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.     

Adsorption of cellulase on cellulose: Effect of physicochemical properties of cellulose on adsorption and rate of hydrolysis

In the cellulase-cellulose reaction system, the adsorption of cellulase on the solid cellulose substrate was found to be one of the important parameters that govern the enzymatic hydrolysis rate of cellulose. The adsorption of cellulase usually parallels the rate of hydrolysis of cellulose. The affinity for cellulase varies depending on the structural properties of cellulose. Adsorption parameters such as the half-saturation constant, the maximum adsorption constant, and the distribution coefficient for both the cellulase and cellulsoe have been experimentally determined for several substrates. These adsorption parameters vary with the source of cellulose and the pretreatment methods and are correlated with the crystallinity and the specific surface area of cellulose substrates. The changing pattern of adsorption profile of cellulase during the hydrolysis reaction has also been elucidated. For practical utilization of cellulosic materials, the cellulose structural properties and their effects on cellulase adsorption, and the rate of hydrolysis must be taken into consideration.     

A novel magnetic affinity support for protein adsorption and purification

A novel magnetic support was prepared by an oxidization-precipitation method with poly(vinyl alcohol) (PVA) as the entrapment material. Transmission electron microscopy indicated that the magnetic particles had a core-shell structure, containing many nanometer-sized magnetic cores stabilized by the cross-linked PVA. The particles showed a high magnetic responsiveness in magnetic field, and no aggregation of the particles was observed after the particles had been treated in the magnetic field. These facts indicated that the particles were superparamagnetic. Cibacron blue 3GA (CB) was coupled to the particles to prepare a magnetic affinity support (MAS) for protein adsorption. Lysozyme was used as a model protein to test the adsorption properties of the MAS. The adsorption equilibrium of lysozyme to the MAS was described by the Langmuir-type isotherm. The capacity for lysozyme adsorption was more than 70 mg/g MAS (wet weight) at a relatively low CB coupling density (3-5 micromol/g). In addition, 1.0 M NaCl solution could be used to dissociate the adsorbed lysozyme. Finally, the MAS was recycled for the purification of alcohol dehydrogenase (ADH) from clarified yeast homogenates. Under proper conditions, the magnetic separation yielded over 5-fold purification of the enzyme with 60% recovery of the enzyme activity.     

Agar-based magnetic affinity support for protein adsorption

Magnetic colloidal particles were prepared by a coprecipitation method. The particles were composed of nanometer-sized superparamagnetic Fe(3)O(4) particles stabilized by lauric acid. Then, magnetic agar gel beads were produced by a water-in-oil emulsification method using a mixture of agar solution and the magnetic colloidal particles as the aqueous phase. A reactive triazine dye, Cibacron blue 3GA (CB), was coupled to the gel to prepare an agar-based magnetic affinity support (MAS) for protein adsorption. The support showed good magnetic responsiveness in a magnetic field. Bovine serum albumin (BSA) was used as a model protein to test adsorption equilibrium and kinetic behavior of the MAS. The adsorption equilibrium of BSA to the MAS was described by the Langmuir-type isotherm. Adsorption capacity of the MAS for BSA was up to 25 mg/mL at a CB coupling density of 1.6 micromol/mL. The effect of ionic strength on BSA adsorption was complex, exhibiting a maximum capacity at an ionic strength of 0.06 mol/L. The adsorption of BSA to the MAS was also influenced by pH. Uptake rate of BSA to the MAS was analyzed using a pore diffusion model. The pore diffusion coefficient was estimated to be 1.75 x 10(-11) m(2)/s. Finally, recycled use of the MAS demonstrated the stability of the MAS in protein adsorption and magnetic responsiveness.     

Course of pH during the formation of amoxicillin by a suspension-to-suspension reaction

Amoxicillin can be produced in an enzymatic suspension-to-suspension reaction in which the substrate(s) and product(s) are mainly present as solid particles, while the reaction takes place in the liquid phase. During these suspension-to-suspension reactions different subprocesses take place, such as dissolution/crystallization of substrates and products, enzymatic synthesis of the product(s), and undesired enzymatic hydrolysis of substrates and/or products. All these subprocesses are influenced by pH and also influence the pH because the reactants are weak electrolytes. This paper describes a quantitative model for predicting pH and concentrations of reactants during suspension-to-suspension reactions. The model is based on mass and charge balances, pH-dependent solubilities of the reactants, and enzyme kinetics. For the validation of this model, the kinetically controlled synthesis of amoxicillin from 6-aminopenicillanic acid and D-(p)hydroxyphenylglycine methyl ester was studied. The pH and the dissolved concentrations took a very different course at different initial substrate amounts. This was described quite reasonably by the model. Therefore, the model can be used as a tool to optimize suspension-to-suspension reactions of weak electrolytes.     

