D-malate production by permeabilized Pseudomonas pseudoalcaligenes; optimization of conversion and biocatalyst productivity

For the development of a continuous process for the production of solid D-malate from a Ca-maleate suspension by permeabilized Pseudomonas pseudoalcaligenes, it is important to understand the effect of appropriate process parameters on the stability and activity of the biocatalyst. Previously, we quantified the effect of product (D-malate2 -) concentration on both the first-order biocatalyst inactivation rate and on the biocatalytic conversion rate. The effects of the remaining process parameters (ionic strength, and substrate and Ca2 + concentration) on biocatalyst activity are reported here. At (common) ionic strengths below 2 M, biocatalyst activity was unaffected. At high substrate concentrations, inhibition occurred. Ca2+ concentration did not affect biocatalyst activity. The kinetic parameters (both for conversion and inactivation) were determined as a function of temperature by fitting the complete kinetic model, featuring substrate inhibition, competitive product inhibition and first-order irreversible biocatalyst inactivation, at different temperatures simultaneously through three extended data sets of substrate concentration versus time. Temperature affected both the conversion and inactivation parameters. The final model was used to calculate the substrate and biocatalyst costs per mmol of product in a continuous system with biocatalyst replenishment and biocatalyst recycling. Despite the effect of temperature on each kinetic parameter separately, the overall effect of temperature on the costs was found to be negligible (between 293 and 308 K). Within pertinent ranges, the sum of the substrate and biocatalyst costs per mmol of product was calculated to decrease with the influent substrate concentration and the residence time. The sum of the costs showed a minimum as a function of the influent biocatalyst concentration.     

The efficiency of recombinant Escherichia coli as biocatalyst for stereospecific epoxidation

Styrene is efficiently converted into (S)-styrene oxide by growing Escherichia coli expressing the styrene monooxygenase genes styAB of Pseudomonas sp. strain VLB120 in an organic/aqueous emulsion. Now, we investigated factors influencing the epoxidation activity of recombinant E. coli with the aim to improve the process in terms of product concentration and volumetric productivity. The catalytic activity of recombinant E. coli was not stable and decreased with reaction time. Kinetic analyses and the independence of the whole-cell activity on substrate and biocatalyst concentrations indicated that the maximal specific biocatalyst activity was not exploited under process conditions and that substrate mass transfer and enzyme inhibition did not limit bioconversion performance. Elevated styrene oxide concentrations, however, were shown to promote acetic acid formation, membrane permeabilization, and cell lysis, and to reduce growth rate and colony-forming activity. During biotransformations, when cell viability was additionally reduced by styAB overexpression, such effects coincided with decreasing specific epoxidation rates and metabolic activity. This clearly indicated that biocatalyst performance was reduced as a result of product toxicity. The results point to a product toxicity-induced biological energy shortage reducing the biocatalyst activity under process conditions. By reducing exposure time of the biocatalyst to the product and increasing biocatalyst concentrations, volumetric productivities were increased up to 1,800 micromol/min/liter aqueous phase (with an average of 8.4 g/L(aq) x h). This represents the highest productivity reported for oxygenase-based whole-cell biocatalysis involving toxic products.     

Kinetic aspects of glucose oxidation by Gluconobacter oxydans cells immobilized in calcium alginate

Summary Gluconobacter oxydans subspecies suboxydans (ATCC 621 H), when growing at high glucose concentrations, oxidizes this substrate incompletely and gluconic acid accumulates in the medium in almost stoichiometric amounts. Such cells were harvested and entrapped in various alginate gels. The preparation with the highest retention of glucose oxidizing activity was used in further studies with the aim of developing an efficient process for continuous gluconic acid production.The retention of activity increases (up to 95%) as the alginate concentration in the gel decreases or the cell/alginate weight ratio is enhanced. In the latter case, however, transport of oxygen to and inside the biocatalyst beads rapidly becomes rate-limiting and thus lowers the efficiency of the biocatalyst. Similarly, the efficiency decreases as the size of the biocatalyst beads increases. In no case rate-limitation by transport of glucose was found. Thus, biocatalyst activity per unit volume of support, diameter of the biocatalyst beads, and aeration efficiency are important parameters for reactor design.     

