Enzyme activation by denaturants in organic solvent systems with a low water content

Biochemical, thermodynamic and structural characteristics of a biotechnologically compatible alkaline protease from a haloalkaliphilic, Nocardiopsis dassonvillei OK-18

This report describes purification strategies, biochemical properties and thermodynamic analysis of an alkaline serine protease from a marine actinomycete, Nocardiopsis dassonvillei strain OK-18. The solvent tolerance, broad thermal-pH stability, favourable kinetics and thermodynamics suggest stability of the enzymatic reaction. The enzyme was active in the range of pH 7-12 and 37-90 °C, optimally at pH 9 and 70 °C. The deactivation rate constant (Kd), half-life (t½), enthalpy (ΔH*), entropy (ΔS*), activation energy (E) and change in free energy (ΔG*) suggested stability and spontaneity of the reaction. β-Sheets as revealed by the Circular dichroism (CD) spectroscopy, were the major elements in the secondary structure of the enzyme, while Fourier-transform infrared spectroscopy (FTIR) indicated the presence of amide I and amide II. Based on the liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QToF-MS) analysis, the amino acid sequence had only 38% similarity with other proteases of Nocardiopsis strains, suggesting its novelty. The Ramachandran Plot revealed the location of the amino acid residues in the most favored region. The blood de-staining, gelatin hydrolysis, silver recovery and deproteinization of crab shells established the biotechnological potential of the enzyme.

Purification and characterization of a urea sensitive lactate dehydrogenase from the liver of the African clawed frog, Xenopus laevis

The African clawed frog, Xenopus laevis, is able to withstand extremely arid conditions by estivating, in conjunction with dehydration tolerance and urea accumulation. Estivating X. laevis reduce their metabolic rate and recruit anaerobic glycolysis, driven by lactate dehydrogenase (LDH; E.C. 1.1.1.27) enzymes that reversibly convert pyruvate and NADH to lactate and NAD(+), to meet newly established ATP demands. The present study investigated purified LDH from the liver of dehydrated and control X. laevis. LDH from dehydrated liver showed a significantly higher K m for L-lactate (1.74 fold), NAD(+) (2.41 fold), and pyruvate (1.78 fold) in comparison to LDH from the liver of control frogs. In the presence of physiological levels of urea found in dehydrated animals, the K m values obtained for dehydrated LDH all returned to control LDH K m values. Dot blot analysis showed post-translational modifications may be responsible for the kinetic modification as the dehydrated form of LDH showed more phosphorylated serine residues (1.54 fold), less methylated lysine residues (0.43 fold), and a higher level of ubiquitination (1.90 fold) in comparison to control LDH. The physiological consequence of dehydration-induced LDH modification appears to adjust LDH function in conjunction with urea levels in dehydrated frogs. When urea levels are high during dehydration, LDH retains its normal function. Yet, as urea levels drop during rehydration, LDH function is reduced, possibly shunting pyruvate to the TCA cycle.

Reversible Equilibrium Unfolding of Triosephosphate Isomerase from Trypanosoma cruzi in Guanidinium Hydrochloride Involves Stable Dimeric and Monomeric Intermediates

The reversible guanidinium hydrochloride-induced unfolding of Trypanosoma cruzi triosephosphate isomerase (TcTIM) was characterized under equilibrium conditions. The catalytic activity was followed as a native homodimeric functional probe. Circular dichroism, intrinsic fluorescence, and size-exclusion chromatography were used as secondary, tertiary, and quaternary structural probes, respectively. The change in ANS fluorescence intensity with increasing denaturant concentrations was also determined. The results show that two stable intermediates exist in the transition from the homodimeric native enzyme to the unfolded monomers: one (N(2*)) is a slightly more expanded, non-native, and active dimer, and the other is a partially expanded monomer (M) that binds ANS. Spectroscopic and activity data were used to reach a thermodynamic characterization. The results indicate that the Gibbs free energies for the partial reactions are 4.5 (N(2) <==> N(2*)), 65.8 (N(2*) <==> 2M), and 17.8 kJ/mol (M <==> U). It appears that TcTIM monomers are more stable than those found for other TIM species (except yeast TIM), where monomer stability is only marginal. These results are compared with those for the guanidinium hydrochloride-induced denaturation of TIM from different species, where despite the functional and three-dimensional similarities, a remarkable heterogeneity exists in the unfolding pathways.