Rapid enrichment of phosphopeptides from tryptic digests of proteins using iron oxide nanocomposites of magnetic particles coated with zirconia as the concentrating probes

Iron oxide nanocomposites of magnetic particles coated with zirconia were used as affinity probes to selectively concentrate phosphopeptides from tryptic digests of alpha- and beta-caseins, milk, and egg white to exemplify the enrichment of phosphopeptides from complex samples. Phosphopeptides, in quantities sufficient for characterization by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), were enriched by the affinity probes within only 30 s. The affinity probe-target species conjugates were separated from the sample solution simply by applying an external magnetic field. The detection limit for tryptic digest of beta-casein using this approach is approximately 45 fmol. Furthermore, we combined this enrichment method with a rapid enzymatic digestion method, that is, microwave-assisted enzymatic digestion using magnetic particles as the microwave absorbers, to speed up the tryptic digest reactions. Thus, we alternatively enriched phosphoproteins on the zirconia-coated particles followed by mixing with trypsin and heated the mixture in a microwave oven for 1 min. The particles remaining in the mixture were used as affinity probes to selectively enrich phosphopeptides from the tryptic digestion product by pipetting, followed by characterization using MALDI MS. Using the bifunctional zirconia-coated magnetic particles as both the affinity probes and the microwave absorbers could greatly reduce the time for the purification and characterization of phosphopeptides from complex samples.     

Kinetic modeling of enzymatic hydrolysis of cellulose in differently pretreated fibers from dairy manure

A kinetic model incorporating dynamic adsorption, enzymatic hydrolysis, and product inhibition was developed for enzymatic hydrolysis of differently pretreated fibers from a nitrogen-rich lignocellulosic material-dairy manure. The effects of manure proteins on the enzyme adsorption profile during hydrolysis have been discussed. Enzyme activity, instead of protein concentration, was used to describe the enzymatic hydrolysis in order to avoid the effect of manure protein on enzyme protein analysis. Dynamic enzyme adsorption was modeled based on a Langmiur-type isotherm. A first-order reaction was applied to model the hydrolysis with consideration being given for the product inhibition. The model satisfactorily predicted the behaviors of enzyme adsorption, hydrolysis, and product inhibition for all five sample manure fibers. The reaction conditions were the substrate concentrations of 10-50 g/L, enzyme loadings of 7-150 FPU/g total substrate, and the reaction temperature of 50 degrees C.     

Protein-bound choline is released from the pneumococcal autolytic enzyme during adsorption of the enzyme to cell wall particles.

The inactive precursor form of the pneumococcal autolytic enzyme cloned in Escherichia coli was isolated by affinity chromatography on Sepharose-linked choline. The enzyme was recovered in an electrophoretically pure and activated form by elution from the affinity column with radioactive choline solution. When radioactive choline was used for elutions, the enzyme protein isolated contained protein-bound choline, at approximately 1 mol of choline per mol of enzyme protein, indicating the presence of a single choline recognition site. Radioactive choline remained bound to the enzyme protein during dialysis, precipitation by trichloroacetic acid or ammonium sulfate, and during gel filtration, but not during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Incubation of the choline-labeled autolysin with pneumococcal cell walls at 0 degrees C resulted in the adsorption of the enzyme to the wall particles and a simultaneous release of free choline from the enzyme protein. It is suggested that the choline molecules that became bound to the enzyme protein during the activation of autolysin are expelled from the choline-binding site and replaced by choline residues from the wall teichoic acid as the autolysin molecules adsorb to their insoluble substrate before the onset of enzymatic wall hydrolysis.     