Immobilization of Candida cylindracea lipase on poly lactic acid,polyvinyl alcohol and chitosan based ternary blend film: Characterization,activity, stability and its application for N-acylation reactions

The ecofriendly ternary blend polymer film was prepared from the chitosan (CH), polylactic acid (PLA) and polyvinyl alcohol (PVA). Immobilization of Candida cylindracea lipase (CCL) was carried out on ternary blend polymer via entrapment methodology. The ternary blend polymer and immobilized biocatalyst were characterized by using N₂ adsorption–desorption isotherm, SEM, FTIR, DSC, and (%) water content analysis through Karl Fischer technique. Biocatalyst was then subjected for the determination of practical immobilization yield, protein loading and specific activity. Immobilized biocatalyst was further applied for the determination of biocatalytic activity for N-acylation reactions. Various reaction parameters were studied such as effect of immobilization support (ratio of PLA:PVA:CH), molar ratio (dibutylamine:vinyl acetate), solvent, biocatalyst loading, time, temperature, and orbital speed rotation. The developed protocol was then applied for the N-acylation reactions to synthesize several industrially important acetamides with excellent yields. Interestingly, immobilized lipase showed fivefold higher catalytic activity and better thermal stability than the crude extract lipase CCL. Furthermore various kinetic and thermodynamic parameters were studied and the biocatalyst was efficiently recycled for four successive reuses. It is noteworthy to mention that immobilized biocatalyst was stable for period of 300 days.     

ENHANCEMENT OF PROPYL GALLATE YIELD IN NONAQUEOUS MEDIUM USING NOVEL CELL-ASSOCIATED TANNASE OF Bacillus massiliensis

Enzymatic synthesis of propyl gallate in organic solvent was studied using cell-associated tannase (EC 3.1.1.20) of Bacillus massiliensis. Lyophilized biomass showing tannase activity was used as the biocatalyst. The effects of solvent, surfactant treatment, and bioimprinting on the propyl gallate synthesis were studied and subsequently optimized. Among various solvents, benzene followed by hexane was found to be the most favorable. Treatment of the biocatalyst with Triton X-100 at a lower concentration (0.2% w/v), before lyophilization, increased the propyl gallate yield by 24.5% compared to the untreated biocatalyst. The biocatalyst was imprinted with various concentrations of gallic acid and tannic acid. Biocatalyst imprinted with tannic acid showed 50% enhancement in the propyl gallate yield compared to the non-imprinted biocatalyst.     

Immobilization of Acetobacter sp. CCTCC M209061 for efficient asymmetric reduction of ketones and biocatalyst recycling

ABSTRACT: BACKGROUND: The bacterium Acetobacter sp. CCTCC M209061 is a promising whole-cell biocatalyst with exclusive anti-Prelog stereoselectivity for the reduction of prochiral ketones that can be used to make valuable chiral alcohols such as (R)-4-(trimethylsilyl)-3-butyn-2-ol. Although it has promising catalytic properties, its stability and reusability are relatively poor compared to other biocatalysts. Hence, we explored various materials for immobilizing the active cells, in order to improve the operational stability of biocatalyst. RESULTS: It was found that Ca-alginate give the best immobilized biocatalyst, which was then coated with chitosan to further improve its mechanical strength and swelling-resistance properties. Conditions were optimized for formation of reusable immobilized beads which can be used for repeated batch asymmetric reduction of 4[prime]-chloroacetophenone. The optimized immobilized biocatalyst was very promising, with a specific activity of 85% that of the free-cell biocatalyst (34.66 mumol/min/g dw of cells for immobilized catalyst vs 40.54 mumol/min/g for free cells in the asymmetric reduction of 4[prime]-chloroacetophenone). The immobilized cells showed better thermal stability, pH stability, solvent tolerance and storability compared with free cells. After 25 cycles reaction, the immobilized beads still retained >50% catalytic activity, which was 3.5 times higher than degree of retention of activity by free cells reused in a similar way. The cells could be recultured in the beads to regain full activity and perform a further 25 cycles of the reduction reaction. The external mass transfer resistances were negligible as deduced from Damkohler modulus Da < <1, and internal mass transfer restriction affected the reduction action but was not the principal rate-controlling step according to effectiveness factors eta < 1 and Thiele modulus 0.3<[empty set] <1. CONCLUSIONS: Ca-alginate coated with chitosan is a highly effective material for immobilization of Acetobacter sp. CCTCC M209061 cells for repeated use in the asymmetric reduction of ketones. Only a small cost in terms of the slightly lower catalytic activity compared to free cells could give highly practicable immobilized biocatalyst.     

