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

Enlarging the substrate binding pocket of penicillin G acylase from Achromobacter sp. for highly efficient biosynthesis of β-lactam antibiotics

Penicillin G acylase (PGA) is a key biocatalyst for the enzymatic production of β-lactam antibiotics, which can not only catalyze the synthesis of β-lactam antibiotics but also catalyze the hydrolysis of the products to prepare semi-synthetic antibiotic intermediates. However, the high hydrolysis and low synthesis activities of natural PGAs severely hinder their industrial application. In this study, a combinatorial directed evolution strategy was employed to obtain new PGAs with outstanding performances. The best mutant βF24G/βW154G was obtained from the PGA of Achromobacter sp., which exhibited approximately a 129.62-fold and a 52.55-fold increase in specific activity and synthesis/hydrolysis ratio, respectively, compared to the wild-type AsPGA. Thereafter, this mutant was used to synthesize amoxicillin, cefadroxil, and ampicillin; all conversions > 99% were accomplished in 90-135 min with almost no secondary hydrolysis byproducts produced in the reaction. Molecular dynamics simulation and substrate pocket calculation revealed that substitution of the smallest glycine residue at βF24 and βW154 expanded the binding pocket, thereby facilitating the entry and release of substrates and products. Therefore, this novel mutant is a promising catalyst for the large-scale production of β-lactam antibiotics.

Immobilization of Penicillin G Acylase on Vinyl Sulfone-Agarose: An Unexpected Effect of the Ionic Strength on the Performance of the Immobilization Process

Penicillin G acylase (PGA) from Escherichia coli was immobilized on vinyl sulfone (VS) agarose. The immobilization of the enzyme failed at all pH values using 50 mM of buffer, while the progressive increase of ionic strength permitted its rapid immobilization under all studied pH values. This suggests that the moderate hydrophobicity of VS groups is enough to transform the VS-agarose in a heterofunctional support, that is, a support bearing hydrophobic features (able to adsorb the proteins) and chemical reactivity (able to give covalent bonds). Once PGA was immobilized on this support, the PGA immobilization on VS-agarose was optimized with the purpose of obtaining a stable and active biocatalyst, optimizing the immobilization, incubation and blocking steps characteristics of this immobilization protocol. Optimal conditions were immobilization in 1 M of sodium sulfate at pH 7.0, incubation at pH 10.0 for 3 h in the presence of glycerol and phenyl acetic acid, and final blocking with glycine or ethanolamine. This produced biocatalysts with stabilities similar to that of the glyoxyl-PGA (the most stable biocatalyst of this enzyme described in literature), although presenting just over 55% of the initially offered enzyme activity versus the 80% that is recovered using the glyoxyl-PGA. This heterofuncionality of agarose VS beads opens new possibilities for enzyme immobilization on this support.

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.

A chemoenzymatic strategy for site-selective functionalization of native peptides and proteins

The emergence of new therapeutic modalities requires complementary tools for their efficient syntheses. Availability of methodologies for site-selective modification of biomolecules remains a long-standing challenge, given the inherent complexity and the presence of repeating residues that bear functional groups with similar reactivity profiles. We describe a bioconjugation strategy for modification of native peptides relying on high site selectivity conveyed by enzymes. We engineered penicillin G acylases to distinguish among free amino moieties of insulin (two at amino termini and an internal lysine) and manipulate cleavable phenylacetamide groups in a programmable manner to form protected insulin derivatives. This enables selective and specific chemical ligation to synthesize homogeneous bioconjugates, improving yield and purity compared to the existing methods, and generally opens avenues in the functionalization of native proteins to access biological probes or drugs.

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.

Three-level hybrid modeling for systematic optimization of biocatalytic synthesis: α-glucosyl glycerol production by enzymatic trans-glycosylation from sucrose

