Stability and Enzymatic Studies of Glucose Dehydrogenase from the Archaeon Haloferax mediterranei in reverse micelles

Reverse micelles were used as a cytoplasmic model to study the kinetics of an extreme halophilic enzyme such as the recombinant glucose dehydrogenase from the Archaeon Haloferax mediterranei. This enzyme was solubilized in reverse micelles of hexadecyltrimethylammoniumbromide in cyclohexane, with 1-butanol as co-surfactant. Glucose dehydrogenase retained its catalytic properties in this organic medium, showing good stability at low water content, even at low salt concentration (125 mM NaCl). The dependence of the enzymatic activity on the molar water surfactant ratio (w₀=[H₂O]/[surfactant]) increased with rising water content. Surprisingly, the activity of this extreme halophilic enzyme did not depend on the salt concentration in reverse micelles. The kinetic of the enzymatic oxidation of β-D-glucose to D-glucono-1,5-lactone using NADP⁺ as coenzyme for the glucose dehydrogenase from Haloferax mediterranei was also studied in the reverse micellar system.     

An extreme halophilic enzyme active at low salt in reversed micelles.

Possible biotechnological applications of extreme halophilic enzymes are strongly determined by their high salt requirement of around 4 M NaCl. Consequently, the use of these in organic media seemed to be unlikely. However, we have succeeded in dissolving a halophilic enzyme, p-nitrophenylphosphate phosphatase from the archaeon Halobacterium salinarum, in an organic medium by creating a reverse micellar system with very low salt concentration. The enzyme retained its catalytic properties in reversed micelles made with an anionic surfactant (dioctyl sodium sulphosuccinate) or with a cationic surfactant (hexadecyltrimethylammonium bromide) in cyclohexane plus 1-butanol as co-surfactant. The dependence of the rate of hydrolysis of p-nitrophenylphosphate phosphate on the molar water/surfactant ratio (w(0) value) showed a bell-shaped curve for each surfactant system. Kinetic parameters were determined in each system. The enzymatic reaction appeared to follow Michaelis-Menten kinetics with the anionic surfactant only. The kinetic behaviour was determined at different concentrations of Mn(2+) in reversed micelles of dioctyl sodium sulphosuccinate as surfactant.     

Enzymatic activity of an extremely halophilic phosphatase from the Archaea Halobacterium salinarum in reversed micelles

Alkaline p-nitrophenylphosphate phosphatase (pNPPase) from the halophilic archaeon Halobacterium salinarum (previously halobium) was solubilized in reversed micelles of cetyltrimethylammonium bromide (CTAB) in cyclohexane with 1-butanol as cosurfactant. The hydrolysis reaction appears to follow Michaelis–Menten kinetics. The dependency of the maximum reaction rate (V_max) on the water content θ (% v/v) (or ω₀ value: molar ratio of water to surfactant concentrations) showed a bell-shaped curve for 0.3 M CTAB, but not for 0.2 M CTAB. The enzyme activity increased with the surfactant concentration at a constant ω₀ value (10.27). When the surfactant concentration was increased at a constant θ, the enzyme activity decreased. The enzyme was more stable in reversed micelles than in aqueous media.     

Reactivity of trypsin in reverse micelles: neglected role of aggregate size compared to water-pool components

The catalytic efficiency of trypsin was estimated in cationic reverse micelles as a function of the concentration of water-pool components and aggregate size to determine their independent influence on enzyme activity. The variation in the aggregate size/water-pool size was achieved by changing both the W0 (mole ratio of water to surfactant) and the headgroup area of surfactant through introduction of hydroxyethyl groups at the polar head. The local molar concentrations of water present inside the water-pool ([H2O]wp) of different cationic reverse micelles across varying W0 was estimated using a modified phenyl cation-trapping protocol. The [H2O]wp in cationic reverse micelles (surfactant/isooctane/n-hexanol/water) increases with W0 and attains the molarity of normal water beyond W0=40 irrespective of the nature of headgroup. Concurrently, the catalytic activity of trypsin compartmentalized within the water-pool increases with the increase in [H2O]wp upto an optimal W0=40 in organized solutions of any surfactant. The aggregate size (determined by static light scattering) also increases expectedly with W0 and noticeably with the area of the surfactant headgroup at similar W0. Since the enzyme activity rises both with the increase in water-pool size and [H2O]wp, trypsin's efficiency was compared with these two parameters across reverse micelles of varying surfactant headgroup size at similar W0 to determine their probable independent influence in regulating the enzyme activity. Noticeably, the efficiency of trypsin rises two to ninefold in spite of the [H2O]wp being distinctly lower in case of hydroxyethyl group substituted surfactants compared to cetyltrimethylammonium bromide w/o microemulsions at similar W0. Thus, the influence of the aggregate size possibly plays an important role alongwith the [H2O]wp in modulating the enzyme activity.     

