Catalytic activity of thermolysin under extremes of pressure and temperature: modulation by metal ions.

The catalytic activity of thermolysin (TL), a Zn-dependent neutral protease from Bacillus thermoproteolyticus, has been studied over a wide interval of pressures (1 bar to 4 kbar) and temperatures (20 degreesC to 80 degreesC) by monitoring hydrolysis of a low-molecular-mass substrate, 3-(2-furylacryloyl)-glycyl-L-leucine amide. This reaction shows a very large negative value for the activation volume and, because of that, simultaneous increase in temperature and pressure leads to a significant (up to 40-fold) acceleration of the reaction. At pressures higher than 2-2.5 kbar, the reaction rate starts to decrease due to disactivation of TL. This disactivation is explained in part by pressure-promoted dissociation of zinc ion from the active site and can be inhibited by adding exogenous zinc. Thus, this thermostable protease does not specifically show a higher stability at high pressure in comparison with small mesophilic proteases.

Testing for diffusion limitations in salt-activated enzyme catalysts operating in organic solvents.

The dramatic activation of serine proteases in nonaqueous media resulting from lyophilization in the presence of KCl is shown to be unrelated to relaxation of potential substrate diffusional limitations. Specifically, lyophilizing subtilisin Carlsberg in the presence of KCl and phosphate buffer in different proportions, ranging from 99% (w/w) enzyme to 1% (w/w) enzyme in the final lyophilized solids, resulted in biocatalyst preparations that were not influenced by substrate diffusion. This result was made evident through use of a classical analysis whereby initial catalytic rates, normalized per weight of total enzyme in the catalyst material, were measured as a function of active enzyme for biocatalyst preparations containing different ratios of active to inactive enzyme. The active enzyme content of a given biocatalyst preparation was controlled by mixing native subtilisin with subtilisin preinactivated with PMSF, a serine protease inhibitor, and lyophilizing the enzyme mixture in the presence of different fractions of KCl and phosphate buffer. Plots of initial reaction rates as a function of percent active subtilisin in the biocatalyst were linear for all biocatalyst preparations. Thus, enzyme activation (reported elsewhere to be as high as 3750-fold in hexane for the transesterification of N-Ac-L-Phe-OEt with n-PrOH) is a manifestation of intrinsic enzyme activation and not relaxation of diffusional limitations resulting from diluted enzyme preparations. Similar activation is reported herein for thermolysin, a nonserine protease, thereby demonstrating that enzyme activation due to lyophilization in the presence of KCl may be a general phenomenon for proteolytic enzymes.

Enzyme-polyelectrolyte complexes in water-ethanol mixtures: negatively charged groups artificially introduced into alpha-chymotrypsin provide additional activation and stabilization effects.

Formation of noncovalent complexes between alpha-chymotrypsin (CT) and a polyelectrolyte, polybrene (PB), has been shown to produce two major effects on enzymatic reactions in binary mixtures of polar organic cosolvents with water. (i) At moderate concentrations of organic cosolvents (10% to 30% v/v), enzymatic activity of CT is higher than in aqueous solutions, and this activation effect is more significant for CT in complex with PB (5- to 7-fold) than for free enzyme (1.5- to 2.5-fold). (ii) The range of cosolvent concentrations that the enzyme tolerates without complete loss of catalytic activity is much broader. For enhancement of enzyme stability in the complex with the polycation, the number of negatively charged groups in the protein has been artificially increased by using chemical modification with pyromellitic and succinic anhydrides. Additional activation effect at moderate concentrations of ethanol and enhanced resistance of the enzyme toward inactivation at high concentrations of the organic solvent have been observed for the modified preparations of CT in the complex with PB as compared with an analogous complex of the native enzyme. Structural changes behind alterations in enzyme activity in water-ethanol mixtures have been studied by the method of circular dichroism (CD). Protein conformation of all CT preparations has not changed significantly up to 30% v/v of ethanol where activation effects in enzymatic catalysis were most pronounced. At higher concentrations of ethanol, structural changes in the protein have been observed for different forms of CT that were well correlated with a decrease in enzymatic activity. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 267-277, 1997.

Application of high hydrostatic pressure for increasing activity and stability of enzymes.