Effect of urea on the enzymatic activity of a lipase entrapped in AOT–heptane–water reverse micellar solutions

A study has been made of the effect of urea upon the hydrolysis of 2-naphthyl acetate (2-NA) catalyzed by lipase from Rhizopus arrhizus in AOT-heptane-water reverse micellar solutions at pH 7. The partition constants, K, of 2-NA between n-heptane and aqueous urea solutions in the absence of micelles were also determined. It was found that K decreases when the concentration of urea increases. In aqueous solution the rate of hydrolysis of 2-NA catalyzed by lipase is dependent on the concentration of urea (at a given 2-NA concentration). This result can be due to a decrease in the magnitude of the association of lipase with 2-NA and/or to changes in the reaction rate of the lipase-2-NA complex. The modifications of the enzymatic activities elicited by addition of urea show a lineal correlation with K, emphasizing the relevance of hydrophobic effects in the loss of activity. Nevertheless, the slope of the line is higher than one, suggesting that changes in the conformation of the enzyme would be also important. Addition of urea to the micellar solutions provokes a decrease of the enzyme activity. From the dependence of the reaction rate with AOT concentration, the partition constant of 2-NA between n-heptane and the micelles, K(p), was obtained. In the presence of 2 M urea a value of K(p)=0.33 M(-1) was derived. This value is lower than that measured in the absence of urea (Aguilar et al., Arch. Biochem. Biophys. 388 (2001) 231), indicating that incorporation of urea to the micellar interface produces a decrease of the association of 2-NA with the micelles. From a comparison of the results obtained in the micellar solution and in aqueous solution, it is concluded that the enzyme is more resistant to denaturation by urea in the micellar solution than in aqueous solution. Furthermore, at intermediate urea concentrations (2 M), the additive produces an increase in the Michaelis constant (K(M)) without a significant decrease (or even a small increase) in the catalytic rate constant (k(cat)).

Activation of enzymes for nonaqueous biocatalysis by denaturing concentrations of urea

Urea is one of the most commonly used denaturants of proteins. However, herein we report that enzymes lyophilized from denaturing concentrations of aqueous urea exhibited much higher activity in organic solvents than their native counterparts. Thus, instead of causing deactivation, urea effected unexpected activation of enzymes suspended in organic media. Activation of subtilisin Carlsberg (SC) in the organic solvents (hexane, tetrahydrofuran, and acetone) increased with increasing urea concentrations up to 8 M. Active-site titration results and activity assays indicated the presence of partially unfolded but catalytically active SC in 8 M urea; however, the urea-modified enzyme retained high enantioselectivity and was ca. 80 times more active than the native enzyme in anhydrous hexane. Likewise, the activity of horseradish peroxidase (HRP) lyophilized from 8 M urea was ca. 56 times and 350 times higher in 97% acetone and water-saturated hexane, respectively, than the activity of HRP lyophilized from aqueous buffer. Compared with the native enzyme, the partially unfolded enzyme may have a more pliant and less rigid conformation in organic solvents, thus enabling it to retain higher catalytic activity. However, no substantial activation was observed for alpha-chymotrypsin lyophilized from urea solutions in which the enzyme retained some activity, illustrating that the activation effect is not completely general.

Klenow fragment and DNA polymerase alpha-primase fromserva calf thymus in water-in-oil microemulsions

The activity of DNA polymerase alpha-primase complex from calf thymus and Klenow fragment of E. coli DNA polymerase 1 has been studied in reverse microemulsions formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT), sodium dodecylsulfate (SDS), cetyl trimethyl ammonium bromide (CTAB), polyoxyethylene 20 cetyl ether (Brij 58), and Triton X-114 in decane. DNA polymerases were not active in AOT, CTAB, and SDS reverse microemulsions, but these enzymes catalyzed DNA synthesis in Brij 58 and its mixture with other surfactants. We have also found the system composed from the Triton X-114, SDS, CTAB, and Brij 58 (concentration of 128, 25, 15, and 10 mM, respectively) in hexanol-decane (1:12 v/v), in which DNA polymerases revealed maximum activity. The above system was optically transparent, fluid, and stable during a few hours with a water-surfactants molar ratio up to 160. The pH dependence of DNA polymerase activity was not significantly different in comparison with water; however, DNA polymerase was sensitive to ionic strength in microemulsions. The dependence of DNA polymerase activity on w0 was the curve with a few optima. DNA polymerases synthesized more products in water than in reverse microemulsions, and the processivity of Klenow fragment decreased. An increase of the water content resulted in an increase of DNA polymerase processivity.