Mechanism-based kinetic models are rigorous tools to analyze enzymatic reactions, but their extension to actual conditions of the biocatalytic synthesis can be difficult. Here, we demonstrate (mechanistic-empirical) hybrid modeling for systematic optimization of the sucrose phosphorylase-catalyzed glycosylation of glycerol from sucrose, to synthesize the cosmetic ingredient α-glucosyl glycerol (GG). The empirical model part was developed to capture nonspecific effects of high sucrose concentrations (up to 1.5 M) on microscopic steps of the enzymatic trans-glycosylation mechanism. Based on verified predictions of the enzyme performance under initial rate conditions (Level 1), the hybrid model was expanded by microscopic terms of the reverse reaction to account for the full-time course of GG synthesis (Level 2). Lastly (Level 3), the application of the hybrid model for comprehensive window-of-operation analysis and constrained optimization of the GG production (~250 g/L) was demonstrated. Using two candidate sucrose phosphorylases (from Leuconostoc mesenteroides and Bifidobacterium adolescentis), we reveal the hybrid model as a powerful tool of "process decision making" to guide rational selection of the best-suited enzyme catalyst. Our study exemplifies a closing of the gap between enzyme kinetic models considered for mechanistic research and applicable in technologically relevant reaction conditions; and it highlights the important benefit thus realizable for biocatalytic process development.

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.

Efficient enzymatic synthesis of cephalexin in suspension aqueous solution system

An efficient method for the enzymatic synthesis of cephalexin (CEX) from 7-amino-3-deacetoxycephalosporanic acid (7-ADCA) and d-phenylglycine methyl ester (PGME) using immobilized penicillin G acylase (IPGA) as catalyst in a suspension aqueous solution system was developed, where the reactant 7-ADCA and product CEX are mainly present as solid particles. The effects of key factors on the enzymatic synthesis were investigated. Results showed that continuous feeding of PGME was more efficient for the synthesis of CEX than the batch mode. Under the optimized conditions, the maximum 7-ADCA conversion ratio of 99.3% and productivity of 200 mmol/L/H were achieved, both of which are much superior to the homogeneous aqueous solution system. Besides, IPGA still retained 95.4% of its initial activity after 10 cycles of enzymatic synthesis, indicating the excellent stability of this approach. The developed approach shows great potential for the industrial production of CEX via an enzyme-based route.

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.

Poly-lysine supported cross-linked enzyme aggregates of penicillin G acylase and its application in synthesis of β-lactam antibiotics

Penicillin G acylase (PGA) from Providencia rettgeri PX04 (PrPGA) was utilized to synthesize β-lactam antibiotics. Poly-lysine supported cross-linked enzyme aggregates (PL-CLEAs) were prepared using PGA. Addition of poly-lysine significantly increased retention of PGA activity in CLEAs, with a decrease in the synthesis/hydrolysis (S/H) ratio. PL-CLEAs with 0.56 mg/mL poly-lysine retained 83% of free PGA activity, and displayed a higher S/H ratio than that of the free enzyme. Both PL-CLEAs and CLEAs exhibited high pH and thermal stabilities. PL-CLEAs possessed the best stability profile, and the lowest α value [(k_cat/K_m)_Ps/(k_cat/K_m)_AD], and was most effective at amoxicillin synthesis. A >94% yield of amoxicillin was achieved using a D-HPGME/6-APA ratio of 1.2:1 (240 mM, 200 mM), with fed-batch addition of D-HPGME. PL-CLEAs displayed excellent operational stability during amoxicillin synthesis. Over 97% of initial conversion was retained after twenty rounds of catalysis. PL-CLEAs exhibited greater potency than CLEAs in practical catalysis, permitting a higher concentration of reactants.

Dynamic Modelling and Optimisation of the Batch Enzymatic Synthesis of Amoxicillin

Amoxicillin belongs to the β-lactam family of antibiotics, a class of highly consumed pharmaceutical products used for the treatment of respiratory and urinary tract infections, and is listed as a World Health Organisation (WHO) “Essential Medicine”. The demonstrated batch enzymatic synthesis of amoxicillin is composed of a desired synthesis and two undesired hydrolysis reactions of the main substrate (6-aminopenicillanic acid (6-APA)) and amoxicillin. Dynamic simulation and optimisation can be used to establish optimal control policies to attain target product specification objectives for bioprocesses. This work performed dynamic modelling, simulation and optimisation of the batch enzymatic synthesis of amoxicillin. First, kinetic parameter regression at different operating temperatures was performed, followed by Arrhenius parameter estimation to allow for non-isothermal modelling of the reaction network. Dynamic simulations were implemented to understand the behaviour of the design space, followed by the formulation and solution of a dynamic non-isothermal optimisation problem subject to various product specification constraints. Optimal reactor temperature (control) and species concentration (state) trajectories are presented for batch enzymatic amoxicillin synthesis.

