Direct measurement of pH profiles in immobilized enzyme carriers during kinetically controlled synthesis using CLSM

Is enzyme immobilization a mature discipline? Some critical considerations to capitalize on the benefits of immobilization

Enzyme immobilization has been developing since the 1960s and although many industrial biocatalytic processes use the technology to improve enzyme performance, still today we are far from full exploitation of the field. One clear reason is that many evaluate immobilization based on only a few experiments that are not always well-designed. In contrast to many other reviews on the subject, here we highlight the pitfalls of using incorrectly designed immobilization protocols and explain why in many cases sub-optimal results are obtained. We also describe solutions to overcome these challenges and come to the conclusion that recent developments in material science, bioprocess engineering and protein science continue to open new opportunities for the future. In this way, enzyme immobilization, far from being a mature discipline, remains as a subject of high interest and where intense research is still necessary to take full advantage of the possibilities.

Intraparticle pH Sensing Within Immobilized Enzymes: Immobilized Yellow Fluorescent Protein as Optical Sensor for Spatiotemporal Mapping of pH Inside Porous Particles

pH is a fundamental variable in enzyme catalysis and its measurement therefore is crucial for understanding and optimizing enzyme-catalyzed reactions. Whereas measurements within homogeneous bulk liquid solution are prominently used, enzymes immobilized inside porous particles often suffer from pH gradients due to partition effects and heterogeneously catalyzed biochemical reactions. Unfortunately, the measurements of intraparticle pH are not available due to the lack of useful suitable methodologies; as a consequence the biocatalyst characterization is hampered. Here, a fully biocompatible methodology for real-time optical sensing of pH within porous materials is described. A genetically encoded ratiometric pH indicator, the superfolder yellow fluorescent protein (sYFP), is used to functionalize the internal surface of enzyme carrier supports. By using controlled, tailor-made immobilization, sYFP is homogeneously distributed within these materials, and so enables, via self-referenced imaging analysis, pH measurements in high accuracy and with useful spatiotemporal resolution. The hydrolysis of penicillin by a penicillin acylase, taking place in solution or confined to the solid surface of the porous matrix is used to show the monitoring of evolution of internal pH. Thus, pH sensing based on immobilized sYFP represents a broadly applicable technique to the study of the internally heterogeneous environment of immobilized enzymes into solid particles.

Model development for enzymatic reactive crystallization of β-lactam antibiotics: a reaction–diffusion-crystallization approach

A mathematical model for production of β-lactam antibiotics via enzymatic reactive crystallization is developed, and its application for catalyst and process design is discussed.

The Microenvironment in Immobilized Enzymes: Methods of Characterization and Its Role in Determining Enzyme Performance

The liquid milieu in which enzymes operate when they are immobilized in solid materials can be quite different from the milieu in bulk solution. Important differences are in the substrate and product concentration but also in pH and ionic strength. The internal milieu for immobilized enzymes is affected by the chemical properties of the solid material and by the interplay of reaction and diffusion. Enzyme performance is influenced by the internal milieu in terms of catalytic rate (“activity”) and stability. Elucidation, through direct measurement of differences in the internal as compared to the bulk milieu is, therefore, fundamentally important in the mechanistic characterization of immobilized enzymes. The deepened understanding thus acquired is critical for the rational development of immobilized enzyme preparations with optimized properties. Herein we review approaches by opto-chemical sensing to determine the internal milieu of enzymes immobilized in porous particles. We describe analytical principles applied to immobilized enzymes and focus on the determination of pH and the O₂ concentration. We show measurements of pH and [O₂] with spatiotemporal resolution, using in operando analysis for immobilized preparations of industrially important enzymes. The effect of concentration gradients between solid particle and liquid bulk on enzyme performance is made evident and quantified. Besides its use in enzyme characterization, the method can be applied to the development of process control strategies.