Reverse Micelle Systems Composed of Water, Triton X-100, and Phospholipids in Organic Solvents

Enzymes entrapped in systems formed with water, phospholipids, toluene, and Triton X-100 show a catalytic activity that is much lower and a thermostability that is much higher than that observed in totally aqueous systems or in other types of reverse micelles. By phase boundary titrations and dynamic light scattering, this work characterizes reverse micelle systems formed in either toluene or propylbenzene with Triton X-100 and water. Four regions with distinct structural features were encountered. Up to one molecule of water per one Triton X-100 molecule, the system was transparent; light scattering measurements of this region indicated that water hydrated Triton X-100 monomers. A turbid region was formed as water content was increased to water:Triton X-100 ratios of 7.6 in toluene and 4.2 in propylbenzene. This thermodynamically unstable region was formed by large polydisperse structures. Transparent systems containing small size (27-150 A) thermodynamically stable reverse micelles were formed when the ratio of water to Triton X-100 molecules in the reverse micelle was in the range of 7.6 to 26.8 in toluene and 4.2 to 15.1 in propylbenzene. In this region, micellar size increased with water content. Water concentrations higher than the latter values resulted in phase separation. A similar titration of the aforementioned systems in the presence of phospholipids revealed that in the first region of transparency up to 10 molecules of water hydrated a phospholipid molecule. The inclusion of phospholipids to the Triton X-100 systems caused a displacement of the boundaries of the second region of transparency toward higher water contents. Copyright 1998 Academic Press. Copyright 1998Academic Press

Reverse Micelle Systems Composed of Water, Triton X-100, and Phospholipids in Organic Solvents

Catalysis, stability, and thermostability of yeast hexokinase were determined in the microenvironments of two organic solvent/Triton X-100/phospholipids systems. In the abscence of enzyme, phase diagrams showed two transparent/turbid transitions, and reverse micelles were only observed in the second region of transparency (T2), where particle size as a function of water content shows a minima (see previous paper in this issue). In the present work, enzyme activity was detected throughout the four regions of the phase diagrams of these systems. Catalysis increased with water content; nevertheless, the maximum activities that were reached in the toluene and propylbenzene systems were 30 and 1.6%, respectively, of the activity in all aqueous media. Because in the T2 region in the propylbenzene system, micelles are much smaller than in toluene (see preceding paper), it would appear that expression of catalysis depends on the size of the micelles. However, a comparison of the dimensions of hexokinase and those of reverse micelles in the T2 region, suggests that in this region, hexokinase entrapment increases the inner volume of the micelle. High enzyme thermostability was only observed in the first transparent region (T1) of the system that contained phospholipids. In this region, hexokinase induced the formation of reverse micelles from dispersed surfactant monomers. There is a striking similarity in the dimensions of hexokinase entrapped in reverse micelles as determined by dynamic light scattering measurements in the T1 region with those of hexokinase as obtained from X ray diffraction studies of the enzyme in a crystalline environment. This suggest that high thermostability, and low catalytic rates result from restrictions in mobility imposed by a low water environment. Copyright 1998 Academic Press. Copyright 1998Academic Press

Charge-shielding and the "paradoxical" stimulation of tubulin polymerization by guanidine hydrochloride

