Categorization of enzyme deactivations using a series-type mechanism

Co-Aggregation of Penicillin G Acylase and Polyionic Polymers: An Easy Methodology To Prepare Enzyme Biocatalysts Stable in Organic Media

A novel type of biocatalyst that combines the good properties of cross-linked enzyme aggregates (CLEAs) and hydrophilic microenvironments has been developed. Dextran sulfate- and polyethyleneimine-coated CLEAs of penicillin acylase (CLEA-GDP) were prepared by adding the polymers of different sizes before the precipitation stage of the enzyme. This study presents the development and optimization of a protocol to produce such a biocatalyst using penicillin acylase as a model. Experiments show that CLEA-GDPs have a highly increased stability in organic media. The average half-life of the preparations was much higher than standard CLEA without a microenvironment (CLEA-G), (e.g., more than 25-fold) in the presence of dioxane. However, their thermal stability was not increased, which leads to the conclusion that the stability of CLEA-GDPs in organic media is due to the hydrophilic microenvironment that surrounds the protein enzyme more than to a conformational stiffening effect. This is further supported by solvation experiments that show a preferential hydration of CLEA when polymers are used to coat the enzyme. CLEA-GDPs are clearly better than other biocatalysts in terms of solvent stability.

Enzyme reactor design under thermal inactivation

Temperature is a very relevant variable for any bioprocess. Temperature optimization of bioreactor operation is a key aspect for process economics. This is especially true for enzyme-catalyzed processes, because enzymes are complex, unstable catalysts whose technological potential relies on their operational stability. Enzyme reactor design is presented with a special emphasis on the effect of thermal inactivation. Enzyme thermal inactivation is a very complex process from a mechanistic point of view. However, for the purpose of enzyme reactor design, it has been oversimplified frequently, considering one-stage first-order kinetics of inactivation and data gathered under nonreactive conditions that poorly represent the actual conditions within the reactor. More complex mechanisms are frequent, especially in the case of immobilized enzymes, and most important is the effect of catalytic modulators (substrates and products) on enzyme stability under operation conditions. This review focuses primarily on reactor design and operation under modulated thermal inactivation. It also presents a scheme for bioreactor temperature optimization, based on validated temperature-explicit functions for all the kinetic and inactivation parameters involved. More conventional enzyme reactor design is presented merely as a background for the purpose of highlighting the need for a deeper insight into enzyme inactivation for proper bioreactor design.

Effects of temperature and pressure on Rhizomucor miehei lipase stability

Both high temperature and high hydrostatic pressure induce irreversible deactivation of enzymes. They enable the enzyme's thermodynamic parameters to be determined and are used to study the mechanisms involved in biochemical systems. The effect of these two factors on the stability of Rhizomucor miehei lipase have been investigated. The stability criterion used was residual hydrolytic activity of the lipase. Experimental and theoretical parameters, obtained by linear regression analysis, were compared with theoretical kinetics in order to validate the series-type inactivation model. The lipase of R. miehei was deactivated by either thermal or pressure treatment. Moreover conformational studies made by fluorescence spectroscopy suggest that the conformational changes induced by pressure were different from those induced by temperature. In addition they show that after thermal deactivation there were less intermolecular hydrogen bonded structures formed than was the case for deactivation by high pressure.

Kinetics of the lipase-catalyzed synthesis of glucose esters in acetone

A simple kinetic model derived from a ping-pong bi-bi mechanism is proposed to describe the lipase-catalyzed esterification of glucose with fatty acids. The mathematical expressions derived from this model have been tested using several sets of data obtained from reactions carried out under different reaction conditions. The predicted values provide very good fits of the experimental data for temperatures from 30 to 60 degrees C, enzyme loadings from 90 to 180 mg, and fatty acid concentrations from 0.33M to 1M. Experiments conducted at different temperatures permit one to estimate an activation energy of approximately 12 kcal/mol for the rate-limiting step of the reaction (formation of the acyl-enzyme complex). The model also considers the kinetics of inactivation of the biocatalyst during the reaction.

Kinetics of thermal deactivation of enzymes: a simple three parameters phenomenological model can describe the decay of enzyme activity, irrespectively of the mechanism

Heat induced enzyme inactivation or protein denaturation is now well documented, due to progresses in methods, instruments and computation resources. Complex mechanisms, rather than the classic simple "one step - two states" model (still in use) are recognized in many cases, leading investigators to manipulate more or less complicated kinetic expressions describing the heat induced decay of enzyme activity.We show that the different kinetic expressions related to different mechanisms among the most frequently encountered can be arranged in a common simple three-parameters biexponential equation.This unifying simplification is of interest for people focusing attention to phenomenological rather than mechanistic description of the kinetics of heat induced enzyme deactivation. Moreover, the reduction in the number of parameters reduces the risk of cross-correlation and allows a better estimation of the apparent rate constants (which are in many cases the pertinent required information). It also illustrates the difficulty to make inference of mechanism from kinetics, since the same equation applies for a variety of mechanisms ("kinetic homeomorphism") - in particular, it stresses out the need of caution when reporting on existence of isoenzymes from deactivation kinetics.Application of this simple 3-parameters biexponential kinetic expression has been validated with a number of results in the Literature and current investigations in our laboratory. Two examples are given.

