Modeling of the enzymatic kinetic synthesis of cephalexin--influence of substrate concentration and temperature

Strategies to Improve the Biosynthesis of β-Lactam Antibiotics by Penicillin G Acylase: Progress and Prospects

β-Lactam antibiotics are widely used anti-infection drugs that are traditionally synthesized via a chemical process. In recent years, with the growing demand for green alternatives, scientists have turned to enzymatic synthesis. Penicillin G acylase (PGA) is the second most commercially used enzyme worldwide with both hydrolytic and synthetic activities toward antibiotics, which has been used to manufacture the key antibiotic nucleus on an industrial level. However, the large-scale application of PGA-catalyzed antibiotics biosynthesis is still in the experimental stage because of some key limitations, such as low substrate concentration, unsatisfactory yield, and lack of superior biocatalysts. This paper systematically reviews the strategies adopted to improve the biosynthesis of β-lactam antibiotics by adjusting the enzymatic property and manipulating the reaction system in recent 20 years, including mining of enzymes, protein engineering, solvent engineering, in situ product removal, and one-pot reaction cascade. These advances will provide important guidelines for the future use of enzymatic synthesis in the industrial production of β-lactam antibiotics.

Enhanced Dissolution of 7-ADCA in the Presence of PGME for Enzymatic Synthesis of Cephalexin

Enzymatic catalysis has been recognized as a green alternative to classical chemical route for synthesis of cephalexin (CEX). However, its industrial practice has been severely limited by the low productivity due to the low solubility of 7-amino-3-deacetoxycephalosporanic acid (7-ADCA) and high hydrolysis of D-phenylglycine methyl ester (PGME). In this work, the enhanced dissolution of 7-ADCA in the presence of PGME for efficient enzymatic synthesis of CEX was investigated. Results showed that the solubility of 7-ADCA in water could be improved by PGME. Moreover, supersaturated solution of 7-ADCA could be created in the presence of PGME by a pH shift strategy. The supersaturated solution of 7-ADCA possess good stability, which could be explained in terms of the inhibition of 7-ADCA precipitation due to the presence of PGME. The interaction between 7-ADCA and PGME is explored by spectroscopic determination and DFT analysis and the mechanism of enhanced dissolution of 7-ADCA in the presence of PGME is discussed and proposed. The feasibility of supersaturated solution of 7-ADCA for the enzymatic synthesis of CEX is evaluated. It was demonstrated that high conversion ratio (> 95.0%) and productivity (> 240.0 mmol/L/h) was obtained under a wide range of reaction conditions, indicating that the supersaturated solution system was highly superior to conventional homogeneous solution system. The information obtained in this work will be helpful to industrial production of CEX via enzymatic route.

On the donor substrate dependence of group-transfer reactions by hydrolytic enzymes: Insight from kinetic analysis of sucrose phosphorylase-catalyzed transglycosylation

Chemical group-transfer reactions by hydrolytic enzymes have considerable importance in biocatalytic synthesis and are exploited broadly in commercial-scale chemical production. Mechanistically, these reactions have in common the involvement of a covalent enzyme intermediate which is formed upon enzyme reaction with the donor substrate and is subsequently intercepted by a suitable acceptor. Here, we studied the glycosylation of glycerol from sucrose by sucrose phosphorylase (SucP) to clarify a peculiar, yet generally important characteristic of this reaction: partitioning between glycosylation of glycerol and hydrolysis depends on the type and the concentration of the donor substrate used (here: sucrose, α-d-glucose 1-phosphate (G1P)). We develop a kinetic framework to analyze the effect and provide evidence that, when G1P is used as donor substrate, hydrolysis occurs not only from the β-glucosyl-enzyme intermediate (E-Glc), but additionally from a noncovalent complex of E-Glc and substrate which unlike E-Glc is unreactive to glycerol. Depending on the relative rates of hydrolysis of free and substrate-bound E-Glc, inhibition (Leuconostoc mesenteroides SucP) or apparent activation (Bifidobacterium adolescentis SucP) is observed at high donor substrate concentration. At a G1P concentration that excludes the substrate-bound E-Glc, the transfer/hydrolysis ratio changes to a value consistent with reaction exclusively through E-Glc, independent of the donor substrate used. Collectively, these results give explanation for a kinetic behavior of SucP not previously accounted for, provide essential basis for design and optimization of the synthetic reaction, and establish a theoretical framework for the analysis of kinetically analogous group-transfer reactions by hydrolytic enzymes.

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.