Low concentrations of guanidine hydrochloride (GuHCl) increase the rate (and to a lesser degree, the extent) of tubulin polymerization as assessed by light scattering. Maximum enhancement occurs at 120-160 mM GuHCl followed by decreases at higher GuHCl. The latent period is decreased, and there is a 3-4 fold reduction in the critical concentration of polymerization. Electronmicrographs reveal microtubules in the controls and an increasing fraction of total polymers present as aberrant microtubules as the GuHCl concentration is increased from 20 to 100 mM. The GuHCl effect is markedly reduced, but not abolished, in tubulin S (in which the anionic C termini of both monomers have been removed). The GuHCl-induced polymerization has an absolute requirement for GTP and taxol or DMSO, is very sensitive to podophyllotoxin inhibition, and can overcome urea-mediated inhibition of polymerization. Guanidinium analogues mimic the GuHCl effect roughly as a function of the number of potential hydrogen bonds. The anions of the guanidine salts superimpose their inhibitory action on the guanidinium cation effect according to the lyotropic series. At higher GuHCl concentrations (peak effect 500-700 mM), a different polymer (type II) is formed that is GTP and taxol independent, but whose polymerization is retarded but not prevented by podophyllotoxin. Its structure resembles the fibrillar network seen in unfolding intermediates of other proteins. We conclude that both charge and hydrogen-bonding ability are major contributors to the GuHCl-induced promotion of tubulin polymerization, and that charge-shielding is likely to be the basis for this effect.

Local unfolding and the stepwise loss of the functional properties of tubulin

Tubulin exhibits a number of characteristic functions that can be used to identify it. They include the ability to polymerize to microtubules, GTPase activity, and the binding of numerous antimitotic drugs and fluorophores. These functions can be differentially modified by low (0.1-1.0M) urea concentrations, and such urea-induced modifications are stable over time periods of minutes to hours. These intermediate states suggest the existence of restricted regions in the protein each of which is associated with a function and its own urea sensitivity. In order of decreasing sensitivity to urea these effects are decreased rate of polymerization of tubulin to microtubules > decreased extent of polymerization approximately decreased GTPase activity > enhanced fluorescence of a rapidly binding analogue of colchicine-MTPT [2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone] approximately decreased proteolysis by trypsin (after alpha Arg339) and by chymotrypsin (after beta Tyr281) > enhanced fluorescence of 1-anilino-8-naphthalenesulfonic acid (ANS). Additional evidence for the independent behavior of the restricted regions stems from the markedly different time dependence of the response to urea. These low urea concentrations do not induce significant changes in tryptophan fluorescence, suggesting that the observed effects are due to local unfolding. At higher urea concentrations (2-4 M), the enhanced fluorescence of the ligands is abolished; MTPT fluorescence decreases at lower urea concentrations than ANS fluorescence. Moreover, tubulin becomes highly susceptible to proteolysis at multiple sites, and tryptophan emission shows a red-shift, as expected. Multistep unfolding in response to denaturants has been reported for some other proteins. Tubulin appears to be an extreme example of such local responses that proceed under milder conditions than the global transition to the unfolded state.

Deamidation of triosephosphate isomerase in reverse micelles: effects of water on catalysis and molecular wear and tear

The specific deamidation of asparagine-71 of triosephosphate isomerase increases upon substrate binding and catalysis. This deamidation at the dimer interface initiates subunit dissociation, unfolding, and protein degradation. The apparent connection between catalysis and terminal marking supports the concept of "molecular wear and tear", and raises questions related to the molecular events that lead to deamidation. In order to explore this interaction, triosephosphate isomerase was entrapped in reverse micelles with different water contents that support different catalytic rates. Deamidation was quantified for the free enzyme, the enzyme in the presence of substrates, and the enzyme which had been covalently modified at the catalytic center with the substrate analogue 3-chloroacetol phosphate (CAP). Both in water and in reverse micelles of cetyltrimethylammonium with 3% and 6% water, substrate binding enhanced deamidation. Studies of the extent of deamidation at various water concentrations showed that deamidation per catalytic turnover was about 6 and 17 times higher in 6% and 3% water than in 100% water, respectively. The enzyme was also entrapped in micelles formed with toluene, phospholipids, and Triton X-100 to explore the process at much lower water concentrations (e.g., 0.3%). Under these conditions, catalysis was very low, and hardly any deamidation took place. Deamidation of the CAP-labeled enzyme was also markedly diminished. At these low-water conditions, the enzyme exhibited markedly increased thermostability and resistance to hydrolysis of the amide bonds. The data suggest that the rate of deamidation not only is dependent on the number of catalytic events but also is related to the time that asparagine-71 exists in a conformation or solvent environment more favorable for deamidation.