The Mathematical Theory of Diffusion and Reaction in Enzymes Immoblized Artificial Membrane. The Theory of the Non-Steady State

In this paper, mathematical model pertaining to the decomposition of enzyme-substrate complex in an artificial membrane is discussed. Here the transport through liquid membrane phases is considered. The model involves the system of non-linear reaction diffusion equations. The non-linear terms in this model are related to Michaelis-Menten reaction scheme. Approximate analytical expressions for the concentrations of substrate and product have been derived by solving the system of non-linear reaction diffusion equations using new approach of homotopy perturbation method for all values of Michaelis-Menten constant, diffusion coefficient, and rate constant. Approximate flux expression for substrate and product for non-steady-state conditions are also reported. A comparison of the analytical approximation and numerical simulation is also presented. The results obtained in this work are valid for the entire solution domain.

Tuning the catalytic properties of lipases immobilized on divinylsulfone activated agarose by altering its nanoenvironment

Lipase from Thermomyces lanuginosus (TLL) and lipase B from Candida antarctica (CALB) have been immobilized on divinylsulfone (DVS) activated agarose beads at pH 10 for 72 h. Then, as a reaction end point, very different nucleophiles have been used to block the support and the effect of the nature of the blocking reagent has been analyzed on the features of the immobilized preparations. The blocking has generally positive effects on enzyme stability in both thermal and organic solvent inactivations. For example, CALB improved 7.5-fold the thermal stability after blocking with imidazole. The effect on enzyme activity was more variable, strongly depending on the substrate and the experimental conditions. Referring to CALB; using p-nitrophenyl butyrate (p-NPB) and methyl phenylacetate, activity always improved by the blocking step, whatever the blocking reagent, while with methyl mandelate or ethyl hexanoate not always the blocking presented a positive effect. Other example is TLL-DVS biocatalyst blocked with Cys. This was more than 8 times more active than the non-blocked preparation and become the most active versus p-NPB at pH 7, the least active versus methyl phenylacetate at pH 5 but the third one most active at pH 9, versus methyl mandelate presented lower activity than the unblocked preparation at pH 5 and versus ethyl hexanoate was the most active at all pH values. That way, enzyme specificity could be strongly altered by this blocking step.

Characterization and lead(II), cadmium(II), nickel(II) biosorption of dried marine brown macro algae Cystoseira barbata

PURPOSE

The objectives of this research are to identify the functional groups and determine corresponding pK (a) values of the acidic sites on dried brown algae Cystoseira barbata using FTIR and potentiometric titrations, and to investigate the biosorption ability of biomass towards divalent nickel, cadmium, and lead ions. Adsorption was studied as a function of solution pH and contact time, and experimental data were evaluated by the Langmuir isotherm model.

METHODS

CaCl(2) pretreatment was applied to the sorbent for enhancing the metal uptake capacity. The effect of solution pH on biosorption equilibrium was investigated in the pH range of 1.5-5.0. Individual as well as competitive adsorption capacity of the sorbent were studied for metal cations and mixtures.

RESULTS

The retention of the tested metal ions was mostly influenced from pH in the range of 1.5-2.5, then stayed almost constant up to 5.0, while Ni(II) uptake showed the highest variation with pH. Potentiometric titrations were performed to find the number of strong and weak acidic groups and their acidity constants. The density of strong and weak acidic functional groups in the biomass were found to be 0.9 and 2.26 mmol/g, respectively. The FTIR spectra of the sorbent samples indicated various functionalities on the biomass surface including carboxyl, hydroxyl, and amino and sulphonate groups which are responsible for the binding of metal ions.

CONCLUSIONS

The capacity of the biomass for single metal ions (around 1 mmol/g) was increased to 1.3 mmol/g in competitive adsorption, Pb(II) showing the highest Langmuir intensity constant. Considering its extremely high abundance and low cost, C. barbata may be potentially important in metal ion removal from contaminated water and industrial effluents.