Continuous enzymatic esterification of glycerol with (poly)unsaturated fatty acids in a packed-bed reactor

Enzymatic synthesis of mono-, di-, and triacyglycerols from (poly)unsaturated fatty acids (linoleic, oleic, and conjugated linoleic acids) has been studied as a solvent-free reaction in a packed-bed reactor containing an immobilized lipase from Mucor miehei. The extents of the esterification reactions of interest are primarily determined by the molar ratio of glycerol to fatty acid because the presence of excess glycerol as a immiscible phase is responsible for reducing the activity of the water produced by the esterification reactions. For molar ratios of fatty acid to glycerol of less than 1.5, the percentage of the fatty acid esterified decreases quasi-linearly with an increase in this molar ratio. By appropriate manipulation of the fluid-residence time, one can control the relative proportions of the various acylglycerols in the effluent stream. At the outlet of the reactor, one observes excellent spontaneous separation of the glycerol and acylglycerol/fatty acid phases. At 50 degrees C and a fluid residence time of 1 hour, as much as 90% of the fatty acid can be esterified when the molar ratio of fatty acid to glycerol is 0.33 or less.

Deactivation and conformational changes of cutinase in reverse micelles

Deactivation data and fluorescence intensity changes were used to probe functional and structural stability of cutinase in reverse micelles. A fast deactivation of cutinase in anionic (AOT) reverse micelles occurs due to a reversible denaturation process. The deactivation and denaturation of cutinase is slower in small cationic (CTAB/1-hexanol) reverse micelles and does not occur when the size of the cationic reverse micellar water-pool is larger than cutinase. In both systems, activity loss and denaturation are coupled processes showing the same trend with time. Denaturation is probably caused by the interaction between the enzyme and the surfactant interface of the reversed micelle. When the size of the empty reversed micelle water-pool is smaller than cutinase (at W0 5, with W0 being the water:surfactant concentration ratio) a three-state model describes denaturation and deactivation with an intermediate conformational state existing on the path from native to denaturated cutinase. This intermediate was clearly detected by an increase in activity and shows only minor conformational changes relative to the native state. At W0 20, the size of the empty water-pool was larger than cutinase and the data was well described by a two-state model for both anionic and cationic reverse micelles. For AOT reverse micelles at W0 20, the intermediate state became a transient state and the deactivation and denaturation were described by a two-state model in which only native and denaturated cutinase were present. For CTAB/1-hexanol reverse micelles at W0 20, the native cutinase was in equilibrium with an intermediate state, which did not suffer denaturation. 1-Hexanol showed a stabilizing effect on cutinase in reverse micelles, contributing to the higher stabilities observed in the cationic CTAB/1-hexanol reverse micelles. Copyright 1998 John Wiley & Sons, Inc.

Irreversible high pressure inactivation of beta-galactosidase from Kluyveromyces lactis: comparison with thermal inactivation

High hydrostatic pressure and high temperature are both shown to induce inactivation of Kluyveromyces lactis beta-galactosidase in deionised water and their respective effects are compared. These two physical parameters lead to similar inactivation kinetics which can be suitably represented by series-type models. The plot of half-lives as a function of pressure is close to the same plot towards temperature. Thus, the same inactivation rate constant can be obtained in two different ways: an increase in pressure at room temperature or an increase in temperature at atmospheric pressure (e.g. 125 MPa at 25 degrees C or 45 degrees C at 0.1 MPa for a kappa 1 value about 28 x 10(-2) min -1). When beta-galactosidase was prepared in 0.1 M potassium phosphate buffer pH 7.3, its stability in extreme conditions of pressure as at high temperature was strongly enhanced. This stabilizing effect of the buffer was essentially attributed to a pH-effect by comparison with the behaviour of the enzyme in a similar buffer but with a 10-fold lower ionic strength.

Role of hydrodynamic shear on activity and structure of proteins

Proteins are important products used in industry. They may be enzymes which catalyze different reactions or they may be required for their biological activities as hormones, growth factors or therapeutics. During production and recovery, proteins are subjected to fluid forces which arise due to operations such as stirring, pumping and centrifugation. The resulting hydrodynamic shear forces may cause damage to the large molecular weight proteins, resulting in denaturation and inactivation of the protein. This is a major concern as it affects the overall efficiency of protein recovery and final yield of the product. A considerable amount of research has been devoted to studying the effects of hydrodynamic shear stress on proteins, especially with respect to the enzymes. Enzymes are subjected to shear stresses during their production in fermentors, during isolation and purification steps in downstream operations and also during their use in enzyme reactors, especially if stirred reactors are employed to perform enzyme catalysed reactions. The present review discusses the effects of fluid shear stress on proteins including enzymes. A brief description on deactivation has been included in order to understand the effect of shear on the deactivation kinetics of proteins. The model systems used to subject proteins to shear and some unit operations during protein processing or use wherein they are exposed to shear stresses have also been presented. The significance of shear effects in designing bioprocesses involving shear sensitive biocatalysts as well as suggestions for future work have also been given.