Structural properties of cellulose and cellulase reaction mechanism

The effects of structural properties and their changes during cellulose hydrolysis on the enzymatic hydrolysis rate have been studied from the reaction mechanism point of view. Important findings are the following: (1) The crystallinity index (CrI) of partially crystalline cellulose increases as the hydrolysis reaction proceeds, and a significant slowing down of the reaction rate during the enzymatic hydrolysis is, in large part, attributable to this structural change of cellulose substrate. (2) The crystallinity of completely disordered cellulose, like phosphoric-acid-treated cellulose, does not change significantly, and a relatively high hydrolysis rate is maintained during hydrolysis. (3) The specific surface area (SSA) of partially crystalline cellulose decreases significantly during enzymatic hydrolysis while the change in SSA of regenerated cellulose is found to be negligible. (4) The value of degree of polymerization (DP) of highly ordered crystalline cellulose remains practically constant whereas the change in DP of disordered regenerated cellulose is found to be very significant. (5) Combination of these structural effects as well as cellulase adsorption, product inhibition, and cellulase deactivation all have important influence on the rate of cellulase reaction during cellulose hydrolysis. More experimental evidence for a two-phase model, which is based on degradation of cellulose by simultaneous actions of cellulase complex on the crystalline and amorphous phases, has been obtained. Based on experimental results from this study and other results accumulated, the mode of cellulase action and a possible reaction mechanism are proposed.     

Continuous protein separations in a magnetically stabilized fluidized bed using nonmagnetic supports

Continuous protein separations were performed using a magnetically stabilized fluidized bed (MSFB) and a commercially available affinity adsorption resin that contained no magnetically susceptible material. These nonmagnetic materials can be stabilized at relatively low fields (<75 G requiring <30 W) if sufficient magnetically susceptible particles are also present in the stabilized bed. The minimum amount of magnetic particles necessary to stabilize the bed is as low as 20% by volume and is a function of various parameters including the size and density of both particles, the magnetic field strength, and the fluidization velocity. Advantages of these beds for performing separations include true continuous, countercurrent liquid-solids contact, mass-transfer efficiencies nearly equal to that of packed beds, and the ability of handle suspended cells or cell debris. A variety of commercially available affinity, ion-exchange, and adsorptive supports can be used in the bed for continuous separations; results are presented for the adsorption and recovery of lysozyme from an aqueous mixture of lysozyme and myoglobin using an affinity resin.     

A mechanistic model for enzymatic saccharification of cellulose using continuous distribution kinetics II: cooperative enzyme action, solution kinetics, and product inhibition

The projected cost for the enzymatic hydrolysis of cellulosic biomass continues to be a barrier for the commercial production of liquid transportation fuels from renewable feedstocks. Predictive models for the kinetics of the enzymatic reactions will enable an improved understanding of current limitations, such as the slow-down of the overall conversion rate, and may point the way for more efficient utilization of the enzymes in order to achieve higher conversion yields. A mechanistically based kinetic model for the enzymatic hydrolysis of cellulose was recently reported in Griggs et al. (2011) (Part I). In this article (Part II), the enzyme system is expanded to include solution-phase kinetics, particularly cellobiose-to-glucose conversion by β-glucosidase (βG), and novel adsorption and product inhibition schemes have been incorporated, based on current structural knowledge of the component enzymes. Model results show cases of cooperative and non-cooperative hydrolysis for an enzyme system consisting of EG(I) and CBH(I). The model is used to explore various potential rate-limiting phenomena, such as substrate accessibility, product inhibition, sterically hindered enzyme adsorption, and the molecular weight of the cellulose substrate.     

Kinetics of milk coagulation: I. The kinetics of kappa casein hydrolysis in the presence of enzyme deactivation

The kinetics of the primary phase of the enzymatic coagulation of milk, i.e., kappa-casein hydrolysis, was investigated in the presence and in the absence of concurrent enzyme deactivation processes. For conditions under which the enzyme is stable, the rate of hydrolysis can be described by Michaelis-Menten kinetics, as has been reported by previous investigators. A mathematical model, experimental data, and parameter estimates are provided for kappa-casein hydrolysis in the presence of concurrent deactivation of enzyme. The model accurately describes the experimental results when porcine pepsin was used as the renneting enzyme. The model and the experimental results indicate that samples of milk treated under conditions where deactivation of enzyme is significant can have fractional conversions of kappa-casein ranging from zero to unity and yet contain no active enzyme at the termination of the treatment.     

