Catalysis and thermostability of mitochondrial F1-ATPase in toluene-phospholipid-low-water systems

Preparation and characterization of single-enzyme nanogels

Enzymes have been incorporated in nanostructures in order to provide robust catalysts for valuable reactions, particularly those performed under harsh and denaturing conditions. This chapter describes the encapsulation of enzymes in polyacrylamide nanogels by a two-step in situ polymerization process for preparing robust biocatalysts. The first step in this process is the generation of vinyl groups on the enzyme surface, while the second step involves in situ polymerization using acrylamide as the monomer. Encapsulation of the enzyme in the hydrophilic, porous, and flexible polyacrylamide gel of several nanometers thick would help to both give a significantly enhanced thermostability and prevent the removal of essential water by polar solvents. The hydrophilic flexible polymer shell also allows the protein structure to undergo necessary conformational transitions during the catalytic reaction and, at the same time, impose marginal mass transfer restrictions for the substrates entering across the polymer shell. The effectiveness of this method is demonstrated with horseradish peroxidase (HRP), carbonic anhydrase, and lipase. The impacts of such an encapsulation on the activity and stability of enzymes are also discussed.

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

Kinetic models in reverse micelles

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Solvent selection for whole cell biotransformations in organic media

Although they were used historically as antimicrobial agents, there is a modern requirement to devise organic solvent systems for exploitation in the biotransformation by intact cells of substrates that are poorly soluble in water. Water-immiscible solvents are normally less cytotoxic than are water-miscible ones. While a unitary mechanism is excluded, damage to the membrane remains the likeliest major mechanism of cytotoxicity, and may be conveniently assessed using an electronic biomass probe. Studies designed to account for the mechanisms of action of general anesthetics and of uncouplers parallel those designed to account for the cytotoxicity of organic solvents. Although there are hundreds of potential physical descriptors of solvent properties, many are broadly similar to each other, such that the intrinsic dimensionality of solvent space is relatively small (< 10). This opens up the possibility of providing a rational biophysical basis for the optimization of the solvents used for biotransformations. The widely used descriptor of solvent behavior, log P (the octanol:water partition coefficient), is a composite of more fundamental molecular descriptors; this explains why there are rarely good correlations between cytotoxicity and log P when a wide variety of solvents is studied. Although the intrinsic dimensionality of solvent space is relatively small, pure solvents still populate it rather sparsely. Thus, mixtures of solvents can and do provide the opportunity of obtaining a solvent optimal for a biotransformation of interest.

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.

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.

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

The effect of urea and guanidine hydrochloride (GdmCl) on the activity of heart lactate dehydrogenase, glycerol-3-phosphate dehydrogenase, hexokinase, inorganic pyrophosphatase, and glyceraldehyde-3-phosphate dehydrogenase was studied in low-water systems. Most of the experiments were made in a system formed with toluene, phospholipids, Triton X-100, and water in a range that varied over 1.0-6.5% (by vol.) [Garza-Ramos, G., Darszon, A., Tuena de Gómez-Puyou, M. & Gómez-Puyou, A. (1990) Biochemistry 29, 751-757]. In such conditions at saturating substrate concentrations, the activity of the enzymes was more than 10 times lower than in all-water media. However the activity of the first four aforementioned enzymes was increased between 4 and 20 times by the denaturants. The most marked activating effect was found with lactate dehydrogenase; with 3.8% (by vol.) water maximal activation was observed with 1.5 M GdmCl (about 20-fold); 4 M urea activated, but to a lower extent. Activation by guanidine thiocyanate was lower than with GdmCl. The activating and inactivating effects of GdmCl on lactate dehydrogenase depended on the amount of water; as the amount of water was increased from 2.0% to 6.0% (by vol.), activation and inactivation took place with progressively lower GdmCl concentrations. When activity was measured as a function of the volume of 1.5 M GdmCl solution, a bell-shaped activation curve was observed. In a low-water system formed with n-octane, hexanol, cetyltrimethylammonium bromide and 3.0% water, a similar activation of lactate dehydrogenase by GdmCl and urea was observed. The water solubility diagrams were modified by GdmCl and urea, and this could reflect on enzyme activity. However, from a comparison of denaturant concentrations on the activity of the enzymes studied, it would seem that, independently of their effect on the characteristics of the low-water systems, denaturants bring about activation through their known mechanism of action on the protein. It is suggested that the effect of denaturants is due to the release of constraints in enzyme catalysis imposed by a low-water environment.