Immobilization of mycobacterial cells onto silicone--assessing the feasibility of the immobilized biocatalyst in the production of androstenedione from sitosterol

Silicone rubbers are hydrophobic, a feature that may prove advantageous if this material is to be used as immobilization matrix in bioconversion systems where hydrophobic species are present, such as sterols and mycobacterial cells. Mycobacterium sp. cells with sitosterol side chain cleavage activity were accordingly effectively adsorbed onto silicone and the potential application of the concept was assessed by matching the behavior of the resulting immobilized biocatalyst with free cells and Celite immobilized cells. Mass transfer, kinetics, thermal and storage stability characterization of a biotransformation system based in the use of the silicone immobilized biocatalyst was performed. The feasibility of biocatalyst reutilization was tentatively explored.     

Continuous desulfurization of dibenzothiophene with Rhodococcusrhodochrous IGTS8 (ATCC 53968)

Desulfurization of a model fuel system consisting of hexadecane and dibenzothiophene (DBT) by Rhodococcus rhodochrous IGTS8 was demonstrated in a 2-L continuous stirred tank reactor (CSTR). The reactor was operated in a semicontinuous and continuous mode with and without recycling of the model fuel. A constant volumetric desulfurization activity A(t), (in mg HBP L(-1) h(-1)) was maintained in the reactor with a feeding strategy of fresh cell suspension based on a first-order decay of the biocatalyst. Maximum desulfurization rates, as measured by specific desulfurization activity, of 1.9 mg HBP/g DCW h were attained. Rates of biocatalyst decay were on the order of 0.072 h(-1). Theoretical predictions of a respiratory quotient (RQ) associated with this biotransformation reaction agree well with experimental data from off-gas analysis. In addition, the ratio of the specific desulfurization activity a(t), (in mg HBP/g DCW h) of recycled and fresh biocatalyst was determined and evaluated.     

Efficient immobilization of epoxide hydrolase onto silica gel and use in the enantioselective hydrolysis of racemic para-nitrostyrene oxide

Epoxide hydrolase from Aspergillus niger (E.C. 3.3.2.3) was immobilized by covalent linking to epoxide-activated silica gel under mild conditions. A very easy procedure allowed to prepare an immobilized biocatalyst with more than 90% retention of the initial enzymatic activity. Immobilized and free enzyme showed very similar behaviour with respect to the effect of pH on activity and stability. One benefit of immobilizing epoxide hydrolase from A. niger on silica gel was the enhanced enzyme stability in the presence of 20% DMSO. The kinetic resolution of racemic para-nitrostyrene oxide was investigated by using this new immobilized biocatalyst. The enantioselectivity of the enzyme was not altered by the immobilization reaction: both unreacted epoxide and formed diol were obtained with very high ee (99 and 92%, respectively). In addition, the biocatalyst could be easily separated from the reaction mixture and re-used for over nine cycles without any noticeable loss of enzymatic activity or change in the enantioselectivity extent. The activity of immobilized AnEH was retained for several months.     