Dependence of Lipase Activity on Water Content and Enzyme Concentration in Reverse Micelles

The activity of Candida rugosa lipase (EC 3.1.1.3) in reverse micelles has been measured at various concentrations of water and enzyme with the aim of answering the question, why is the enzyme activity affected by the molar ratio of water to surfactant (w₀ = [H₂O]/[Surfactant])? In the low range of water content (below w₀ ≈ 6), the activity increases with increasing water content, indicating the requirement of a minimum amount of water for the full expression of enzymatic activity. The minimal w₀-value for obtaining maximal activity depends on the enzyme concentration: The higher the enzyme concentration, the higher w_{0, max}. In addition, it was found that, at least for the case of Candida rugosa lipase, the measured dependence of enzyme activity on w₀ does not represent a true chemical equilibrium. Changing the w₀-value during the reaction does not change the activity as expected on the basis of the w₀-activity profile obtained for single w₀ point measurements. All these observations, however, cannot be directly generalized to all enzymes in reverse micelles, due to the peculiarity of lipase. In particular, the enzyme seems to inactivate irreversibly during the solubilization process.     

Esterification reactions of lipase in reverse micelles

The activities of lipase from Candida cylindracea and Rhizopus delemar have been investigated in water/AOT/iso-octane reverse micellar media through the use of two esterification reactions: fatty acid-alcohol esterification and glyceride synthesis. Such media promotes the occurrence of these two lipase-catalyzed reactions due to its low water content. The effect of various parameters on the activity of lipase from C. cylindracea in reverse micelles was determined and compared to results where alternate media were employed. It was observed that the structure of the media, as dictated by the type and concentration of the substrates and products and by the water/AOT ratio, w(0), had a strong impact on enzyme activity. Strong deactivation of both typase types occurred in reverse micelles, especially in the absence of substrates and for w(0) values greater than 3.0. Glyceride synthesis was realized with lipase from R. delemar, but not with that from C. cylindracea; the temperature and concentration of substrates and water strongly dictated the reaction rate and the percent conversion.     

Increased stability of malate dehydrogenase from Halobacterium salinarum at low salt concentration in reverse micelles

The stability of malate dehydrogenase (hMDH) from Halobacterium salinarum in aqueous medium at low salt concentrations (1 and 0.5 M NaCl) was studied at 4 degrees and 25 degrees C. The results showed that hMDH was more stable at the higher salt concentration and the low temperature. hMDH was introduced into reverse micelles of hexadecyltrimethylammonium bromide in cyclohexane with 1-butanol as co-surfactant. The hMDH stability in this system was studied at two omega(0) ([H(2)O]/[surfactant]) values and the effects of salt concentration, presence of substrate and dilution before or after its introduction into reverse micelles were examined. The results showed that the half-life of hMDH dissolved in buffer with 1 M NaCl was 12-50 days in reverse micelles (depending on the various conditions), in contrast to only about 1 day in aqueous medium at 25 degrees C. These observations indicate that reverse micelles provide a microenvironment that allows a much greater stability of this enzyme compared with an aqueous medium.     

Activity and conformational changes of alpha-chymotrypsin in reverse micelles studied by spin labeling.

alpha-Chymotrypsin (CT), spin-labeled at the active site by using an acylating label which constitutes a substrate for this protein, has been investigated in reverse micelles formed by AOT in isooctane. The electron spin resonance spectra provided information on conformation, dynamics and deacylation activity. The dynamics of the label bound to CT appears to be more hindered in reverse micelles than in aqueous solution, probably owing to the effect of the micellar environment on protein conformation. The deacylation rate in reverse micelles does not show the characteristic bell-shaped dependence on water content which is generally found for CT enzymatic activity.     

