Modelling of the enzymatic kinetically controlled synthesis of cephalexin: influence of diffusion limitation

Stabilization of enzymes via immobilization: Multipoint covalent attachment and other stabilization strategies

The use of enzymes in industrial processes requires the improvement of their features in many instances. Enzyme immobilization, a requirement to facilitate the recovery and reuse of these water-soluble catalysts, is one of the tools that researchers may utilize to improve many of their properties. This review is focused on how enzyme immobilization may improve enzyme stability. Starting from the stabilization effects that an enzyme may experience by the mere fact of being inside a solid particle, we detail other possibilities to stabilize enzymes: generation of favorable enzyme environments, prevention of enzyme subunit dissociation in multimeric enzymes, generation of more stable enzyme conformations, or enzyme rigidification via multipoint covalent attachment. In this last point, we will discuss the features of an "ideal" immobilization protocol to maximize the intensity of the enzyme-support interactions. The most interesting active groups in the support (glutaraldehyde, epoxide, glyoxyl and vinyl sulfone) will be also presented, discussing their main properties and uses. Some instances in which the number of enzyme-support bonds is not directly related to a higher stabilization will be also presented. Finally, the possibility of coupling site-directed mutagenesis or chemical modification to get a more intense multipoint covalent immobilization will be discussed.

Dynamic Optimization of a Fed-Batch Nosiheptide Reactor

Nosiheptide is a sulfur-containing peptide antibiotic, showing exceptional activity against critical pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE) with livestock applications that can be synthesized via fed-batch fermentation. A simplified mechanistic fed-batch fermentation model for nosiheptide production considers temperature- and pH-dependence of biomass growth, substrate consumption, nosiheptide production and oxygen mass transfer into the broth. Herein, we perform dynamic simulation over a broad range of possible feeding policies to understand and visualize the region of attainable reactor performances. We then formulate a dynamic optimization problem for maximization of nosiheptide production for different constraints of batch duration and operability limits. A direct method for dynamic optimization (simultaneous strategy) is performed in each case to compute the optimal control trajectories. Orthogonal polynomials on finite elements are used to approximate the control and state trajectories allowing the continuous problem to be converted to a nonlinear program (NLP). The resultant large-scale NLP is solved using IPOPT. Optimal operation requires feedrate to be manipulated in such a way that the inhibitory mechanism of the substrate can be avoided, with significant nosiheptide yield improvement realized.

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.

Determination of temporal and spatial concentration gradients in hydrogel beads using multiphoton microscopy techniques

Multiphoton microscopy is a promising technique to detect spatially and temporally resolved concentration gradients of chemical compounds, e.g., reactants in hydrogel-encapsulated biocatalysts. In contrast to current techniques, the improved spatial and temporal resolution of this method in data acquisition and its ability to measure hydrogel beads facilitates the identification of various kinetic phenomena. To our knowledge, multiphoton microscopy is used here for the first time to examine diffusion, mass transfer, and reaction in immobilized hydrogel systems. In a first step, the phenomena of diffusion and diffusion-coupled mass transfer through the phase interface are investigated in the bead center. Finally, the complete system--consisting of diffusion, mass transfer, and enzymatic reaction--is observed by measuring concentration gradients along the bead radius with temporal and spatial resolution. This metrology enables a subsequent mechanistic model identification, which in turn leads to an enhanced knowledge of reaction kinetics and supports the design of biotechnological processes. This task was only possible due to excellent spatial (25 microm) and temporal (5 s) resolution and the accuracy (+/-1%) achieved by using a multiphoton microscopy setup.

Hydrogel coated monoliths for enzymatic hydrolysis of penicillin G

The objective of this work was to develop a hydrogel-coated monolith for the entrapment of penicillin G acylase (E. coli, PGA). After screening of different hydrogels, chitosan was chosen as the carrier material for the preparation of monolithic biocatalysts. This protocol leads to active immobilized biocatalysts for the enzymatic hydrolysis of penicillin G (PenG). The monolithic biocatalyst was tested in a monolith loop reactor (MLR) and compared with conventional reactor systems using free PGA, and a commercially available immobilized PGA. The optimal immobilization protocol was found to be 5 g l(-1) PGA, 1% chitosan, 1.1% glutaraldehyde and pH 7. Final PGA loading on glass plates was 29 mg ml(-1) gel. For 400 cpsi monoliths, the final PGA loading on functionalized monoliths was 36 mg ml(-1) gel. The observed volumetric reaction rate in the MLR was 0.79 mol s(-1) m(-3) (monolith). Apart from an initial drop in activity due to wash out of PGA at higher ionic strength, no decrease in activity was observed after five subsequent activity test runs. The storage stability of the biocatalysts is at least a month without loss of activity. Although the monolithic biocatalyst as used in the MLR is still outperformed by the current industrial catalyst (immobilized preparation of PGA, 4.5 mol s(-1) m(-3) (catalyst)), the rate per gel volume is slightly higher for monolithic catalysts. Good activity and improved mechanical strength make the monolithic bioreactor an interesting alternative that deserves further investigation for this application. Although moderate internal diffusion limitations have been observed inside the gel beads and in the gel layer on the monolith channel, this is not the main reason for the large differences in reactor performance that were observed. The pH drop over the reactor as a result of the chosen method for pH control results in a decreased performance of both the MLR and the packed bed reactor compared to the batch system. A different reactor configuration including an optimal pH profile is required to increase the reactor performance. The monolithic stirrer reactor would be an interesting alternative to improve the performance of the monolith-PGA combination.