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.

Respiratory electron transfer activity in an asolectin-isooctane reverse micellar system

Bovine heart submitochondrial particles (SMP) were solubilized in an asolectin isooctane reverse micellar system and the functionality of the respiratory chain was tested by spectroscopic and amperometric techniques. Electron transfer rate supported by NADH was very slow as evidenced by the low cytochrome reduction levels attained over long incubation periods. In the presence of KCN, NADH caused 34% and 12.5% reduction of the cytochromes aa3 and c, respectively, and negligible reduction of cytochrome b. Supplementation of the system with menadione rose the NADH-dependent reduction of all the cytochromes to levels that were close to the total content. However, no measurable O2 uptake activity took place in the presence of NADH plus menadione, or with ascorbate (or NADH) plus TMPD reducing systems. Therefore, it is suggested that in the organic medium, electron transfer from NADH to O2 is arrested at the terminal oxidase step. Cytochrome oxidase reduced by ascorbate (or NADH) plus TMPD seems to be trapped in its half reduced state (ie, a2+ a3(3+)). Although it is poorly reactive with O2, it can transfer electrons back to cytochrome c and TMPD. The electron transfer block to O2 was overcome when PMS was used instead of TMPD. This seems to be due to the recognized capacity of PMSH2 to carry out simultaneous reduction of both a CuA and a3 CuB redox centers of cytochrome oxidase. The cytochrome oxidase reaction in the organic solvent was highly sensitive to KCN (Ki 1.9 microM) and showed bell-shaped kinetics towards the PMS concentration and a sigmoidal response to water concentration, reaching its maximal turnover number (18 s-1) at 4 mM PMS and 1.1% (v/v) water.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzyme function in organic solvents

Enzyme catalysis in organic solvents is being increasingly used for a variety of applications. Of special interest are the cases in which the medium is predominantly non-aqueous and contains little water. A display of enzyme activity, even in anhydrous solvents (water less than 0.02% by vol.), perhaps reflects that the minimum necessity for water is for forming bonds with polar amino acids on the enzyme surface. The rigidity of enzyme structure at such low water content results in novel substrate specificities, pH memory and the possibility of techniques such as molecular imprinting. Limited data indicates that, while enhanced thermal stability invariably results, the optimum temperature for catalysis may not change. If true in general, this enhanced thermostability would have extremely limited benefits. Medium engineering and biocatalyst engineering are relevant techniques to improve the efficiency and stability of enzymes in such low water systems. Most promising, as part of the latter, is the technique of protein engineering. Finally, this review provides illustrations of applications of such systems in the diverse areas of organic synthesis, analysis and polymer chemistry.

High concentrations of guanidine chloride activate lactate dehydrogenase in low water media

The effect of guanidine chloride on the activity of bovine heart lactate dehydrogenase transferred to a system that was made with toluene, phospholipids, Triton X-100 and 3.8% water (v/v) was studied. The activity of the enzyme in the latter system was about 30 times lower than in standard water mixtures. In the low water system, 1.5 and 2.0 M guanidine chloride increased the activity by approximately 20 times. These concentrations of guanidine chloride caused complete inactivation of the enzyme in conventional water systems. The activating effect of the denaturant was independent of enzyme concentration. It is suggested that the increase in activity produced by guanidine chloride was due to a facilitation of the protein-solvent interactions that operate in a catalytic cycle.

Phospholipid-based reverse micelles

Physicochemical investigations on the aggregation of phospholipids (mainly phosphatidylcholines) in organic solvents are reviewed and compared with the aggregation behaviour of phospholipids in aqueous medium. In particular we review the data showing that phosphatidylcholines (lecithins) form reverse micellar structures in certain apolar solvents. In these systems not only low molecular weight compounds but also catalytically active enzymes and entire cells can be solubilized. In addition, highly viscous phosphatidylcholine gels can be obtained in organic solvents upon solubilizing a critical amount of water. Generally, phospholipid-based reverse micelles can be regarded as thermodynamically stable models for inverted micellar lipid structures possibly occurring in biological membranes.