Elevated hydrostatic pressure has been used to increase catalytic activity and thermal stability of alpha-chymotrypsin (CT). For an anilide substrate, characterized by a negative value of the reaction activation volume (DeltaV( not equal)), an increase in pressure at 20 degrees C results in an exponential acceleration of the hydrolysis rate catalyzed by CT reaching a 6.5-fold increase in activity at 4700 atm (4.7 kbar). Due to a strong temperature dependence of DeltaV( not equal), the acceleration effect of high pressure becomes more pronounced at high temperatures. For example, at 50 degrees C, under a pressure of 3.6 kbar, CT shows activity which is more than 30 times higher than the activity at normal conditions (20 degrees C, 1 atm). At pressures of higher than 3.6 kbar, the enzymatic activity is decreased due to a pressure-induced denaturation.Elevated hydrostatic pressure is also efficient for increasing stability of CT against thermal denaturation. For example, at 55 degrees C, CT is almost instantaneously inactivated at atmospheric pressure, whereas under a pressure of 1.8 kbar CT retains its anilide-hydrolyzing activity during several dozen minutes. Additional stabilization can be achieved in the presence of glycerol, which is most effective for protection of CT at an intermediate concentration of 40% (v/v). There has been observed an additivity in stabilization effects of high pressure and glycerol: thermal inactivation of pressure-stabilized CT can be decelerated in a supplementary manner by addition of 40% (v/v) glycerol. The protection effect of glycerol on the catalytic activity and stability of CT becomes especially pronounced when both extreme factors of temperature and pressure reach critical values. For example, at approximately 55 degrees C and 4.7 kbar, enzymatic activity of CT in the presence of 40% (v/v) glycerol is severalfold higher than in aqueous buffer.The results of this study are discussed in terms of the hypotheses which explain the action of external and medium effects on protein structure, such as preferential hydration and osmotic pressure.

High pressure effects on protein structure and function.

Many biochemists would regard pressure as a physical parameter mainly of theoretical interest and of rather limited value in experimental biochemistry. The goal of this overview is to show that pressure is a powerful tool for the study of proteins and modulation of enzymatic activity.

Kinetics of butyrylcholinesterase in reversed micelles under high pressure.

The combined effects of high pressure and reversed micelles have been studied to modulate the catalytic behaviour of butyrylcholinesterase. The purpose of this study was to determine whether the conformational plasticity of the enzyme is altered by entrapment in reversed micelles. The presence of soman, an irreversible inhibitor of cholinesterase was used to bring to the fore a possible modification of the enzyme behaviour in this system under pressure. Results show differences between enzyme in conventional medium and in reversed micelles regarding the mechanism of BuChE catalyzed hydrolysis of acetylthiocholine. In both systems, the enzyme displays a non-Michaelian behaviour with this substrate. In conventional medium the kinetics is multiphasic with an activation phase followed by an inhibition phase at high concentration. In reversed micelles there is inhibition by excess substrate but the activation phase is missing. This behaviour may be the result of a change of the enzyme conformational plasticity when is entrapped in reversed micelles.

High-pressure stabilization of alpha-chymotrypsin entrapped in reversed micelles of aerosol OT in octane against thermal inactivation.

alpha-Chymotrypsin (CT) solubilized in reversed micelles of sodium bis-(2-ethylhexyl)-sulfosuccinate (AOT) undergoes thermal inactivation and the enzyme stability decreases significantly when temperature increases (25-40 degrees C). The half-life of CT in micelles shows a bell-shaped dependence on the degree of hydration of AOT (wo) analogous to the previously obtained dependence on wo for the enzyme activity. The optima of catalytic activity and thermal stability have been observed under conditions where the diameter of the inner aqueous cavity of the micelle is close to the size of the enzyme molecule (wo = 10). Application of high hydrostatic pressure in the range of 1-1500 atm (bar) stabilizes CT against thermal inactivation at all hydration degrees (wo) from 7 to 20; the stabilization effect is most pronounced under the experimental conditions being far from the optimum for catalytic activity.

Pressure effects on enzyme reactions in mainly organic media: alpha-chymotrypsin in reversed micelles of Aerosol OT in octane.