Naringinase-catalyzed hydrolysis of naringin adsorbed on macroporous resin

Naringenin, a natural plant flavonoid found in citrus fruits, has been reported to exhibit a wide range of pharmacological functions, including anticancer, antioxidant, antiatherogenic, antithrombotic, and vasodilator activities. Naringenin can be produced from the naringinase (NGase)-catalyzed enzymatic hydrolysis of naringin. However, the poor solubility of naringin in aqueous systems considerably limits the efficiency of naringenin biocatalysis. In this work, a novel substrate adsorption system was proposed for naringin adsorption to increase the efficiency of naringin hydrolysis and naringenin production. Three Amberlite macroporous resins, namely, XAD-4, XAD-7HP and XAD-16, were investigated for their naringin adsorption capacities and effects on NGase hydrolysis. Results indicated that the physical properties of the resins played a critical role in naringin adsorption and naringenin enzymatic synthesis. Naringin hydrolysis was carried out using free and adsorbed substrates. The substrate adsorption strategy could increase the catalytic efficiency at a high naringin concentration. In addition, the reaction conditions for enzymatic naringenin synthesis were optimized, and naringenin was prepared at a liter scale with a high substrate concentration. These results suggested that substrate adsorption is a promising strategy to increase the enzymatic hydrolysis efficiency of naringenin in aqueous systems.     

Kinetics of ATP synthesis in pea cotyledon submitochondrial particles

A kinetic study of oxidative phosphorylation by pea submitochondrial particles gave two K_m values for ADP, one low, the other high. The high value probably reflected a damaged site or a population of leaky mitochondria. Only the high affinity site with a low K_m for ADP was involved in ATP synthesis. α,β-Methylene ADP was found to be a competitive inhibitor of ATP synthesis. The inorganic phosphate analog, thiophosphate, decreased the apparent K_m of ADP while the rate of the reaction remained approximately the same. Adenyl imidodiphosphate, a specific inhibitor of ATP hydrolysis activity, had little effect on oxidative phosphorylation. A slight decrease in the K_m of the high affinity binding site for ADP was noted. Aurovertin was found to be a potent inhibitor of oxidative phosphorylation in pea submitochondrial particles. The K_m of the high affinity site was increased 10-fold. Also, the inhibition normally exerted by ADP on ATPase activity was severely reduced by aurovertin. In contrast, increasing the concentration of aurovertin only slightly affected the level of inhibition caused by adenyl imidodiphosphate on ATP hydrolysis.     

Optimization and simulation of continuous affinity-recycle extraction (care).

Simulation and optimization of continuous affinity recycle extraction (CARE), a protein purification unit operation based on protein adsorption to solid phase adsorbents, is described in this paper. Rather than packing conventional adsorbent particles in a fixed bed (column), solid/liquid contact is carried out in well-mixed reactors. Continuous operation is achieved by recirculation of the adsorbent particles between two or more contactors. The feasibility of this purification scheme was established with the recovery and isolation of the enzyme beta-galactosidase from E.coli, using the affinity support PABTG/Agarose. A mathematical model describing system performance was developed. The mathematical model was used to optimize several facets of the system design and operation. The base two-stage contractor design was modified by the addition of an intermediate wash stage as well as the incorporation of multiple adsorption stages. These design modifications serve to increase purification, concentration and recovery while utilizing the same amount of adsorbent. The methodology for defining and optimizing objective functions was developed and experimentally validated. Finally, optimum system start-up protocols, minimizing the time required to reach steady-state operation, were developed and experimentally validated. The impact of early introduction of adsorptive purification in a downstream processing sequence, with CARE, was evaluated and is described. Through the early introduction of a highly specific adsorptive step, significant purification is achieved simultaneously with clarification and concentration. In addition, purification performance in CARE was contrasted with that achievable in conventional column chromatography.     

Effect of particle size on the rate of enzymatic hydrolysis of cellulose

The effect of particle size on enzymatic hydrolysis of cellulose has been investigated. The average size of microcrystalline cotton cellulose has been reduced to submicron scale by using a media mill. The milled products were further subjected to hydrolysis using cellulase. High cellulose concentration (7%) appeared to retard the size reduction and resulted in greater particles and smaller specific surface areas than those at low concentration (3%) with the same milling time. Initial rate method was employed to explore the rate of enzymatic hydrolysis of cellulose. The production rate of cellobiose was increased at least 5-folds due to the size reduction. The yield of glucose was also significantly increased depending upon the ratio of enzyme to substrate. A high glucose yield (60%) was obtained in 10-h hydrolysis when the average particle size was in submicron scale.     