Continuous sucrose hydrolysis by yeast cells immobilized to wool

A novel immobilized biocatalyst with invertase activity was prepared by adhesion of yeast cells to wool using glutaraldehyde. Yeast cells could be immobilized onto wool by treating either the yeast cells or wool or both with glutaraldehyde. Immobilized cells were not desorbed by washing with 1 M KCl or 0.1 M buffers, pH 3.5–7.5. The biocatalyst shows a maximum enzyme activity when immobilized at pH 4.2–4.6 and 7.5–8.0. The immobilized biocatalyst was tested in a tubular fixed-bed reactor to investigate its possible application for continuous full-scale sucrose hydrolysis. The influence of temperature, sugar concentration and flow rate on the productivity of the reactor and on the specific productivity of the biocatalyst was studied. The system demonstrates a very good productivity at a temperature of 70 °C and a sugar concentration of 2.0 M. The increase of the volume of the biocatalyst layer exponentially increases the productivity. The productivity of the immobilized biocatalyst decreases no more than 50% during 60 days of continuous work at 70 °C and 2.0 M sucrose, but during the first 30 days it remains constant. The cumulative biocatalyst productivity for 60 days was 4.8 × 10³kg inverted sucrose/kg biocatalyst. The biocatalyst was proved to be fully capable of continuous sucrose hydrolysis in fixed-bed reactors. Received: 8 November 1996 / Received revision: 31 January 1997 / Accepted: 31 January 1997     

Selection of a whole-cell biocatalyst for methyl parathion biodegradation

Whole-cell biocatalyst has the potential to become a cost-effective alternative to conventional enzyme methods for solving ecological and energy issues. However, cytosolic-expressing biocatalyst systems are critically disadvantaged due to the low permeability of the cell membrane. To overcome substrate transport barrier, periplasmic secretion and surface display biocatalysts were developed by expressing signal peptides or anchor proteins in Escherichia coli. In this work, six carriers were compared in regard to whole-cell activity of methyl parathion hydrolase (MPH). Our results indicate that the surface display systems yielded one to three times whole-cell activity than the periplasmic secretion systems. Although periplasmic secretion systems showed generally more stable than surface display systems, surface display appeared more suitable for whole-cell biocatalyst. It should note that the applicability of the DsbA/PhoA/AIDA-I leader to MPH expression is shown here for the first time. In addition, the result provided a useful reference for other whole-cell biocatalyst selection.     

Improvement of the enzymatic synthesis of ethyl valerate by esterification reaction in a solvent system

The present study reports the improved enzymatic synthesis of ethyl valerate (green apple flavor) by esterification reaction of ethanol and valeric acid in heptane medium. Lipase from Thermomyces lanuginosus (TLL) was immobilized by physical adsorption on polyhydroxybutyrate (PHB) particles and used as a potential biocatalyst. The effect of certain parameters that influence the ester synthesis was evaluated by factorial design. The experimental conditions that maximized the synthesis of ethyl valerate were 30.5°C, 18% m/v of biocatalyst (TLL–PHB), absence of molecular sieves, agitation of 234?rpm, and 1,000?mM of each reactant (ethanol and valeric acid). Under these conditions, conversion percentage ≈92% after 105?min of reaction was observed. Soluble TLL was also used as biocatalyst and the highest conversion was of 82% after 120?min of reaction. Esterification reaction performed in a solvent-free system exhibited conversion of 13% after 45?min of reaction catalyzed by immobilized lipase, while the soluble lipase did not exhibit catalytic activity. The synthesis of the ester was confirmed by Fourier transform infrared spectroscopy and gas chromatography–mass spectrometry analyses. After six consecutive cycles of ethyl valerate synthesis, the prepared biocatalyst retained ≈86% of its original activity.     

Methane hydroxylation by <Emphasis Type="Italic">Methylosinus trichosporium</Emphasis> OB3b: Monitoring the biocatalyst activity for methanol production optimization in an innovative membrane bioreactor

A quasi-total loss of the bacterial hydroxylating activity was identified to be responsible for methanol production stop. Different strategies acting on the reaction mixture were implemented to apprehend the biocatalyst behavior in view to extend methanol production. Activity monitoring showed first that sodium formate addition did not maintain the biocatalyst activity and even disrupted bacterial equilibrium when added into the reaction mixture with still active biocatalysts. Reaction medium renewals had no influence on methanol production and highlighted a limited hydroxylating potential of the biocatalyst while addition of fresh biocatalysts in the reaction mixture resulted in methanol consumption. Finally, performing hydroxylation directly in the native bacterial culture appeared as a way to enhance methanol production by both release of intracellular methanol accumulated in the cells during cultivation and effective production by methane hydroxylation.     