Biocatalytic transformations in reversed micelles formed by anionic surfactant Aerosol OT in octane have been studied at high pressures by an example of alpha-chymotrypsin-catalyzed hydrolysis of N-carbobenzoxy-L-tyrosine p-nitrophenyl ester and N-succinyl-L-phenylalanine p-nitroanilide. For the first time it has been found that the enzyme retains high activity in these water-in-oil microemulsions up to a pressure of 2 kbar. The value of the activation volume (delta V*) for the enzyme reactions shows a dependence on the water content in the system. When the size of the micellar aqueous inner cavity (as evaluated at 1 atm) approaches the molecular size of alpha-chymotrypsin, delta V* becomes significantly different from the value in aqueous solution and in the micelles with a larger size. Possibilities of regulating the enzyme activity by pressure in systems with a low content of water are discussed.

[Native, modified, and immobilized chymotrypsin in chaotropic media. Stabilization limits]. / Nativnyi, modifitsirovannyi i immobilizovannyi khimotripsin v khaotropnykh sredakh. Predely stabilizatsii.

To stabilize alpha-chymotrypsin against irreversible thermal inactivation at high temperatures, methods of covalent modification and multi-point immobilization in combination with the addition of salting-in compounds were used. The upper limit of the protein stability proved to be the same for a combination of the modification and salting-in media and for each of these methods separately. The limit of stabilization reached by means of covalent immobilization is higher than the limit of stabilization reached by two other methods. The greatest stabilization of immobilized alpha-chymotrypsin by the salting-in media (a 10000 fold increase in the native enzyme's stability level) takes place only in the case of the protein with the minimum number of bonds with the support. Stabilization of the enzyme by these methods is explained in terms of the suppression of the conformational inactivation processes.

[The effect of hydrophobic surface properties of protein on its resistance to denaturation by organic solvents (using modified alpha-chymotrypsin as an example]. / Vliianie gidrofobnykh svoistv poverkhnosti belka na ego ustoichivost' k denaturatsii organicheskimi rastvoriteliami (na primere modifitsirovannogo alpha-khimotripsina).

Catalytic activity of covalently modified alpha-chymotrypsin in water/cosolvent solutions was investigated. The stability of chymotrypsin increases upon modification with hydrophilic reagents, such as glyceraldehyde, pyrometallic and succinic anhydrides, and glucosamine. Correlation was observed between the protein's stability in organic solvents and the degree of hydrophilization of the protein's surface. The protein is the more stable, the higher are the modification degree and the hydrophilicity of the modifying residue. At a certain critical hydrophilization degree of chymotrypsin a limit of stability is achieved. The stabilization effect can be accounted for by the fact that the interaction between water molecules on the surface and protein's functional groups become stronger in the hydrophilized protein.

Reversible conformational transition gives rise to 'zig-zag' temperature dependence of the rate constant of irreversible thermoinactivation of enzymes.

We have obtained unusual 'zig-zag' temperature dependencies of the rate constant of irreversible thermoinactivation (k(in)) of enzymes (alpha-chymotrypsin, covalently modified alpha-chymotrypsin, and ribonuclease) in a plot of log k(in) versus reciprocal temperature (Arrhenius plot). These dependencies are characterized by the presence of both ascending and descending linear portions which have positive and negative values of the effective activation energy (Ea), respectively. A kinetic scheme has been suggested that fits best for a description of these zig-zag dependencies. A key element of this scheme is the temperature-dependent reversible conformational transition of enzyme from the 'low-temperature' native state to a 'high-temperature' denatured form; the latter form is significantly more stable against irreversible thermoinactivation than the native enzyme. A possible explanation for a difference in thermal stabilities is that low-temperature and high-temperature forms are inactivated according to different mechanisms. Existence of the suggested conformational transition was proved by the methods of fluorescence spectroscopy and differential scanning calorimetry. The values of delta H and delta S for this transition, determined from calorimetric experiments, are highly positive; this fact underlies a conclusion that this heat-induced transition is caused by an unfolding of the protein molecule. Surprisingly, in the unfolded high-temperature conformation, alpha-chymotrypsin has a pronounced proteolytic activity, although this activity is much smaller than that of the native enzyme.

Correlation of high-temperature stability of alpha-chymotrypsin with 'salting-in' properties of solution.