Design, characterisation and application of alginate-based encapsulated pig liver esterase

Encapsulation of hydrolases in biopolymer-based hydrogels often suffers from low activities and encapsulation efficiencies along with high leaching and unsatisfactory recycling properties. Exemplified for the encapsulation of pig liver esterase the coating of alginate and chitosan beads have been studied by creating various biopolymer hydrogel beads. Enzyme activity and encapsulation efficiency were notably enhanced by chitosan coating of alginate beads while leaching remained nearly unchanged. This was caused by the enzymatic reaction acidifying the matrix, which increased enzyme retention through enhanced electrostatic enzyme-alginate interaction but decreased activity through enzyme deactivation. A practical and ready-to-use method for visualising pH in beads during reaction by co-encapsulation of a conventional pH indicator was also found. Our method proves that pH control inside the beads can only be realised by buffering. The resulting beads provided a specific activity of 0.267 μmol ∙ min^-1 ∙ mg^-1, effectiveness factor 0.88, encapsulation efficiency of 88%, 5% leaching and good recycling properties. This work will contribute towards better understanding and application of encapsulated hydrolases for enzymatic syntheses.

Biobased, Internally pH-Sensitive Materials: Immobilized Yellow Fluorescent Protein as an Optical Sensor for Spatiotemporal Mapping of pH Inside Porous Matrices

The pH is fundamental to biological function and its measurement therefore crucial across all biosciences. Unlike homogenous bulk solution, solids often feature internal pH gradients due to partition effects and confined biochemical reactions. Thus, a full spatiotemporal mapping for pH characterization in solid materials with biological systems embedded in them is essential. In here, therefore, a fully biocompatible methodology for real-time optical sensing of pH within porous materials is presented. A genetically encoded ratiometric pH sensor, the enhanced superfolder yellow fluorescent protein (sYFP), is used to functionalize the internal surface of different materials, including natural and synthetic organic polymers as well as silica frameworks. By using controlled, tailor-made immobilization, sYFP is homogenously distributed within these materials and so enables, via self-referenced imaging analysis, pH measurements in high accuracy and with useful spatiotemporal resolution. Evolution of internal pH is monitored in consequence of a proton-releasing enzymatic reaction, the hydrolysis of penicillin by a penicillin acylase, taking place in solution or confined to the solid surface of the porous matrix. Unlike optochemical pH sensors, which often interfere with biological function, labeling with sYFP enables pH sensing without altering the immobilized enzyme's properties in any of the materials used. Fast response of sYFP to pH change permits evaluation of biochemical kinetics within the solid materials. Thus, pH sensing based on immobilized sYFP represents a broadly applicable technique to the study of biology confined to the internally heterogeneous environment of solid matrices.

Microenvironmental pH changes in immobilized cephalosporin C acylase during a proton-producing reaction and regulation by a two-stage catalytic process

Cephalosporin C acylase (CCA), a proton-producing enzyme, was covalently bound on an epoxy-activated porous support. The microenvironmental pH change in immobilized CCA during the reaction was detected using pH-sensitive fluorescein labeling. The high catalytic velocity of the initial stage of conversion resulted in a sharp intraparticle pH gradient, which was likely the key factor relating to low operational stability. Accordingly, a novel strategy for a two-stage catalytic process was developed to reduce the reaction rate of stage I at a low temperature to preserve enzymatic activity and to shorten the duration of catalysis at a high reaction temperature in stage II. The reaction using the two-stage catalytic process (10-37°C shift at 30min) showed significantly improved stability compared with that of the single-temperature reaction at 37°C (29 batches versus five batches, respectively) and a shorter catalytic period than the reaction at 10°C (40min versus 70min, respectively).

Dual lifetime referencing enables pH-control for oxidoreductions in hydrogel-stabilized biphasic reaction systems

pH-shifts are a serious challenge in cofactor dependent biocatalytic oxidoreductions. Therefore, a pH control strategy was developed for reaction systems, where the pH value is not directly measurable. Such a reaction system is the biphasic aqueous-organic reaction system, where the oxidoreduction of hydrophobic substrates in organic solvents is catalysed by hydrogel-immobilized enzymes, and enzyme-coupled cofactor regeneration is accomplished via formate dehydrogenase, leading to a pH-shift. Dual lifetime referencing (DLR), a fluorescence spectroscopic method, was applied for online-monitoring of the pH-value within the immobilizates during the reaction, allowing for a controlled dosage of formic acid. It could be shown that by applying trisodium 8-hydroxypyrene-1, 3, 6-trisulfonate as pH indicator and Ru(II) tris(4, 7-diphenyl-1, 10-phenantroline) (Ru[dpp]) as a reference luminophore the control of the pH-value in a macroscopic gel-bead-stabilized aqueous/organic two phase system in a range of pH 6.5 to 8.0 is possible. An experimental proof of concept could maintain a stable pH of 7.5 ± 0.15 during the reaction for at least 105 h. With these results, it could be shown that DLR is a powerful tool for pH-control within reaction systems with no direct access for conventional pH-measurement.