Improved X-ray diffraction from Bacillus megaterium penicillin G acylase crystals through long cryosoaking dehydration

Penicillin G acylase from Bacillus megaterium (BmPGA) is currently used in the pharmaceutical industry as an alternative to PGA from Escherichia coli (EcPGA) for the hydrolysis of penicillin G to produce 6-aminopenicillanic acid (6-APA), a penam nucleus for semisynthetic penicillins. Despite the significant differences in amino-acid sequence between PGAs from Gram-positive and Gram-negative bacteria, a representative PGA structure of Gram-positive origin has never been reported. In this study, crystallization and diffraction studies of BmPGA are described. Poor diffraction patterns with blurred spots at higher resolution were typical for BmPGA crystals cryocooled after a brief immersion in cryoprotectant solution. Overnight soaking in the same cryo-solution substantially improved both the mosaicity and resolution limit through the establishment of a new crystal-packing equilibrium. A crystal of BmPGA diffracted X-rays to 2.20 Å resolution and belonged to the monoclinic space group P2(1) with one molecule of BmPGA in the asymmetric unit.

Redirecting the inactivation pathway of penicillin amidase and increasing amoxicillin production via a thermophilic molecular chaperone

We have previously shown that a single-subunit thermosome from Methanocaldococcus jannaschii (rTHS) can stabilize enzymes in semi-aqueous media (Bergeron et al., 2008b). In the present study, rTHS was used to stabilize penicillin amidase (PGA) in methanol-water mixtures. Including methanol in the reaction medium for amoxicillin synthesis can suppress unwanted hydrolysis reactions but inactivate PGA. Inactivation and reactivation pathways proposed for PGA illustrate the predictability of enzyme stabilization by rTHS in co-solvents. Calcium was necessary for reversible dissociation of the two PGA subunits in methanol-water and the presence of calcium resulted in an enhancement of chaperone-assisted stabilization. rTHS also acted as a stabilizer in the enzymatic synthesis of the beta-lactam antibiotic amoxicillin. rTHS stabilized PGA, increasing its half-life in 35% methanol by fivefold at 37 degrees C. Stabilization by rTHS was enhanced but did not require the presence of ATP. Including rTHS in fed-batch reactions performed in methanol-water resulted in nearly 4 times more amoxicillin than when the reaction was run without rTHS, and over threefold higher selectivity towards amoxicillin synthesis compared to aqueous conditions without rTHS. The thermosome and other thermophilic chaperones may thus be generally useful for stabilizing enzymes in their soluble form and expanding the range of conditions suitable for biocatalysis.

In-line system containing porous polymer monoliths for protein digestion with immobilized pepsin, peptide preconcentration and nano-liquid chromatography separation coupled to electrospray ionization mass spectroscopy

The use of two different monoliths located in capillaries for on-line protein digestion, preconcentration of peptides and their separation has been demonstrated. The first monolith was used as support for covalent immobilization of pepsin. This monolith with well-defined porous properties was prepared by in situ copolymerization of 2-vinyl-4,4-dimethylazlactone and ethylene dimethacrylate. The second, poly(lauryl methacrylate-co-ethylene dimethacrylate) monolith with a different porous structure served for the preconcentration of peptides from the digest and their separation in reversed-phase liquid chromatography mode. The top of the separation capillary was used as a preconcentrator, thus enabling the digestion of very dilute solutions of proteins in the bioreactor and increasing the sensitivity of the mass spectrometric detection of the peptides using a time-of-flight mass spectrometer with electrospray ionization. Myoglobin, albumin, and hemoglobin were digested to demonstrate feasibility of the concept of using the two monoliths in-line. Successive protein injections confirmed both the repeatability of the results and the ability to reuse the bioreactor for at least 20 digestions.

6-aminopenicillanic acid production by intact cells of E. coli containing penicillin G acylase (PGA)