A correlation between the stability of alpha-chymotrypsin against irreversible thermal inactivation at high temperatures (long-term stability) and the coefficient of Setchenov equation as a measure of salting-in/out efficiency of solutes in the Hofmeister series has been found. An increase in the concentration of salting-in solutes (KSCN, urea, guanidinium chloride, formamide) leads to a many-fold decrease of the inactivation rate of the enzyme. In contrast, addition of salting-out solutes has a small effect on the long-term stability of alpha-chymotrypsin at high temperatures. The effects of solutes are additive with respect to their salting-in/out capacities; the stabilizing action of the solutes is determined by the calculated Setchenov coefficient of solution. The correlation is explained by a solute-driven shift of the conformational equilibrium between the 'low-temperature' native and the 'high-temperature' denatured forms of the enzyme within the range of the kinetic scheme put forward in the preceding paper in this journal: irreversible inactivation of the high-temperature form proceeds much more slowly compared with the low-temperature form.

Mechanism-based strategies for protein thermostabilization.

Strategies for stabilizing enzymes can be derived from a two-step model of irreversible inactivation that involves preliminary reversible unfolding, followed by an irreversible step. Reversible unfolding is best prevented by covalent immobilization, whereas methods such as covalent modification of amino acid residues or 'medium engineering' (by the addition of low-molecular-weight compounds) are effective against irreversible 'incorrect' refolding. Genetic modification of the protein sequence is the most effective approach for preventing chemical deterioration.

[Correlation between the stability of alpha-chymotrypsin at high temperatures and "salting in" of a strong solution]. / Korreliatsiia mezhdu stabil'nost'iu alpha-khimotripsina pri vysokikh temperaturakh i "vsalivaiushchei" siloi rastvora.

A correlation was found between the thermal stability of alpha-chymotrypsin and the coefficient Ks of the Sechenov equation as a quantitative measure of the "salting-in" or "salting-out" capacity of solutes. At high temperatures, an increase in the concentration of "salting-in" agents (KSNC, GuHCl, urea, formamide) resulted in thermal stabilization of alpha-chymotrypsin. The maximal (about 100-fold) stabilizing effect in concentrated solutions of salting-in agents was comparable with those induced by covalent modification with hydrophilic reagents or immobilization. Conversely, an increase in the concentration of "salting-out" agents stabilized the enzyme only marginally at high temperatures. An additivity of solutes' action on the thermal stability of the protein has been demonstrated. The observed correlation was explained in terms of the solutes' action on the reversible conformational transition of the enzyme native form into a much more stable form existing at high temperatures.

High stability to irreversible inactivation at elevated temperatures of enzymes covalently modified by hydrophilic reagents: alpha-Chymotrypsin.

Based on the idea that proteins can be stabilized by a decrease in the thermodynamically unfavorable contact of the hydrophobic surface clusters with water, alpha-chymotrypsin (CT) was acylated with carboxylic acid anhydrides or reductively alkylated with aliphatic aldehydes. Modification of CT with hydrophilic reagents leads to 100-1000-fold increase in stability against the irreversible thermoinactivation. The correlation holds: the greater the hydrophilization increment brought about by the modification, the higher is the protein thermostability. After some limiting value, however, a further increase in hydrophilicity does not change thermostability.It follows from the dependence of the thermoinactivation rate constants on temperature that for hydrophilized CT there is the conformational transition at 55-65 degrees C into an unfolded state in which inactivation is much slower than that of the low-temperature conformation. The thermodynamic analysis and fluorescent spectral data confirm that the slow inactivation of hydrophilized CT at high temperatures proceeds via a chemical mechanism rather than Incorrect refolding operative for both the native and low-temperature form of the modified enzyme. Hence, the hydrophilization stabilizes the unfolded high-temperature conformation by eliminating the incorrect refolding.

Homogeneous solutions of hydrophilic enzymes in nonpolar organic solvents. New systems for fundamental studies and biocatalytic transformations.

A typical hydrophilic enzyme, CT, can be dissolved in nonpolar organic solvents (n-octane, cyclohexane and toluene) up to microM concentrations. In the homogeneous solution obtained, the enzyme possesses catalytic activity and enormously high thermostability. It does not lose this activity even after several hours refluxing in octane (126 degrees C) or cyclohexane (81 degrees C).