Immobilization of a recombinant cutinase by entrapment and by covalent binding. Kinetic and stability studies

Fusarium solani pisi recombinant cutinase, immobilized by entrapment in calcium alginate and by covalent binding on porous silica, was used to catalyze the hydrolysis of tricaprylin. The influence of relevant parameters on the catalytic activity such as pH, temperature, and the substrate concentration were studied. Cutinase immobilized by entrapment presented a Michaelis-Menten kinetics for tricaprylin concentrations up to 200 mM. At higher concentrations of substrate, inhibition was observed. For covalent binding immobilization, diffusional limitations were observed at low substrate concentrations and substrate inhibition occurred for concentrations higher than 150 mM. The stability of immobilized cutinase was also evaluated. The enzyme immobilized by entrapment showed a high stability, in contrast to the immobilization on porous silica.

Superactivity of peroxidase solubilized in reversed micellar systems

Vaccinium mirtyllus peroxidase solubilized in reversed micelles was used for the oxidation of guaiacol. Some relevant parameters for the enzymatic activity, such as pH, w(o) (molar ratio water/surfactant), surfactant type and concentration, and cosurfactant concentration, were investigated. The peroxidase showed higher activities in reversed micelles than in aqueous solution. The stability of the peroxidase in reversed micelles was also studied, namely, the effect of w(o) and temperature on enzyme deactivation. The peroxidase displayed higher stabilities in CTAB/hexanol in isooctane reversed micelles, with half-life times higher than 500 h.

Triglyceride hydrolysis and stability of a recombinant cutinase from Fusarium solani in AOT-iso-octane reversed micelles

A recombinant cutinase from Fusarium solani was encapsulated in AOT reversed micelles. Physicochemical parameters of the system were optimized relative to triolein hydrolysis. Kinetic studies of triglyceride hydrolysis showed a decrease in specificity with increase of the acyl chain length. Stability of cutinase in the system under study is lower than in aqueous solution and decreases with increase in the water content in the system (W0 = [H2O]/[AOT]). The products of triolein hydrolysis had little effect on the cutinase stability. Although glycerol did not alter the stability, oleic acid decreases the enzyme stability. The increase in log P of solvent (from iso-octane to n-dodecane) decreased the stability. Deactivation profiles were fitted with the Henley and Sadana model (1).

Influence of polyhydroxylic cosolvents on papain thermostability

Papain thermostability was studied, and non-first-order deactivation kinetics were observed. The results obtained were analyzed by a two-step series-type deactivation model involving the native and active enzyme, an active intermediate enzyme state, and a final inactive state, with excellent agreement. The influence of different polyhydroxylic cosolvents (ethylene glycol, glycerol, erythritol, xylitol and sorbitol) on the thermostability of papain at 60 degrees C was also studied. Analysis of the results by the assayed model showed that the main protective effect of cosolvents was observed in the second step of the deactivation profile. The results obtained were analyzed as a function of both the thermodynamic parameters and a protective effect, defined as the ratio of papain half-lives (with and without cosolvents) for the second deactivation step, showing in both cases an important stabilizing effect of these cosolvents on the enzyme. The overall protective effect of cosolvents was also related simultaneously to their concentration and their water activity-depressing power.

Effect of pH on subunit association and heat protection of soybean alpha-galactosidase

Soybeans contain the enzyme alpha-galactosidase, which hydrolyzes alpha-1, 6 linkages in stachyose and raffinose to give sucrose and galactose. We have found that galactose, a competitive product inhibitor of alpha-galactosidase, strongly promotes the heat stability of the tetrameric form of the enzyme at pH 4.0 and at temperatures of up to 70 degrees C for 60 min. Stachyose and raffinose also protect alpha-galactosidase from denaturation at pH 4.0 although to a lesser extent. Glucose and mannose have little effect. At pH 7.0 the enzyme is a monomer, and galactose has no effect on the heat stability of the enzyme. In the absence of heat protection of the enzyme by added sugars, a series deactivation mechanism was found to describe the deactivation data. In comparison, a unimolecular, non-first order deactivation model applies at pH 4.0, where heat protection effects were observed. At a temperature above 60 degrees C, simple deactivation is a suitable model. The results suggest that alpha-galactosidase conformation and heat stability are directly related.

Kinetics and mechanisms of reactions catalysed by immobilized lipases

This review focuses on the kinetics and mechanisms of reactions catalysed by immobilized lipases. The effects of pH, temperature, and various substances on the catalytic properties of immobilized lipases and on the processes by which they are deactivated are reviewed and discussed.