-phase systems, such as an emulsified product stream and phase toxicity. Its working principle is to recirculate a gas phase through a bioreaction compartment and a saturator/absorber module where the product accumulates as a clear organic solution. A wide range of bioconversions should be possible in this set-up, and proof of concept was established for the epoxidation of 1,7-octadiene to (R)-1,2-epoxyoct-7-ene by a native strain of Pseudomonas oleovorans. This reaction represents a group of terminal alkene epoxidations where the bioconversion substrate does not support growth of the microorganism. Practical results at a 5l-scale are presented for this bioconversion for both batch and continuous operation with respect to the aqueous phase, showing continuous stable epoxidation at productivities >14 micromol min(-1) L(-1) (U L(-1)). The results confirm that the metabolism does not allow a simple optimization strategy, because growth and biotransformation substrates compete for the same enzyme sites, and conversely growth on a substrate using this very enzyme system is necessary for longterm bioconversion. Integrated removal of the CO(2) formed via the liquid overflow was estimated from theory and verified in experimental work.

Production of enantiopure styrene oxide by recombinant Escherichia coli synthesizing a two-component styrene monooxygenase

A whole cell biocatalytic process was developed to enable the efficient oxidation of styrene to chiral (S)-styrene oxide with an enantiomeric excess better than 99%. Recombinant Escherichia coli cells were employed to express the genes styAB encoding the styrene monooxygenase of Pseudomonas sp. strain VLB120 from an expression plasmid utilizing the alk regulatory system of P. oleovorans GPo1. The strains reached specific activities of up to 70 U* (g cell dry weight)(-1) in shake-flask experiments with glucose as the carbon source. An efficient two-liquid phase fed-batch process was established for the production of (S)-styrene oxide with hexadecane as an apolar carrier solvent and a nutrient feed consisting of glucose, magnesium sulfate, and yeast extract. Engineering of the phase fraction and the composition of organic phase and feed led to a 2-L scale process with maximal volumetric productivities of 2.2 g (S)-styrene oxide per liter liquid volume per hour. This optimized process was based completely on defined medium and used bis(2-ethylhexyl)phthalate as the apolar carrier solvent, which together with substrate and inducer consisted of 50% of the total liquid volume. Using this system, we were able to produce per liter liquid volume 11 g of enantiopure (S)-styrene oxide in 10 h.

Appearance of a stress-response protein, phage-shock protein A, in Escherichia coli exposed to hydrophobic organic solvents

A 28 kDa protein associated with the inner membrane was induced strongly in Escherichia coli K-12 cells grown in the presence of a hydrophobic organic solvent, n-hexane or cyclooctane. These organic solvents suppressed the growth (growth rate and yield) of E. coli. A partial amino acid sequence showed that this protein was the phage-shock protein PspA. PspA is known to be induced in E. coli cells under extreme stress conditions. The results suggest that E. coli cells are subject to strong stress in the presence of organic solvents. Introduction of a multi-copy plasmid vector carrying the psp operon into E. coli improved the survival frequency of cells exposed suddenly to n-hexane but not the growth rate of cells growing in the presence of n-hexane.

Solvent selection for whole cell biotransformations in organic media

Although they were used historically as antimicrobial agents, there is a modern requirement to devise organic solvent systems for exploitation in the biotransformation by intact cells of substrates that are poorly soluble in water. Water-immiscible solvents are normally less cytotoxic than are water-miscible ones. While a unitary mechanism is excluded, damage to the membrane remains the likeliest major mechanism of cytotoxicity, and may be conveniently assessed using an electronic biomass probe. Studies designed to account for the mechanisms of action of general anesthetics and of uncouplers parallel those designed to account for the cytotoxicity of organic solvents. Although there are hundreds of potential physical descriptors of solvent properties, many are broadly similar to each other, such that the intrinsic dimensionality of solvent space is relatively small (< 10). This opens up the possibility of providing a rational biophysical basis for the optimization of the solvents used for biotransformations. The widely used descriptor of solvent behavior, log P (the octanol:water partition coefficient), is a composite of more fundamental molecular descriptors; this explains why there are rarely good correlations between cytotoxicity and log P when a wide variety of solvents is studied. Although the intrinsic dimensionality of solvent space is relatively small, pure solvents still populate it rather sparsely. Thus, mixtures of solvents can and do provide the opportunity of obtaining a solvent optimal for a biotransformation of interest.

Oxidative Bioconversion of Cholesterol by Pseudomonas sp. Strain ST-200 in a Water-Organic Solvent Two-Phase System

Pseudomonas sp. strain ST-200, which is capable of conversion of cholesterol, was isolated from humus soil. This organism effectively modified cholesterol dissolved in an organic solvent by dehydrogenation and oxygenation. When the organism was grown in a medium overlaid with a 10% volume of a mixed organic solvent ( p -xylene and diphenylmethane; 3:7, vol/vol) containing cholesterol (20 mg/ml), the cholesterol concentration in the organic solvent was reduced to only 0.4 mg/ml after 8 days. Although the organism did not assimilate cholesterol, 98% of the cholesterol initially present disappeared. The organic solvent layer contained two major and three minor compounds converted from cholesterol. The major compounds were 6β-hydroxycholest-4-en-3-one (8.9 mg/ml) and cholest-4-ene-3,6-dione (7.6 mg/ml). The concentrations of these compounds were equivalent to 43 and 37% of the cholesterol initially present. This organism would provide an effective and convenient system to oxidize the C-3 and -6 positions of cholesterol by introduction of a hydroxyl or ketone group.