Backward extraction of reverse micellar encapsulated proteins using a counterionic surfactant

The back-extraction of proteins encapsulated in AOT reverse micelles was performed by adding a counterionic surfactant, either TOMAC or DTAB. This novel backward transfer method gave higher backward extraction yields compared to the conventional method with high salt and high pH of the aqueous stripping solution. The protein activity was maintained in the resulting aqueous phase, which in this case had a near neutral pH and low salt concentration. A sharp decrease of the water content was observed in the organic phase corresponding to protein back-extraction using TOMAC. The backward transfer mechanism was postulated to be caused by electrostatic interaction between oppositely charged surfactant molecules, which lead to the collapse of the reverse micelles. The back-extraction process with TOMAC was found to be very fast; more than 100 times faster than back-extraction with the conventional method, and as much as 3 times faster than forward extraction. The formation of 1:1 complexes of AOT and TOMAC in the solvent phase was observed, and these hydrophobic complexes could be efficiently removed from the solvent using adsorption onto Montmorillonite in order for the organic solvent to be reused. A second cationic surfactant, DTAB, confirmed the general applicability of counterionic surfactants for the backward transfer of proteins.     

Dependence of Lipase Activity on Water Content and Enzyme Concentration in Reverse Micelles

The activity of Candida rugosa lipase (EC 3.1.1.3) in reverse micelles has been measured at various concentrations of water and enzyme with the aim of answering the question, why is the enzyme activity affected by the molar ratio of water to surfactant (w₀ = [H₂O]/[Surfactant])? In the low range of water content (below w₀ ≈ 6), the activity increases with increasing water content, indicating the requirement of a minimum amount of water for the full expression of enzymatic activity. The minimal w₀-value for obtaining maximal activity depends on the enzyme concentration: The higher the enzyme concentration, the higher w_{0, max}. In addition, it was found that, at least for the case of Candida rugosa lipase, the measured dependence of enzyme activity on w₀ does not represent a true chemical equilibrium. Changing the w₀-value during the reaction does not change the activity as expected on the basis of the w₀-activity profile obtained for single w₀ point measurements. All these observations, however, cannot be directly generalized to all enzymes in reverse micelles, due to the peculiarity of lipase. In particular, the enzyme seems to inactivate irreversibly during the solubilization process.     

The use of reverse micelles for the simultaneous extraction of oil and proteins from vegetable meal

A method for the simultaneous extraction of oil and proteins from vegetable meals is presented. The method uses hydrocarbon reverse micelles, so that the oil is extracted directly into the hydrocarbon phase and the proteins are solubilized in the water pools of the reverse micelles. The surfactant used is bis (2-ethylhexyl) sodium sulfosuccinate (AOT) in isooctane at variable w(0) values (w(0) measures the amount of water in the system, where w(0) = [H(2)O]/[AOT]). A comparison with the usual extraction methods is offered. It is shown that with the micelle system the extraction of oil is as large as with the usual methods, and it is independent of w(0). However the amount and type of proteins extracted depends strongly on w(0). At w(0) values below 6, no protein and only low molecular weight compounds (i.e. chlorogenic acid) are extracted, at larger water content (i.e. by increasing the dimension of the micelle water pool), also proteins are solubilized in a significant amount and with a molecular weight which increases by increasing W(0). The protein solubilized in the microemulsion system can be recovered into an aqueous phase with a back-transfer step.     

DNA polymerase β reveals enhanced activity and processivity in reverse micelles

Water is essential for the stability and functions of proteins and DNA. Reverse micelles are simple model systems where the structure and dynamics of water are controlled. We have estimated the size of complex reverse micelles by light scattering technique and examined the local microenvironment using fluorescein as molecular probe. The micelle size and water polarity inside reverse micelles depend on water volume fraction. We have investigated the different hydration and confinement effects on activity, processivity, and stability of mammalian DNA polymerase β in reverse micelles. The enzyme displays high processivity on primed single-stranded M13mp19 DNA with maximal activity at 10% of water content. The processivity and activity of DNA polymerase strongly depend on the protein concentration. The enzyme reveals also the enhanced stability in the presence of template-primer and at high protein concentration. The data provide direct evidence for strong influence of microenvironment on DNA polymerase activity.     

Modeling stability and flexibility of α-Chymotrypsin in room temperature ionic liquids