Activity and fluorescence changes of lactate dehydrogenase induced by guanidine hydrochloride in reverse micelles

Denaturants activate several multimeric enzymes in reverse micelles [Garza-Ramos, G., Darszon, A., Tuena de Gómez-Puyou, M. & Gómez-Puyou, A. (1992) Eur. J. Biochem. 205, 509-517]. Here, the effect on activity and intrinsic fluorescence of pig heart lactate dehydrogenase (LDH) in reverse micelles [formed with 0.2 M cetyltrimethylammonium bromide in octane/hexanol (8.6:1, by vol.)] was explored at various water and guanidine hydrochloride (Gdn/HCl) concentrations. Emission fluorescence spectra of LDH in aqueous media and in micelles were similar. As in all aqueous media, 1.0 M Gdn/HCl in the water phase of reverse micelles produced fluorescence quenching and a blue shift of the maximal emission. In 5.0 M Gdn/HCl, instead of the red shift and significant quenching seen in water, the maximum emission further shifted to the blue and was only slightly quenched. Gdn/HCl titrations of activity and fluorescence changes of LDH in micelles with different water contents showed that at Wo ([H2O]/[surfactant]) of 6.6, 8.3, or 12.5, increasing concentrations of Gdn/HCl up to 0.6 M produced small changes in fluorescence, whereas activity increased several-fold. At higher denaturant concentrations, activity decreased with significant fluorescence changes. In reverse micelles with 1 M Gdn/HCl, Vmax but not Km of LDH decreased with time. Under these conditions, there was progressive quenching of LDH fluorescence. The results show that in reverse micelles different Gdn/HCl concentrations induce variations in activity with or without alterations of the intrinsic fluorescence of LDH. The results also indicate that in reverse micelles, concentrations of Gdn/HCl below 1.0 M cause an enhancement of protein flexibility; this is accompanied by a marked increase in activity without important changes in intrinsic fluorescence. 1.0 M Gdn/HCl produces perturbations of inter-subunit contacts that lead to fluorescence quenching and loss of catalytic activity, probably as consequence of dimerization of tetrameric LDH.

Inhibition and activation studies on sheep liver sorbitol dehydrogenase

Reversible inhibition and activation, as well as protection against affinity labelling with DL-2-bromo-3-(5-imidazolyl)propionic acid, of sheep liver sorbitol dehydrogenase have been studied. The results presented are discussed in terms of enzyme active-site properties and may have potential applications for drug design. Kinetics with mainly sorbitol competitive inhibitors reveals that aliphatic thiols are generally the most potent inhibitors of enzyme activity. Inhibition and inactivation by heterocyclics parallel that seen previously with sorbitol dehydrogenase from other sources as well as with alcohol dehydrogenase from yeast. However, there are significant differences in relation to the structurally similar horse liver alcohol dehydrogenase, as the catalytic zinc of sorbitol dehydrogenase is more easily removed by chelating molecules. Several aldose reductase inhibitors are shown to also inhibit sorbitol dehydrogenase, but at concentrations unlikely to be reached clinically. Enzyme activation has been observed with various compounds, in particular halo-alcohols and detergents. Several inhibitors provide competitive protection against enzyme inactivation by DL-2-bromo-3-(5-imidazolyl)propionic acid. This enables the dissociation constants for binary enzyme-inhibitor complexes to be determined. NADH protects noncompetitively against inactivation. The presence of some binary and ternary enzyme-NADH complexes is indicated from fluorescence emission spectra, as a shift in the fluorescence maximum and intensity is observed due to their formation.

Are there different water requirements in different steps of a catalytic cycle? Hydration effects at the E1 and E2 conformers of sarcoplasmic reticulum Ca(2+)-ATPase studied in organic solvents with low amounts of water