The aim of present study was to optimize conditions for conversion of penicillin G into 6-APA using intact crude cells of locally collected PGA producing bacterial strains as biocatalyst. Corn steep liquor medium supplemented with phenylacetic acid was used for PGA production. For enzymatic conversion of penicillin G into 6-APA by PGA impregnated bacterial cells, a maximum reaction time of 4 h was found adequate. The procedure for extraction and crystallization of 6-APA from the enzyme reaction mixture was standardized. Isolation process was carried out under controlled pH conditions and 6-APA crystals were recovered from the reaction mixture via filtration, concentration and drying. The maximum PGA activity was observed in Escherichia coli strain BDCS-N-FMu12 (6.4 mg 6-APA h(-1) mg(-1) wet cells) whereas Bacillus megaterium (ATCC 14945 used as check) exhibited only 2.4 mg 6-APA h(-1) mg(-1) wet cells. The overall yield of 6-APA crystals obtained after enzymatic conversion of penicillin G ranged between 37-55 and 47-68% in foreign and local strains, respectively. BDCS-N-FMu12 was identified as the best PGA producer with 68% 6-APA conversion whereas ATCC 14945 showed the lowest conversion (37%). The recovery of 6-APA (68%) obtained by using crude intact cells as cheap biocatalyst appeared promising. The process of enzyme fermentation and 6-APA crystallization optimized during this study seems cost-effective and environment-friendly. However, further studies are required to scale up the 6-APA biosynthesis reaction for achieving 80-90% conversion of penicillin G into 6-APA by PGA hyper-producing locally collected strains of E. coli.

Online filtering of CO2 signals from a bioreactor gas outflow using a committee of constructive neural networks

This work proposes a committee of Cascade Correlation neural networks as an online smoother for random measurement noise. The goals of this paper are twofold: first it intends to explore the possibilities of using a constructive neural network algorithm to learn how to filter typical noisy data from a bioreactor, CO(2) mol fractions of the effluent gas during the aerobic cultivation of Bacillus megaterium to produce the enzyme penicillin G acylase. Second, to propose a committee of such networks for achieving more realistic results, capturing the inherent trend of the process. In order to do that this paper discusses the advantages of using a constructive neural network algorithm, describes how the committee of NNs operates and evaluates its performance using real CO(2) online data obtained in laboratorial experiments. The paper also presents results obtained with classical filtering algorithms, for comparison.

Calmodulin-mediated reversible immobilization of enzymes

This work demonstrates the use of the protein calmodulin, CaM, as an affinity tag for the reversible immobilization of enzymes on surfaces. Our strategy takes advantage of the of the reversible, calcium-mediated binding of CaM to its ligand phenothiazine and of the ability to produce fusion proteins between CaM and a variety of enzymes to reversibly immobilize enzymes in an oriented fashion to different surfaces. Specifically, we employed two different enzymes, organophosphorus hydrolase (OPH) and beta-lactamase and two different solid supports, a silica surface and cellulose membrane modified by covalently attaching a phenothiazine ligand, to demonstrate the versatility of our immobilization method. Fusion proteins between CaM-OPH and CaM-beta-lactamase were prepared by using genetic engineering strategies to introduce the calmodulin tail at the N-terminus of each of the two enzymes. In the presence of Ca(2+), CaM adopts a conformation that favors interaction between hydrophobic pockets in CaM and phenothiazine, while in the presence of a Ca(2+)-chelating agent such as EGTA, the interaction between CaM and phenothiazine is disrupted, thus allowing for removal of the CaM-fusion protein from the surface under mild conditions. CaM also acts as a spacer molecule, orienting the enzyme away from the surface and toward the solution, which minimizes enzyme interactions with the immobilization surface. Since the method is based on the highly selective binding of CaM to its phenothiazine ligand, and this is covalently immobilized on the surface, the method does not suffer from ligand leaching nor from interference from other proteins present in the cell extract. An additional advantage lies in that the support can be regenerated by passing through EGTA, and then reused for the immobilization of the same or, if desired, a different enzyme. Using a fusion protein approach for immobilization purposes avoids the use of harsh conditions in the immobilization and/or regeneration steps, which could cause inactivation of the immobilized enzyme. Moreover, we have demonstrated that the CaM affinity tag allows immobilization of enzymes on a variety of surfaces without compromising their enzymatic activity substantially; for example, the immobilized OPH retained more than 80% of the activity of the free enzyme. Our results with beta-lactamase showed the feasibility of using a phenothiazine surface in several consecutive loading and regeneration cycles. This can be advantageous when expensive and/or difficult to obtain immobilization surfaces have to be employed; the immobilization surface could be reused to immobilize the same or a different enzyme using the CaM affinity tail. We also determined that the phenothiazine-modified silica particles are stable for long periods of time, i.e., up to 2 years when stored at 4 degrees C. It is envisioned that this type of reversible immobilization may find applications in the development of reversible, reusable biosensors and bioreactors endowed with the additional advantage that the biological element at the surface of the sensor or bioreactor could be replaced under mild conditions when needed to sense or process a different target molecule.