Theoretical study on enzyme synthesis of cephalexin in a parallel-flow microreactor combined with electrically driven ATPS microextraction

A mathematical model of a microfluidic device with two aqueous phases for the simultaneous cephalexin production and its separation from a reaction mixture was developed. The model anticipates the continuous cephalexin synthesis and enzyme recyclation.

Biotechnological advances on penicillin G acylase: pharmaceutical implications, unique expression mechanism and production strategies

In light of unrestricted use of first-generation penicillins, these antibiotics are now superseded by their semisynthetic counterparts for augmented antibiosis. Traditional penicillin chemistry involves the use of hazardous chemicals and harsh reaction conditions for the production of semisynthetic derivatives and, therefore, is being displaced by the biosynthetic platform using enzymatic transformations. Penicillin G acylase (PGA) is one of the most relevant and widely used biocatalysts for the industrial production of β-lactam semisynthetic antibiotics. Accordingly, considerable genetic and biochemical engineering strategies have been devoted towards PGA applications. This article provides a state-of-the-art review in recent biotechnological advances associated with PGA, particularly in the production technologies with an emphasis on using the Escherichia coli expression platform.

Application of modeling and simulation tools for the evaluation of biocatalytic processes: a future perspective

Modeling and simulation techniques have for some time been an important feature of biocatalysis research, often applied as a complement to experimental studies. In this short review, we report on the state-of-the-art process and kinetic modeling for biocatalysis with the aim of identifying future research needs. We have particularly focused on four aspects of modeling: (i) the model purpose, (ii) the process model boundary, (iii) the model structure, and (iv) the model identification procedure. First, one finds that most of the existing models describe biocatalyst behavior in terms of enzyme selectivity, mechanism, and reaction kinetics. More recently, work has focused on extending these models to obtain process flowsheet descriptions. Second, biocatalysis models remain at a relatively low level of complexity compared with the trends observed in other engineering disciplines. Hence, there is certainly room for additional development, i.e., detailed mixing and hydrodynamics, more process units (e.g., biorefinery). Third, biocatalysis models have been only partially subjected to formal statistical analysis. In particular, uncertainty analysis is needed to ascertain reliability of the predictions of the process model, which is necessary to make sound engineering decisions (e.g., the optimal process flowsheet, control strategy, etc). In summary, for modeling studies to be more mature and successful, one needs to introduce Good Modeling Practice and that asks for (i) a standardized and systematic guideline for model development, (ii) formal identifiability analysis, and (iii) uncertainty analysis. This will advance the utility of models in biocatalysis for more rigorous application within process design, optimization, and control strategy evaluation.

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.

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.

Conjugation of penicillin acylase with the reactive copolymer of N-isopropylacrylamide: a step toward a thermosensitive industrial biocatalyst

Conjugation of penicillin acylase (PA) to poly-N-isopropylacrylamide (polyNIPAM) was studied as a way to prepare a thermosensitive biocatalyst for industrial applications to antibiotic synthesis. Condensation of PA with the copolymer of NIPAM containing active ester groups resulted in higher coupling yields of the enzyme (37%) compared to its chemical modification and copolymerization with the monomer (9% coupling yield) at the same NIPAM:enzyme weight ratio of ca. 35. A 10-fold increase of the enzyme loading on the copolymer resulted in 24% coupling yield and increased by 4-fold the specific PA activity of the conjugate. Two molecular forms of the conjugate were found by gel filtration on Sepharose CL 4B: the lower molecular weight fraction of ca. 10(6) and, presumably, cross-linked protein-polymer aggregates of MW > 10(7). Michaelis constant for 5-nitro-3-phenylacetamidobenzoic acid hydrolysis by the PA conjugate (20 microM) was found to be slightly higher than that of the free enzyme (12 microM), and evaluation of V(max) testifies to the high catalytic efficiency of the conjugated enzyme. PolyNIPAM-cross-linked PA retained its capacity to synthesize cephalexin from d-phenylglycin amide and 7-aminodeacetoxycephalosporanic acid. The synthesis-hydrolysis ratios of free and polyNIPAM-cross-linked enzyme in cephalexin synthesis were 7.46 and 7.49, respectively. Thus, diffusional limitation, which is a problem in the industrial production of beta-lactam antibiotics, can be successfully eliminated by cross-linking penicillin acylase to a smart polymer (i.e., polyNIPAM).