We investigate the structure and dynamics of α-Chymotrypsin in five room temperature ionic liquids (RTILs) sharing a common cation, hydrated with different water percentages (w/w) (weight of water over protein). Results from molecular dynamics simulations are correlated with experimental evidences from studies on the activity of enzymes in RTILs. α-Chymotrypsin protein structure is closer to its native crystallographic structure in RTILs than in aqueous environment. We show that the structural properties of α-Chymotrypsin were affected by the water concentration assayed in a typical bell-shaped profile, which is also frequently reported for organic solvents. The protein structure was more native like at 10–20% of water (w/w) for all RTILs except for [BMIM][Cl]. We found that the fluctuations of the main chain in [BMIM][BF₄] and [BMIM][TfO] were not significantly affected by the increasing amount of water. However, we were able to show that the flexible regions were the ones more hydrated, indicating that water is responsible for the flexibility of the protein. The solvation of the enzyme in water-immiscible RTILs, such as [BMIM][PF₆] and [BMIM][Tf₂N] lead to higher enzyme flexibility at increased water content. Enzyme solvation by [BMIM][Cl] resulted in ion penetration in the core enzyme structure, causing incremented flexibility and destabilization at low water percentages. All RTILs stripped water molecules from the protein surface, following a similar behavior also found in organic solvents. Anions formed structured arrangements around the protein, which allowed non-stripped water molecules to localize on the protein surface.     

Enzymatic hydrolysis of microcrystalline cellulose in reverse micelles

The activities of cellulases from Trichoderma reesei entrapped in three types of reverse micelles have been investigated using microcrystalline cellulose as the substrate. The reverse micellar systems are formed by nonionic surfactant Triton X-100, anionic surfactant Aerosol OT (AOT), and cationic surfactant cetyltrimethyl ammonium bromide (CTAB) in organic solvent media, respectively. The influences of the molar ratio of water to surfactant omega0, one of characteristic parameters of reverse micelles, and other environmental conditions including pH and temperature, on the enzymatic activity have been studied in these reverse micellar systems. The results obtained indicate that these three reverse micelles are more effective than aqueous systems for microcrystalline cellulose hydrolysis, and cellulases show "superactivity" in these reverse micelles compared with that in aqueous systems under the same pH and temperature conditions. The enzymatic activity decreases with the increase of omega0 in both AOT and Triton X-100 reverse micellar systems, but reaches a maximum at omega0 of 16.7 for CTAB reverse micelles. Temperature and pH also influence the cellulose hydrolysis process. The structural changes of cellulases in AOT reverse micelles have been measured by intrinsic fluorescence method and a possible explanation for the activity changes of cellulases has been proposed.     

Activity and stability of lipase in Aerosol-OT/isooctane reverse micelles

The stability of Candida rugosa lipase, which catalyzes the hydrolysis reaction of olive oil in AOT/isooctane reverse micelles, decreased with the increase of ₀ (defined as the molar ratio of water to surfactant) and Aerosol-OT concentration. The addition of a non-ionic cosurfactant, tetraethylene glycol dodecyl ether (C₁₂E₄), preserved enzymatic activity. The residual activity of the lipase was 53% after 24 h, while the enzyme completely lost its activity within 6 h in the absence of C₁₂E₄ addition. The stabilizing effect of C₁₂E₄ resulted in the increase of conversion. The enhancement of the activity and stability of lipase in reverse micelles by the addition of C₁₂E₄ may contribute to increase the rigidity of the micellar matrix stabilizing the enzyme structure.     

Renaturation and interaction of ribonuclease A with AOT surfactant in reverse micelles

This study extended works on effects of solute on the percolation of reverse micelles to the effects of interactions between protein and surfactants on protein refolding by reverse micelles. The changes in percolation behavior were identified and attributed to the position of solutes in the core aqueous phase and the interaction between the solute and the surfactants. The percolation behavior of reverse micelles with solutes was related to protein renaturation and the reverse micelle. This study aims to highlight the involvement of the interface and the interaction of the protein with the surfactant during protein refolding. Ribonuclease A and AOT reverse micelles together constitute a model system considered here. The systemic parameters of the reverse micelle, water content (W(o)) and pH value, were applied to modify the interaction between the denatured protein molecules and the surfactant interface. The interactions and the locations of the protein molecules were determined from changes in percolation temperature measured by conductivity. The percolation and protein activity show that a stronger interaction of the protein molecules with surfactant corresponds to superior recovery of protein activity. The investigation concludes that the refolding of protein by reverse micelles is not only facilitated by the isolation of reverse micelles but also by the interaction due to the interface of the reverse micelle.     