Polyethyleneglycol diacrylate microspheres: a novel carrier for laccase immobilisation

Laccase was immobilised on polyethyleneglycol diacrylate (PEGDA) microspheres. The optimal preparation conditions of PEGDA microspheres were as follows: 3.0% (w/v) 2,2-azobisisobutyro-nitrite (AIBN), 4.0-5.0% (w/v) polyvinylpyrrolidone (PVP), 5.0-8.0% (w/v) glucose and 4.0% (w/v) PEGDA in glucose solution. The volume ratio of PEGDA solution, glucose/PVP solution and AIBN solution was 25: 100: 1. Microspheres obtained exhibited good characteristics with small sizes (1-4 µm). The immobilised laccase showed a higher stability in a wide pH range. Thermal stability and storage stability of immobilised laccase were enhanced. The activity of immobilised laccase was 45.0% after six cycles uses. Only 62.7% of the activity remained for free laccase while there was a 60.4% increased for immobilised laccase with storage at 4 °C for 25 d. The Km value of laccase increased from 21.9 to 114.0 µmol/l after immobilisation.

Shine a light on immobilized enzymes: real-time sensing in solid supported biocatalysts

Enzyme immobilization on solid supports has been key to biotransformation development. Although technologies for immobilization have largely reached maturity, the resulting biocatalysts are not well understood mechanistically. One limitation is that their internal environment is usually inferred from external data. Therefore, biological consequences of the immobilization remain masked by physical effects of mass transfer, obstructing further development. Work reviewed herein shows that opto-chemical sensing performed directly within the solid support enables the biocatalyst's internal environment to be uncovered quantitatively and in real time. Non-invasive methods of intraparticle pH and O2 determination are presented, and their use as process analytical tools for development of heterogeneous biocatalysts is described. Method diversification to other analytes remains a challenging task for the future.

Dual-lifetime referencing (DLR): a powerful method for on-line measurement of internal pH in carrier-bound immobilized biocatalysts

Background

Industrial-scale biocatalytic synthesis of fine chemicals occurs preferentially as continuous processes employing immobilized enzymes on insoluble porous carriers. Diffusional effects in these systems often create substrate and product concentration gradients between bulk liquid and the carrier. Moreover, some widely-used biotransformation processes induce changes in proton concentration. Unlike the bulk pH, which is usually controlled at a suitable value, the intraparticle pH of immobilized enzymes may deviate significantly from its activity and stability optima. The magnitude of the resulting pH gradient depends on the ratio of characteristic times for enzymatic reaction and on mass transfer (the latter is strongly influenced by geometrical features of the porous carrier). Design and selection of optimally performing enzyme immobilizates would therefore benefit largely from experimental studies of the intraparticle pH environment. Here, a simple and non-invasive method based on dual-lifetime referencing (DLR) for pH determination in immobilized enzymes is introduced. The technique is applicable to other systems in which particles are kept in suspension by agitation.

Results

The DLR method employs fluorescein as pH-sensitive luminophore and Ru(II) tris(4,7-diphenyl-1,10-phenantroline), abbreviated Ru(dpp), as the reference luminophore. Luminescence intensities of the two luminophores are converted into an overall phase shift suitable for pH determination in the range 5.0-8.0. Sepabeads EC-EP were labeled by physically incorporating lipophilic variants of the two luminophores into their polymeric matrix. These beads were employed as carriers for immobilization of cephalosporin C amidase (a model enzyme of industrial relevance). The luminophores did not interfere with the enzyme immobilization characteristics. Analytical intraparticle pH determination was optimized for sensitivity, reproducibility and signal stability under conditions of continuous measurement. During hydrolysis of cephalosporin C by the immobilizate in a stirred reactor with bulk pH maintained at 8.0, the intraparticle pH dropped initially by about 1 pH unit and gradually returned to the bulk pH, reflecting the depletion of substrate from solution. These results support measurement of intraparticle pH as a potential analytical processing tool for proton-forming/consuming biotransformations catalyzed by carrier-bound immobilized enzymes.