Maximizing the stability of metabolic engineering‐derived whole‐cell biocatalysts

A systematic and powerful knowledge‐based framework exists for improving the activity and stability of chemical catalysts and for empowering the commercialization of respective processes. In contrast, corresponding biotechnological processes are still scarce and characterized by case‐by‐case development strategies. A systematic understanding of parameters affecting biocatalyst efficiency, that is, biocatalyst activity and stability, is essential for a rational generation of improved biocatalysts. Today, systematic approaches only exist for increasing the activity of whole‐cell biocatalysts. They are still largely missing for whole‐cell biocatalyst stability. In this review, we structure factors affecting biocatalyst stability and summarize existing, yet not completely exploited strategies to overcome respective limitations. The factors and mechanisms related to biocatalyst destabilization are discussed and demonstrated inter alia based on two case studies. The factors are similar for processes with different objectives regarding target molecule or metabolic pathway complexity and process scale, but are in turn highly interdependent. This review provides a systematic for the stabilization of whole‐cell biocatalysts. In combination with our knowledge on strategies to improve biocatalyst activity, this paves the way for the rational design of superior recombinant whole‐cell biocatalysts, which can then be employed in economically and ecologically competitive and sustainable bioprocesses.     

Use of immobilized Bacillus subtilis for the stereospecific hydrolysis of d,l-menthyl acetate

Bacillus subtilis possessing a stereospecific menthyl esterase has been immobilized in a polyurethane foam and used for the hydrolysis of menthyl ester. The specific activity of cells immobilized in polyurethane foam decreased as the cell loading was increased. The aqueous content of the immobilized biocatalyst particles influenced the activity. The activity half-life of about 400 h observed for the immobilized biocatalyst is 66 × greater than that observed for free bacteria.     

Comparison of batch and repeated batch operation of lactulose synthesis with cross-linked aggregates of Bacillus circulans β-galactosidase

Synthesis of lactulose with crosslinked aggregates of Bacillus circulans β-galactosidase (CLAGs) has been compared in batch and repeated-batch operation for the first time. The effect of the type of the precipitating agent and its concentration, the crosslinker concentration and the time of crosslinking were evaluated for their effect on the parameters: immobilization yield, specific activity and thermal stability of the biocatalysts. The type and concentration of the precipitating agent were the variables that produced a significant variation in the immobilization parameters of the biocatalyst. CLAGs were obtained with a specific activity of 7790 IU_H⋅g⁻¹ at an immobilization yield of 46.2 % using 50 % v/v of propanol as precipitating agent, 5.5 g_{glutarldehyde} g_protein⁻¹ for crosslinking and 1 h of crosslinking time. This biocatalyst was more stable than the free enzyme with a stabilization factor of 11.3 h at 50 °C. Highest yield of lactulose synthesis with CLAGs was 0.42 g g⁻¹ for a fructose/lactose molar ratio of 8. Repeated-batch operation allowed a significant increase in lactulose production per unit mass of biocatalyst and in cumulative productivity with respect to batch operation, yielding an efficiency of biocatalyst use of 2.43 kg_lactulose g_{biocatalyst protein}⁻¹.     

Continuous sucrose hydrolysis by an immobilized whole yeast cell biocatalyst

An immobilized biocatalyst with invertase activity prepared by immobilization of whole yeast cells without use of any insoluble carrier was tested in tubular fixed-bed reactors from the point of view of possible application for continuous full-scale sucrose hydrolysis. At inlet sucrose concentration above 60% (w/w) and reaction temperature 60–70°C, total sucrose hydrolysis was achieved at a flow rate of 0.6–1.5 bed volumes per hour. At a flow rate about 10 bed volumes per hour, the conversion was still 0.5. The specific productivity of the biocatalyst was 3–25 h⁻¹; the productivity of the reactor was 1–9 kg l⁻¹ h⁻¹. The half-life of the biocatalyst invertase activity was 815 h at 70°C. The specific pressure drop over the biocatalyst bed was less than 23 kPa m⁻¹. The biocatalyst was proved to be fully capable of continuous sucrose hydrolysis in fixed-bed reactors.     