Catalysis by Laccase (From Coriolus uersicolor) in Microheterogeneous Media of the Water/Organic Solvent/Surfactant Type

Catalysis by laccase from Coriolus uersicolor solubilized in the ternary systems of surfactant/water/organic solvent type, namely, Aerosol OT/water/octane, Brij 56/water/cyclohexane and egg lecithin/water/octane + pentanol + methanol mixture, has been studied. The laccase activity is found to depend, in principle, not only on the water/surfactant molar ratio, but on the surfactant concentration (with its hydration degree being constant) as well. The following inferences should be emphasized. Firstly, in all the systems under study, the catalytic activity (k_cat) of laccase entrapped into surfactant reversed micelles increases more than 50 times (when the surfactant concentration is extrapolated to zero) compared with the k_cat value in aqueous solution. Secondly, the catalytic activity (k_cat) of laccase entrapped in hydrated Aerosol OT aggregates, having lamellar, reversed cylindrical (hexagonal) and reversed micellar structure, depends greatly on the aggregate type. In other words, the phase transitions, i.e. an alteration in the packing of hydrated Aerosol OT molecules, evokes a sharp reversible change in the enzymatic activity. Thirdly, in the same phase, the catalytic activity of the solubilized enzyme depends on the linear dimensions of water cavities inside the surfactant aggregates (i.e. on the water content in the system under study). All these effects, regulating enzymatic activity, are probably caused by an alteration of the conformational mobility of laccase molecules incorporated into the inner polar cavities inside the surfactant aggregates.     

A novel approach to study of action of water-insoluble inhibitors of enzymic reactions

The effect of water-insoluble compounds on enzyme catalytic properties was studied using a colloidal solution of water in organic solvent as reaction medium. In this microheterogeneous medium enzyme is entrapped into hydrated reversed micelles of a surfactant, the dimensions of the internal hole of the micelles being dependent on the ratio of water to surfactant. At sufficiently low values of this ratio the molecule of entrapped enzyme has limited mobility in the micelle. Because of this the interaction of the enzyme with water-insoluble compound which is added in assay solution and intercalated in the surface layer of the micelle may be manifested. The suggested method was used to study the inhibitory action of dihydroriboflavin esters on D-amino acid oxidase from pig kidney and soybean lipoxygenase. The reaction medium was hydrated reversed micelles of Aerosol OT in octane. The method of sedimentation in an analytical ultracentrifuge has shown the dihydroriboflavin esters to be completely included into reversed micelles.     

Stability of an extreme halophilic alkaline phosphatase from Halobacterium salinarium in non-conventional medium

Alkaline p-nitrophenylphosphate phosphatase from the halophilic archaeon Halobacterium salinarum (earlier halobium) was solubilised in organic medium using reversed micelles of hexadecyltrimethylammonium bromide in cyclohexane, with 1-butanol as co-surfactant. The stability of alkaline p-nitrophenylphosphate phosphatase in this system was studied at different conditions, w(0) ([H(2)O]/[surfactant]), salt concentration, with and without Mn(+2). At all the conditions assayed, alkaline p-nitrophenylphosphate phosphatase was more stable in reversed micelles than in bulk aqueous solution (at 25 degrees C). The stabilisation effect of the reversed micelles was dramatic when the enzyme was dialysed against Mn(+2)-free buffer since the enzyme lost all the activity within 90 min in aqueous medium, but it retained approximately 72% of the initial enzymatic activity for 90 min in reversed micelles.     

Membrane proteins in reverse micelles: myelin basic protein in a membrane-mimetic environment

The solubility, reactivity, and conformational dynamics of myelin basic protein (MBP) from bovine brain were studied in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-isooctane and water. Such a membrane-mimetic system resembles the aqueous spaces of native myelin sheath in terms of physicochemical properties as reflected in the high affinity of MBP for interfacial bound water. This is marked by the unusual profile of the solubility curve of the protein in reverse micelles, which shows optimal solubility at a much lower molar ratio of water to surfactant ([ H2O]/[AOT] = w0) than that reported for other water-soluble proteins. The role of counterions and/or charged polar head groups in the solubilization process is revealed by comparison of the solubility of MBP in nonionic surfactant micellar solutions. Whereas MBP is unfolded in aqueous solutions, insertion into reverse micelles generates a more folded structure, characterized by the presence of 20% alpha-helix. This conformation is unaffected by variations in the water content of the system (in the 2.0-22.4 w0 range). The reactivity of epsilon-amino groups of lysine residues with aqueous solutions of o-phthalaldehyde demonstrates that segments of the peptide chain are accessible to water. Similar results were obtained with the sequence involved in heme binding. In contrast, the sole tryptophan residue, Trp-117, is shielded from the aqueous solvent, as indicated by lack of reaction with N-bromosuccinimide. The invariance of the wavelength maximum emission in the fluorescence spectra as a function of w0 is consistent with this result.(ABSTRACT TRUNCATED AT 250 WORDS)