Enzymatic synthesis of amoxicillin: avoiding limitations of the mechanistic approach for reaction kinetics

A recurrent doubt that occurs to the enzyme-kinetics modeler is, When should I stop adding parameters to my mechanistic model in order to fit a non-conventional behavior? This problem becomes more and more involving when the complexity of the reaction network increases. This work intends to show how the use of artificial neural networks may circumvent the need of including an overwhelming number of parameters in the rate equations obtained through the classical, mechanistic approach. We focus on the synthesis of amoxicillin by the reaction of p-OH-phenylglycine methyl ester and 6-aminopenicillanic acid, catalyzed by penicillin G acylase immobilized on glyoxyl-agarose, at 25 degrees C and pH 6.5. The reaction was carried on a batch reactor. Three kinetic models of this system were compared: a mechanistic, a semi-empiric, and a hybrid-neural network (NN). A semi-empiric, simplified model with a reasonable number of parameters was initially built-up. It was able to portray many typical process conditions. However, it either underestimated or overestimated the rate of synthesis of amoxicillin when substrates' concentrations were low. A more complex, full-scale mechanistic model that could span all operational conditions was intractable for all practical purposes. Finally, a hybrid model, that coupled artificial neural networks (NN) to mass-balance equations was established, that succeeded in representing all situations of interest. Particularly, the NN could predict with accuracy reaction rates for conditions where the semi-empiric model failed, namely, at low substrate concentrations, a situation that would occur, for instance, at the end of a fed-batch industrial process.

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

In this study the influence of diffusion limitation on enzymatic kinetically controlled cephalexin synthesis from phenylglycine amide and 7-aminodeacetoxycephalosporinic acid (7-ADCA) was investigated systematically. It was found that if diffusion limitation occurred, both the synthesis/hydrolysis ratio (S/H ratio) and the yield decreased, resulting in lower product and higher by-product concentrations. The effect of pH, enzyme loading, and temperature was investigated, their influence on the course of the reaction was evaluated, and eventually diffusion limitation was minimised. It was found that at pH >or=7 the effect of diffusion limitation was eminent; the difference in S/H ratio and yield between free and immobilised enzyme was considerable. At lower pH, the influence of diffusion limitation was minimal. At low temperature, high yields and S/H ratios were found for all enzymes tested because the hydrolysis reactions were suppressed and the synthesis reaction was hardly influenced by temperature. The enzyme loading influenced the S/H ratio and yield, as expected for diffusion-limited particles. For Assemblase 3750 (the number refers to the degree of enzyme loading), it was proven that both cephalexin synthesis and hydrolysis were diffusion limited. For Assemblase 7500, which carries double the enzyme load of Assemblase 3750, these reactions were also proven to be diffusion limited, together with the binding-step of the substrate phenylglycine amide to the enzyme. For an actual process, the effects of diffusion limitation should preferably be minimised. This can be achieved at low temperature, low pH, and high substrate concentrations. An optimum in S/H ratio and yield was found at pH 7.5 and low temperature, where a relatively low reaction pH can be combined with a relatively high solubility of 7-ADCA. When comparing the different enzymes at these conditions, the free enzyme gave slightly better results than both immobilised biocatalysts, but the effect of diffusion limitation was minimal.

Integrated reactor concepts for the enzymatic kinetic synthesis of cephalexin

Integrated process concepts for enzymatic cephalexin synthesis were investigated by our group, and this article focuses on the integration of reactions and product removal during the reactions. The last step in cephalexin production is the enzymatic kinetic coupling of activated phenylglycine (phenylglycine amide or phenylglycine methyl ester) and 7-aminodeacetoxycephalosporanic acid (7-ADCA). The traditional production of 7-ADCA takes place via a chemical ring expansion step and an enzymatic hydrolysis step starting from penicillin G. However, 7-ADCA can also be produced by the enzymatic hydrolysis of adipyl-7-ADCA. In this work, this reaction was combined with the enzymatic synthesis reaction and performed simultaneously (i.e., one-pot synthesis). Furthermore, in situ product removal by adsorption and complexation were investigated as means of preventing enzymatic hydrolysis of cephalexin. We found that adipyl-7-ADCA hydrolysis and cephalexin synthesis could be performed simultaneously. The maximum yield on conversion (reaction) of the combined process was very similar to the yield of the separate processes performed under the same reaction conditions with the enzyme concentrations adjusted correctly. This implied that the number of reaction steps in the cephalexin process could be reduced significantly. The removal of cephalexin by adsorption was not specific enough to be applied in situ. The adsorbents also bound the substrates and therewith caused lower yields. Complexation with beta-naphthol proved to be an effective removal technique; however, it also showed a drawback in that the activity of the cephalexin-synthesizing enzyme was influenced negatively. Complexation with beta-naphthol rendered a 50% higher cephalexin yield and considerably less byproduct formation (reduction of 40%) as compared to cephalexin synthesis only. If adipyl-7-ADCA hydrolysis and cephalexin synthesis were performed simultaneously and in combination with complexation with beta-naphthol, higher cephalexin concentrations also were found. In conclusion, a highly integrated process (two reactions simultaneously combined with in situ product removal) was shown possible, although further optimization is necessary.