The Ca(2+)-ATPase from sarcoplasmic reticulum was transferred in an active form to a low-water system composed of toluene, phospholipids, and Triton X-100 (TPT). The Ca(2+)-ATPase activity in the TPT system with 4.0% water (by vol. was about 50% of the activity observed in all-aqueous mixtures. Phosphate formation was linear with time up to 20% of ATP hydrolysis and, as expected from an enzyme-catalysed reaction, activity was linear with protein concentration. No ATPase activity was detected in the presence of 3 mM EGTA, indicating that the enzyme retained its Ca2+ dependence in the TPT system. A hyperbolic response to ATP concentration was observed with a Km of 0.15 mM. There was no detectable ATPase activity at water concentrations below 1.5% (by vol.). With 2.0% water, activity became detectable and increased as the water content was progressively raised to 7.0% (by vol.). Higher amounts of water produced unstable emulsions. Enzyme phosphorylation by ATP and dephosphorylation took place in the TPT system. The velocities of both enzyme phosphorylation and dephosphorylation increased with increments in the water content. The enzyme could also be phosphorylated in the TPT system by inorganic phosphate. However, in comparison to ATP, phosphorylation by phosphate took place with significantly lower amounts of water. It is suggested that at low amounts of water, the enzyme is in a relatively rigid conformation and, as the water content is increased, the ATPase acquires more flexibility and, hence, the capacity to carry out catalysis at higher rates. Nevertheless, the release of conformational constraints of the catalytic site of the E2 conformer takes place at water concentrations much lower than those needed for the expression of catalytic activity by the E1 conformer.

Dimerization and reactivation of triosephosphate isomerase in reverse micelles

The reactivation of the homodimeric enzyme triosephosphate isomerase (TPI) was studied in reverse micelles. The enzyme was denatured in conventional aqueous mixtures with guanidine hydrochloride and transferred to reverse micelles formed with cetyltrimethylammonium bromide, hexanol, n-octane and water. In the transfer step, denatured TPI monomers distributed in single micelles, and guanidine hydrochloride was diluted more than 100 times. Under optimal reactivation conditions, 100% of the enzyme activity could be recovered. The rate of appearance of the catalytic activity increased with the concentration of protein, which indicated that catalysis required the formation of the dimer. The rate of TPI reactivation also increased with increasing protein concentration in the system with denatured TPI covalently derivatized at the catalytic site with the substrate analogue 3-chloroacetol phosphate. Thus, reactivation could take place via the formation of dimers composed of an inactive and an active subunit. Reactivation critically depended on the amount of water in the reverse micelles. The plot of the extent of reactivation versus the amount of water (2.5-7.0%) was markedly sigmoidal. Less than 20% reactivation took place with water concentrations below 3.5%, due to the formation (in less than 30 s) of stable inactive structures. The results indicate that reverse micelles provide a useful system to probe the events involved in the transformation of unfolded monomers to polymeric enzymes.

Activity of heart and muscle lactate dehydrogenases in all-aqueous systems and in organic solvents with low amounts of water. Effect of guanidine chloride

The effect of urea and guanidine hydrochloride (GdmCl) on the activity of lactate dehydrogenases from heart and muscle was studied in standard water mixtures and in reverse micelles formed with n-octane, hexanol, cetyltrimethylammonium bromide and water in a concentration that ranged over 2.5-6.0% (by vol.). In all water mixtures GdmCl (0.15-0.75 M) and urea (0.5-3.0 M) inhibited the activity of the enzymes at non-saturating pyruvate concentrations. At concentrations of pyruvate that proved inhibitory for enzyme activity due to the formation of a ternary enzyme-NAD-pyruvate complex, GdmCl and urea increased the activity of the enzymes. This increase correlated with a decrease of the ternary complex, as evidenced by its absorbance at 320-325 nm. In the low-water system it was found that: (a) at all concentrations of pyruvate tested (0.74-30 mM), GdmCl enhanced the activity of the heart enzyme to a similar extent; (b) in the muscle enzyme, GdmCl inhibited or increased the activity through a process that depended on the concentration of pyruvate and GdmCl; (c) under optimal conditions, the activation by GdmCl was about two times lower in the muscle than in the heart enzyme, although in all-water media the activity of the muscle enzyme was twice as high. The expression of lactate dehydrogenase activity in the low-water system was higher with the heart than with the muscle enzyme compared to their activities in all-water media (about 260 and 600 mumol min-1 mg-1 in the heart and muscle enzymes respectively). Apparently for catalysis, the water requirement in the heart enzyme is lower than in the muscle enzyme. It is likely that the different response of the two enzymes to solvent is due to their distinct structural features.