Biocatalysts: Measurement, modelling and design of heterogeneity

Multiple phenomena are involved in conversions by immobilized biocatalysts. A paradox is identified between analytical desires on one hand and analytical boundary conditions on the other: while the study of interdependent phenomena would call for their simultaneous analysis in an integrated context, the available experimental options may impose a series of separate and dedicated analyses. From this analysis, bottlenecks in particle performance may be identified, if possible supported by a mechanistic model and performance criteria. Subsequently, a strategy for further biocatalyst development may be chosen. Finally, possibilities for future improvement of biocatalysts are discussed for various fields of research. Some examples of recent developments in enzyme and matrix characteristics, reactor operation, and micro-technology are discussed.

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.

Kinetics of β-lactam antibiotics synthesis by penicillin G acylase (PGA) from the viewpoint of the industrial enzymatic reactor optimization

Competition with well-established, fine-tuned chemical processes is a major challenge for the industrial implementation of the enzymatic synthesis of beta-lactam antibiotics. Enzyme-based routes are acknowledged as an environmental-friendly approach, avoiding organochloride solvents and working at room temperatures. Among different alternatives, the kinetically controlled synthesis, using immobilized penicillin G acylase (PGA) in aqueous environment, with the simultaneous crystallization of the product, is the most promising one. However, PGA may act either as a transferase or as a hydrolase, catalyzing two undesired side reactions: the hydrolysis of the acyl side-chain precursor (an ester or amide, a parallel reaction) and the hydrolysis of the antibiotic itself (a consecutive reaction). This review focuses specially on aspects of the reactions' kinetics that may affect the performance of the enzymatic reactor.

Enzyme distribution and matrix characteristics in biocatalytic particles

In a study of Assemblase, an industrial immobilized penicillin-G acylase, various electron microscopic techniques were used to relate intra-particle enzyme heterogeneity with the morphological heterogeneity of the support material at various levels of detail. Transmission electron microscopy was used for the study of intra-particle penicillin-G acylase distribution in Assemblase particles of various sizes; it revealed an abrupt increase in enzyme loading at the particle surface (1.4-fold) and in the areas (designated halo's) surrounding internal macro-voids (7.7-fold). Cryogenic field-emission scanning electron microscopy related these abrupt local enzyme heterogeneities to local heterogeneity of the support material by revealing the presence of dense top layers surrounding both the particle exterior and the internal macro-voids. Furthermore, it showed a very distinct morphological appearance of the halo. Most probably, all these regions contained relatively more chitosan than gelatin (the polymers Assemblase was constructed of), which suggested local polymer demixing during particle production. A basic thermodynamic line of reasoning suggested that a difference in hydrophilicity between the two polymers induced local demixing. In the future, thermodynamic knowledge on such polymer interactions resulting in matrix heterogeneity may be used as a tool for biocatalyst design.

Field-emission scanning electron microscopy analysis of morphology and enzyme distribution within an industrial biocatalytic particle

Field-emission scanning electron microscopy (FESEM) was used in a technical feasibility study to obtain insight into the internal morphology and the intraparticle enzyme distribution of Assemblase, an industrial biocatalytic particle containing immobilized penicillin-G acylase. The results were compared with previous studies based on light and transmission electron microscopic techniques. The integrated FESEM approach yielded the same quantitative results as the microscopic techniques used previously. Given this technical equivalence, the integrated approach offers several advantages. First, the single preparation method and detection system avoids interpretation discrepancies between corresponding areas that were examined for different properties with different detection techniques in different samples. Second, the specimen size suitable for whole particle study is virtually unlimited, which simplifies sectioning and puts less stringent demands on the embedding technique. Furthermore, the sensitivity toward enzyme presence and distribution increases because the epitopes inside thick sections become available for labeling. Quick and unambiguous analysis of the relation between particle morphology and enzyme distribution is important because this information may be used in the future for the design of enzyme distributions in which the particle morphology can be used as a control parameter.

Novel approach to quantify immobilized-enzyme distributions

The quantitative intraparticle enzyme distribution of Assemblase, an industrially employed polydisperse immobilized penicillin-G acylase, was measured. Because of strong autofluorescence of the carrier, the generally applied technique of confocal scanning microscopy could not be used; light microscopy was our method of choice. To do so, Assemblase particles of various sizes were sectioned, labeled with antibodies specifically against the enzyme, and analyzed light microscopically. Image analysis software was developed and used to determine the intraparticle enzyme distribution, which was found to be heterogeneous, with most enzyme located in the outer regions of the particles. Larger particles showed steeper gradients than smaller ones. A mathematical representation of the intraparticle profiles, based on in-stationary enzyme diffusion into the particles, was validated successfully for a broad range of particle sizes using data for volume-averaged particle size and enzyme loading. The enzyme gradients determined in this work will be used as input for a physical model that quantitatively describes the complex behavior of Assemblase. Such a physical model will lead to identification of the current bottlenecks in Assemblase and can serve as a starting point for the design of improved biocatalysts that also may be based on intelligent use of enzyme gradients.