Quantitative characterization of the nucleophile reactivity in penicillin acylase-catalyzed acyl transfer reactions

Nucleophile reactivity of two most known nuclei of penicillins and cephalosporins, 6-aminopenicillanic (6-APA) and 7-aminodesacetoxycephalosporanic (7-ADCA) acids, was quantitatively characterized. In penicillin acylase (PA)-catalyzed acyl transfer reactions the relative reactivity of the added nucleophile compared to the water (i.e. nucleophile reactivity) is defined by two complex kinetic parameters beta(0) and gamma, and depends on the nucleophile concentration. In turn, parameters beta(0) and gamma were shown to be dependent on the structure of both reactants involved: nucleophile and acyl donor. Analysis of the kinetic scheme revealed that nucleophile reactivity is one of a few key parameters controlling efficiency of PA-catalyzed acyl transfer to the added nucleophile in an aqueous medium. Computation of the maximum nucleophile conversion to the product using determined nucleophile reactivity parameters in the synthesis of three different antibiotics, ampicillin, amoxicillin and cephalexin, showed good correlation with the results of corresponding synthetic experiments. Suggested approach can be extended to the quantitative description and optimization of PA-catalyzed acyl transfer reactions in a wide range of experimental conditions.

A two-step, one-pot enzymatic synthesis of cephalexin from D-phenylglycine nitrile

A cascade of two enzymatic transformations is employed in a one-pot synthesis of cephalexin. The nitrile hydratase (from R. rhodochrous MAWE)-catalyzed hydration of D-phenylglycine nitrile to the corresponding amide was combined with the penicillin G acylase (penicillin amidohydrolase, E.C. 3.5.1.11)-catalyzed acylation of 7-ADCA with the in situ-formed amide to afford a two-step, one-pot synthesis of cephalexin. D-Phenylglycine nitrile appeared to have a remarkable selective inhibitory effect on the penicillin G acylase, resulting in a threefold increase in the synthesis/hydrolysis (S/H) ratio. 1,5-Dihydroxynaphthalene, when added to the reaction mixture, cocrystallized with cephalexin. The resulting low cephalexin concentration prevented its chemical as well as enzymatic degradation; cephalexin was obtained at 79% yield with an S/H ratio of 7.7.

Advantages of using non-isothermal bioreactors for the enzymatic synthesis of antibiotics: the penicillin G acylase as enzyme model

A new hydrophobic and catalytic membrane was prepared by immobilizing Penicillin G acylase (PGA, EC.3.5.1.11) from E. coli on a nylon membrane, chemically grafted with butylmethacrylate (BMA). Hexamethylenediamine (HMDA) and glutaraldehyde (Glu) were used as a spacer and coupling agent, respectively. PGA was used for the enzymatic synthesis of cephalexin, using D(-)-phenylglycine methyl ester (PGME) and 7-amino-3-deacetoxycephalosporanic acid (7-ADCA) as substrates. Several factors affecting this reaction, such as pH, temperature, and concentrations of substrates were investigated. The results indicated good enzyme-binding efficiency of the pre-treated membrane, and an increased stability of the immobilized PGA towards pH and temperature. Calculation of the activation energies showed that cephalexin production by the immobilized biocatalyst was limited by diffusion, resulting in a decrease of enzyme activity and substrate affinity. Temperature gradients were employed as a way to reduce the effects of diffusion limitation. Cephalexin was found to linearly increase with the applied temperature gradient. A temperature difference of about 3 degrees C across the catalytic membrane resulted into a cephalexin synthesis increase of 100% with a 50% reduction of the production times. The advantage of using non-isothermal bioreactors in biotechnological processes, including pharmaceutical applications, is also discussed.

The use of enzymes in the chemical industry in Europe

Many European chemical industries are in a phase of reorganization resulting in a general opening towards life sciences. Traditional chemical markets are served increasingly with products derived from bioprocesses or hybrid chemical/biocatalytic processes. Biocatalytic steps are already being used to produce a wide range of products, including agricultural chemicals, organics, drugs and plastic materials, to name but a few. Apart from the rapidly growing number of commercialized bioprocesses, a partial survey of exploratory activities points to future applications of enzymes in the European chemical industry, which will bring new products and technologies and, in some cases, replace traditional syntheses.