Preparation of L-aspartic acid by means of immobilized Alcaligenes metalcaligenes cells

Two types of biocatalysts based on immobilized cells of Alcaligenes metalcaligenes exhibiting aspartate ammonia-lyase activity (EC 4.3.1.1) were developed for the enzymic preparation of L-aspartic acid from ammonium fumarate. The first type of the biocatalyst consists in individual covalently crosslinked and permeabilized cells(I), while the second type is represented by cell aggregates (II). For the above preparation, biocatalyst I can be used only discontinuously in a mixed reactor. After termination of the reaction between individual cycles of its use, the biocatalyst is returned to the reactor in the form of a highly concentrated cell suspension or paste. Biocatalyst II can be used discontinuously or continuously in a fixed-bed column of the catalyst. The effects of pH, substrate concentration and temperature on the reaction velocity and effectivity of enzymic conversion was investigated. Optimal parameters of the reaction are as follows: pH 8.5, initial substrate concentration, 1.35 mol/L, temperature for discontinuous process, 37 degrees C, and temperature for continuous process, 25 degrees C. Under these conditions the enzymic conversion of substrate to product is quantitative. Under optimal toring conditions, the specific activity of both catalysts does not change within a period of one year. The operational half-life of the biocatalyst II during continuous use in a fixed-bed column of the catalyst under standard reaction conditions depends on the quality of the substrate. The discontinuous preparation of L-asparatic acid with the aid of biocatalyst I and continuous preparation of this product with the aid of biocatalyst II have been verified under pilot-plant conditions.     

Characterisation of a novel support for biocatalysis in supercritical carbon dioxide

The work presents a characterisation study of Accurel EP100 (polypropylene based hydrophobic granules) as support material for lipase (Lypozyme 10,000 l, from native Rhizomucor miehei) operating as biocatalyst in supercritical CO₂ as solvent. The study involved assay of biocatalytic activity and operational stability as functions of system pressure and temperature. Furthermore, the presence of diffusion limitations was tested, by varying the bed diameter and support particle size. In addition, SEM and Gas Absorption were employed to test the mechanical stability. Results were compared with the commercially available biocatalyst Lipozyme™ IM60.

Pressure did not have a significant effect on the activity or the stability, while temperature had a positive effect on the activity and negative effect on the stability. As expected, an ‘optimum’ value of system water content gave maximum catalytic activity for each biocatalyst. External- and internal-diffusion limitations were both found negligible. The mechanical stability analysis demonstrated little (if any) effect of supercritical carbon dioxide (scCO₂) on the structural integrity of Accurel EP100, although subtle increases in pore volume and surface area were observed.     

Development of a magnetic biocatalyst useful for the synthesis of ethyloleate

Candida antarctica Lipase B was successfully immobilized on magnetite (Fe₃O₄) nanoparticles functionalized with chitosan and glutaraldehyde. The obtained magnetic catalyst was characterized and its performance was evaluated in solvent-free synthesis of ethyl oleate at room temperature. The performance of this biocatalyst was compared with the commercial Novozym 435, as a tool to estimate the efficiency of immobilization. It was found that using 33 mg of the biocatalyst it was possible to reach almost the same activity that was obtained using 12 mg of Novozym 435. Furthermore, this new biocatalyst presents the advantages of not being degraded by short alcohols, being easily recovered from the reaction media by magnetic decantation, and low fabrication cost. The possibility of reutilization was also studied, keeping a significant activity up to eight cycles. A special sampling protocol was also developed for the multiphasic reaction system, to assure accurate results. This novel biocatalyst is an interesting alternative for potential industrial applications, considering the above-mentioned advantages.