Conclusions

Fluorescein and Ru(dpp) constitute a useful pair of luminophores in by DLR-based intraparticle pH monitoring. The pH range accessible by the chosen DLR system overlaps favorably with the pH ranges at which enzymes are optimally active and stable. DLR removes the restriction of working with static immobilized enzyme particles, enabling suspensions of particles to be characterized also. The pH gradient developed between particle and bulk liquid during reaction steady state is an important carrier selection parameter for enzyme immobilization and optimization of biocatalytic conversion processes. Determination of this parameter was rendered possible by the presented DLR method.

Buffer salt effect on pH in the interior of an anion exchange resin

The internal pH of Q Sepharose Fast Flow anion exchange resin in equilibrium with a bis-tris acetate buffer solution is investigated as a function of buffer salt concentration. Direct evidence of a resin phase pH shift is presented. At low buffer salt concentrations of 20 mM NaCl the resin phase pH is found to be as much as 1.1 pH units greater than that of the buffer phase, approaching to within 0.1 units of the buffer phase at salt concentrations greater than 250 mM. An ideal model with no adjustable parameters based on the Boltzmann distribution and the electroneutrality condition provides excellent agreement with experimental observations. The model assumes that small ions do not bind to the resin fixed charge sites and the agreement between the model predictions and observed resin internal pH suggests that strong electrolytes do not form ion pairs with the resin fixed charge sites.

A multicomponent reaction–diffusion model of a heterogeneously distributed immobilized enzyme

A physical model was derived for the synthesis of the antibiotic cephalexin with an industrial immobilized penicillin G acylase, called Assemblase. In reactions catalyzed by Assemblase, less product and more by-product are formed in comparison with a free-enzyme catalyzed reaction. The model incorporates reaction with a heterogeneous enzyme distribution, electrostatically coupled transport, and pH-dependent dissociation behavior of reactants and is used to obtain insight in the complex interplay between these individual processes leading to the suboptimal conversion. The model was successfully validated with synthesis experiments for conditions ranging from heavily diffusion limited to hardly diffusion limited, including substrate concentrations from 50 to 600 mM, temperatures between 273 and 303 K, and pH values between 6 and 9. During the conversion of the substrates into cephalexin, severe pH gradients inside the biocatalytic particle, which were previously measured by others, were predicted. Physical insight in such intraparticle process dynamics may give important clues for future biocatalyst design. The modular construction of the model may also facilitate its use for other bioconversions with other biocatalysts.

New insights in the spatially resolved dynamic pH measurement in macroscopic large absorbent particles by confocal laser scanning microscopy

Both, experimental investigation of protein adsorption processes and mathematical models describing such processes indicate, that the pH in the absorbent particle might be the key factor for an improved understanding of these chromatographic processes. Thus, a technique aiming at the spatially resolved pH measurement in macroscopic large absorbent particles is presented. The first application of this method, being based on confocal laser scanning microscopy (CLSM), revealed an apparent dependence of the pH calibration curve on the scanning depth. By a model-based approach, factors distorting the measurement signal are identified: The wavelength-dependent light scattering and the re-absorption of emitted light. The resulting consequences for further development and application of CLSM based techniques to measure pH in macroscopic large absorbent particles are illustrated and discussed.

Direct observation of intraparticle equilibration and the rate-limiting step in adsorption of proteins in chromatographic adsorbents with confocal laser scanning microscopy