Kinetic studies of enzymatic hydrolysis of insoluble cellulose: analysis of the initial rates

A study was conducted on the kinetics of enzymatic hydrolysis of pure insoluble cellulose using unpurified culture filtrate Trichoderma reesei, with the emphasis on the initial reaction period. The initial hydrolysis rate and extent of enzyme (soluble protein)adsorption, either apparent or initial, were evaluated under various experimental conditions. It has been found that the various mass-transfer steps do not control the overall hydrolysis rate and that the hydrolysis rate is mainly controlled by the surface reaction step promoted by the adsorbed enzyme. It has also been found that the initial hydrolysis rate strongly depends on the initial extent of soluble protein adsorption and the effectiveness of the adsorbed soluble protein to promote the hydrolysis. The initial extent of soluble protein adsorption, in turn, is related to the initial cellulose concentration, enzyme concentration, and specific surface area of cellulose, whereas the effectiveness of the initially adsorbed soluble protein to promote the derived to interrelate these parameters without resorting to the Michaelis-Menten kinetics. The present result appear to imply that the role of enzyme-substrate complex formation should not be ignored in deriving a mechanistic kinetic model for enzymatic hydrolysis of cellulose.     

Enzymatic resolution of (S)-(+)-naproxen in a continuous reactor

An enzymatic method for the continuous production of (S)-(+)-2-(6-methoxy-2-naphthyl) propionic acid (Naproxen) has been developed. The process consists of a stereoselective hydrolysis of the racemic Naproxen ethoxyethyl ester catalyzed by Candida cylindracea lipase. The reaction has been carried out in a continuous-flow closed-loop column bioreactor packed with Amberlite XAD-7, a slightly polor resin on which the lipase has been immobilized by adsorption. Various immobilization conditions as well as the properties of the immobilized lipase have been studied. The performance and the productivity of the bioreactor were evaluated as a function of the critical reaction parameters such as temperature, substrate concentration, and product inhibition. By using a 500-mL column bioreactor, 1.8 kg of optically pure (S)-(+)-Naproxen were produced after 1200 h of continuous operation with a slight loss of the enzymatic activity.     

Kinetics of the enzymatic hydrolysis of cellulose: 2. A mathematical model for the process in a plug-flow column reactor

The kinetics of enzymatic cellulose hydrolysis in a plug-flow column reactor catalysed by cellulases [see 1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.4] from Trichoderma longibrachiatum adsorbed on cellulose surface have been studied. The maximum substrate conversion achieved was 90–94%. The possibility of enzyme recovery for a reactor of this type is discussed. A mathematical model for enzymatic cellulose hydrolysis in a plug-flow column reactor has been developed. The model allows for the component composition of the cellulase complex, adsorption of cellulases on the substrate surface, inhibition by reaction products, changes in cellulose reactivity and the inactivation of enzymes in the course of hydrolysis. The model affords a reliable prediction of the kinetics of d-glucose and cellobiose formation from cellulose in a column reactor as well as the degree of substrate conversion and reactor productivity with various amounts of adsorbed enzymes and at various flow rates.     

Interaction of heavy chain binding protein (BiP/GRP78) with adenine nucleotides.

Immunoglobulin heavy chain binding protein (BiP/GRP78) is a resident endoplasmic reticulum protein that binds tightly to a number of incompletely assembled or aberrant proteins. BiP also binds ATP and can be purified by ATP affinity chromatography. Here we show that an ATPase activity co-purifies with BiP prepared from canine pancreas. The BiP-associated ATPase has a high affinity for ATP but a low turnover number, suggesting a regulatory, rather than an enzymatic role. We also show that submicromolar levels of ATP or ADP decrease the rate of adsorption of [125I]BiP to nitrocellulose filters coated with protein or non-ionic detergents. In contrast, micromolar levels of AMP increase the rate of adsorption. Furthermore, ATP and ADP decrease the susceptibility of BiP to proteolytic degradation, whereas AMP was found to enhance degradation slightly. Adenine nucleotides may therefore induce or stabilize different conformations of BiP even when ATP hydrolysis does not occur.