Selection of Xenobiotic-Degrading Microorganisms in a Biphasic Aqueous-Organic System

Microbial selection on mixtures of chlorinated and nonchlorinated compounds that are poorly soluble in water and/or toxic to growing microbial cells was examined in both biphasic aqueous-organic and monophasic aqueous systems. A biphasic system in which silicone oil was used as the organic phase permitted the acceleration of acclimation, leading to rapid selection and to an increase in xenobiotic compound degradation. In contrast, acclimation, selection, and degradation were very slow in the monophasic aqueous system. The variation in microbial growth rate with the degree of dispersion (i.e., dispersion at different silicone oil concentrations and agitation rates), and cell adhesion to the silicone oil indicate that the performance of the biphasic aqueous-organic system is dependent on the interfacial area between the two phases and that microbial activity is important at this interface. Therefore, the biphasic water-silicone oil system could be used for microbial selection in the presence of xenobiotic compounds that are toxic and have low water solubility.

Whole cell biocatalysis in nonconventional media

In this paper biocatalytic reactions carried out by whole cells in nonconventional media are reviewed. Similar relationships are observed between solvent hydrophobicity and catalytic activity in reactions carried out by isolated enzymes and whole cells. In addition to the effect of organic solvent on biocatalyst stability, microbial cells are susceptible to damaging effects caused by the organic phase. In general, more hydrophobic solvents manifest lower toxicity towards the cells. Whole cell biocatalysts require more water than isolated enzymes and two-phase systems have been most widely used to study whole cell biocatalysis. Immobilization makes cell biocatalysts more resistant to organic solvents and helps achieve homogeneous biocatalyst dispersion. Cell entrapment methods have been widely used with organic solvent systems and mixtures of natural and/or synthetic polymers allow adjustment of the hydrophobicity-hydrophilicity balance of the support matrix. Some examples of stereoselective catalysis using microbial cells in organic solvent media are presented.

Strategies for enzymatic esterification in organic solvents: Comparison of microaqueous, biphasic, and micellar systems

The kinetics of butyl butyrate synthesis by a lipase from Mucor miehei in different types of organic media were investigated. The three systems studied were a microaqueous medium containing enzyme in suspension in hexane, a water-hexane two-phase system, and reverse micelles. The synthesis of butyl butyrate was possible in all cases because of a favorable partition of the ester into the organic solvent. A sufficient stirring rate was necessary to achieve good reaction rates in the case of the liquid-liquid biphasic medium. The effect of water content was different according to the type of system used. The dependence of reaction rate and of conversion yield on enzyme and substrate concentrations was also investigated. From an applied point of view, the best performances were obtained with either microaqueous or liquid-liquid two-phase systems. The use of reverse micelles can be advocated only in particular conditions, such as low enzyme concentration, compatible with the specific constraints it involves.

Microbial enzymes for oxidation of organic molecules

Enzymatic systems employed by microorganisms for oxidative transformation of various organic molecules include laccases, ligninases, tyrosinases, monooxygenases, and dioxygenases. Reactions performed by these enzymes play a significant role in maintaining the global carbon cycle through either transformation or complete mineralization of organic molecules. Additionally, oxidative enzymes are instrumental in modification or degradation of the ever-increasing man-made chemicals constantly released into our environment. Due to their inherent stereo- and regioselectivity and high efficiency, oxidative enzymes have attracted attention as potential biocatalysts for various biotechnological processes. Successful commercial application of these enzymes will be possible through employing new methodologies, such as use of organic solvents in the reaction mixtures, immobilization of either the intact microorganisms or isolated enzyme preparations on various supports, and genetic engineering technology.

Biocatalysis in nonaqueous media. Patents and literature

Biocatalysis in nonaqueous media is being used in increasing regularity both in academic and industrial research. A variety of biocatalysts have been used in organic media including enzymes, multi-enzyme systems, and whole cells. In addition, the nonaqueous media has encompassed both monophasic and biphasic solvent systems, enzymes and whole cells in reversed micelles, enzymes and cells in nearly anhydrous (no added water) solvents, and enzymes catalytically active in supercritical fluids and the gas phase. Recent US and overseas patents and scientific literature on biocatalysis in nonaqueous media are surveyed. Patent abstracts are summarized individually, and literature references are divided into major subheadings.

Biocatalysis in multi-phase reaction mixtures containing organic liquids

A wide range of enzymes and whole microbial cells will act as catalysts in reaction mixtures that contain 2 or more phases, one of which is an organic liquid (either a reactant or including water-immiscible organic solvents). These "biphasic" systems have a variety of structures, knowledge of which aids predictions about biocatalyst activity and stability. There is often a dilute aqueous solution phase (containing the biocatalyst), which may be emulsified with the organic phase, or "trapped" within catalyst particles; sometimes however there may only be traces of water adsorbed to the enzyme or cells. These reaction systems offer several advantages for industrial applications, notably the higher solubilities of many reactants of interest, and the ability of readily available hydrolytic enzymes to catalyse syntheses. The most non-polar organic liquids are least likely to inactivate biocatalysts, though many do remain active with relatively polar solvents. Modification of the biocatalyst may stabilise against inactivation, especially where this is due to direct contact with the phase interface. The mass transfer processes required in these systems remain poorly understood, particularly because the interfacial area is often unknown. Attractive continuous reactors may be operated using a packed bed of catalyst with a trapped aqueous phase.