The adsorption of different proteins in a single biospecific and hydrophobic adsorbent particle for preparative protein chromatography has been observed directly by confocal laser scanning microscopy as a function of time at a constant bulk concentration c(b). The bulk concentration was in the non-linear part of the adsorption isotherm. At all times the concentration of free protein at the particle surface was almost equal to the bulk content indicating that external mass transfer resistance is not rate limiting for the adsorption under these conditions. Inside the particles a distinct maximum in adsorbed and free protein concentration that moved inside to a distance of approximately 0.2 R (R particle radius) from the particle surface, was observed. This is due to a decreasing solid-phase density and adsorptive capacity in the particle between 0.8 R and R indicating that the fraction of macropores (or void space) is larger in the outer than in the inner part of the adsorbent particles. By increasing the bulk concentration by a factor of 10 the equilibration time was reduced by about the same magnitude. This is in agreement with the concentration dependence of the effective pore diffusion coefficient D(p,eff)=D(p)/[epsilon(p)[1+nK/(K +c)(2)]] derived from the mass conservation relations describing the adsorption process. The time dependence protein adsorption up to approximately 90% of the equilibration value q* could be described by a bilinear free driving force model. The rapid equilibration in the outer part of the particle with a half-life time of approximately 100 s in the studied systems accounted for 0.3-0.4 q*. The slower equilibration with a up to ten times longer half-life time, was the adsorption in the inner part of the particle that outside 0.5 R accounts for 0.5-0.6 q*. These data were compared with literature data for batch adsorption of proteins in biospecific, hydrophobic and ion-exchange adsorbents. They could also be described by a bilinear free driving force model, with about the same quantitative results as obtained for similar conditions in the single particle experiments. The static adsorption parameters, maximum binding site concentration n, and dissociation constant for the protein binding to a binding site K, were determined from Scatchard plots. For the same protein-adsorbent system the plots changed from linear to non-linear with increasing n. This change occurred when the average distance between adjacent binding sites become of the same order of magnitude as the size of the binding site or adsorbed protein. This causes a shielding of free binding sites increasing with n and the concentration of adsorbed protein, yielding a concentration dependence in K. These results show that for a high throughput and rapid adsorption in preparative chromatography, the adsorption step should be carried out in the non-linear part of the adsorption isotherm with concentrations up to c(b) where q*/c(b)>/=10 to obtain high protein recoveries. To avoid tailing due to the flow of adsorbed proteins in the inner part of the particles further into the particles at the start of the desorption, and to speed up desorption rates, protein adsorption in the particle within 0.5 R from the particle center should be avoided. This requires the further development of suitable pellicular particles for preparative protein chromatography that meet this requirement.

Assessment of intraparticle biocatalytic distributions as a tool in rational formulation

Research has shown that the intraparticle biocatalytic distribution has extensive effects on the properties of various (industrial) biocatalytic particles and their performance in (bio-) chemical reactions. In recent years, advances in molecular chemistry have led to the development of many different specific (immuno-) labeling and light-microscopic detection techniques. Furthermore, high-quality image-digitizing devices and enhanced computing power have made image analysis readily accessible. These technologies may lead to the assessment and improvement of the internal biocatalyst profile as an integral part of biocatalytic particle optimization.

Active site titration as a tool for the evaluation of immobilization procedures of penicillin acylase

Native and immobilized preparations of penicillin acylase from Escherichia coli and Alcaligenes faecalis were studied using an active site titration technique. Knowledge of the number of active sites allowed the calculation of the average turnover rate of the enzyme in the various preparations and allowed us to quantify the contribution of irreversible inactivation of the enzyme to the loss of catalytic activity during the immobilization procedure. In most cases a loss of active sites as well as a decrease of catalytic activity per active site (turnover rate) was observed upon immobilization. Immobilization techniques affected the enzymes differently. The effect of increased loading of penicillin acylase on the average turnover rate was determined by active site titration to assess diffusion limitations in the carrier.

Evaluation of the performance of immobilized penicillin G acylase using active-site titration

Penicillin G acylase from Escherichia coli was immobilized on Eupergit C with different enzyme loading. The activity of the immobilized preparations was assayed in the hydrolysis of penicillin G and was found to be much lower than would be expected on the basis of the residual enzyme activity in the immobilization supernatant. Active-site titration demonstrated that the immobilized enzyme molecules on average had turnover rates much lower than that of the dissolved enzyme. This was attributed to diffusion limitations of substrate and product inhibition. Indeed, when the immobilized preparations were crushed, the activity increased from 587 U g-1 to up to 974 U g-1. The immobilized preparations exhibited up to 15% lower turnover rates than the dissolved enzyme in cephalexin synthesis from 7-ADCA and D-(-)-phenylglycine amide. The synthesis over hydrolysis ratios of the immobilized preparations were also much lower than that of the dissolved enzyme. This was partly due to diffusion limitations but also to an intrinsic property of the immobilized enzyme because the synthesis over hydrolysis ratio of the crushed preparations was much lower than that of the dissolved enzyme.