[Interconnection of physico-chemical characteristics of organic solvents and their denaturing ability in relation to proteins]. / Vzaimosviaz' fiziko-khimicheskikh kharakteristik organicheskikh rastvoritelei s ikh denaturiruiushchei sposobnost'iu po otnosheniiu k belkam.

Reversible denaturation of several proteins (alpha-chymotrypsin, trypsin, laccase, chymotrypsinogen, cytochrome c and myoglobin) by a broad series of organic solvents of different nature was studied. The regularities of this process were analyzed, employing both experimental and literary data based on the results of kinetic and spectroscopic measurements. In all the systems under study denaturation proceeded in a threshold manner, i. e., an abrupt change in the catalytic and/or spectroscopic properties of the dissolved proteins was observed after a certain threshold concentration of the organic solvent had been reached. To account for the observed features of the denaturation process, a thermodynamic model of reversible protein denaturation by organic solvents was proposed. This model is based on the widely accepted viewpoint that the undisturbed water shell around the protein globule is necessary for maintaining the dissolved protein in the native state. Quantitative analysis of the model led to an equation establishing a relationship between the threshold concentration of an organic solvent and its physico-chemical characteristics, such as hydrophobicity, solvating ability and molecular geometry. This equation fits well in the experimental data for all the proteins tested. Based on the above thermodynamic model of protein denaturation, a novel quantitative parameter characterizing the denaturing strength of organic solvents (termed as the denaturation capacity or DC) was proposed. Different organic solvents arranged according to their DC values form the DC scale of organic solvents which permits to predict theoretically the threshold concentration of any organic solvent for a given protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between surface hydrophilicity of a protein and its stability against denaturation by organic solvents.

The stability of alpha-chymotrypsin covalently modified with a strongly hydrophilic modifier, pyromellitic dianhydride, against solvent-induced denaturation in water-organic solvent binary mixtures has been studied. It was found that the hydrophilization results in a strong stabilization of the enzyme against denaturation by organic solvents. The stabilizing effect is explained in terms of the enhanced ability of the hydrophilized enzyme to keep its hydration shell, which is indispensable for supporting the native protein conformation, from denaturing stripping by organic solvents.

Denaturation capacity: a new quantitative criterion for selection of organic solvents as reaction media in biocatalysis.

The process of reversible denaturation of several proteins (alpha-chymotrypsin, trypsin, laccase, chymotrypsinogen, cytochrome c and myoglobin) by a broad series of organic solvents of different nature was investigated using both our own and literature data, based on the results of kinetic and spectroscopic measurements. In all systems studied, the denaturation proceeded in a threshold manner, i.e. an abrupt change in catalytic and/or spectroscopic properties of dissolved proteins was observed after a certain threshold concentration of the organic solvent had been reached. To account for the observed features of the denaturation process, a thermodynamic model of the reversible protein denaturation by organic solvents was developed, based on the widely accepted notion that an undisturbed water shell around the protein globule is a prerequisite for the retention of the native state of the protein. The quantitative treatment led to the equation relating the threshold concentration of the organic solvent with its physicochemical characteristics, such as hydrophobicity, solvating ability and molecular geometry. This equation described well the experimental data for all proteins tested. Based on the thermodynamic model of protein denaturation, a novel quantitative parameter characterizing the denaturing strength of organic solvents, called the denaturation capacity (DC), was suggested. Different organic solvents, arranged according to their DC values, form the DC scale of organic solvents which permits theoretical prediction of the threshold concentration of any organic solvent for a given protein. The validity of the DC scale for this kind of prediction was verified for all proteins tested and a large number of organic solvents. The experimental data for a few organic solvents, such as formamide and N-methylformamide, did not comply with equations describing the denaturation model. Such solvents form the group of so-called 'bad' solvents; reasons for the occurrence of 'bad' solvents are not yet clear. The DC scale was further extended to include also highly nonpolar solvents, in order to explain the well-known ability of enzymes to retain catalytic activity and stability in biphasic systems of the type water/water-immiscible organic solvent. It was quantitatively demonstrated that this ability is accounted for by the simple fact that nonpolar solvents are not sufficiently soluble in water to reach the inactivation threshold concentration.