A theoretical study on the expression of enzymic activity in reverse micelles

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.

Protein-protein interactions in reverse micelles: trypsin shows superactivity towards a protein substrate alpha-chymotrypsinogen A in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in isooctane

Protein-protein interactions in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT), in isooctane containing varying amounts of Tris buffer are studied by using activation of alpha-chymotrypsinogen A by trypsin (EC 3.4.21.4) to alpha-chymotrypsin (EC 3.4.21.1) as a model reaction. It has been found that protein-protein interactions are strongly dependent on the water content of the medium. At an optimum water content the activation reaction in reverse micelles is faster than reaction in water by a factor of 21.3. At lower water content both reaction rates and the conversion of alpha-chymotrypsinogen A into alpha-chymotrypsin decrease with decrease in water content of the reaction medium.

Application of active-phase plot to the kinetic analysis of lipoxygenase in reverse micelles

A new plot for explaining the complex expression of the enzymic activity in reverse micelles has been developed as an extension of the theoretical model described by our group [Bru, Sánchez-Ferrer & García-Carmona (1990) Biochem. J. 268, 679-684]. The plot describes the changes in the relative volume, amount of enzyme (mumoles), enzyme concentration (microM) and substrate concentration (microM) in the phase where the enzyme is active. To illustrate the usefulness of this plot, the complex activity of soya bean lipoxygenase in reverse micelles acting on its interfacial substrate, octadecadienoic acid, was studied. It showed the key parameters ruling the activity profiles of lipoxygenase with respect to micelle size (omega 0), micelle concentration (theta) and the substrate/surfactant molar ratio (rho), which have never been described before.

Substrate effects on the enzymatic activity of alpha-chymotrypsin in reverse micelles

Six different substrates have been used for measuring the activity of alpha-chymotrypsin in reverse micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in isooctane. The substrates were glutaryl-Phe p-nitroanilide, succinyl-Phe p-nitroanilide, acetyl-Phe p-nitroanilide, succinyl-Ala-Ala-Phe p-nitroanilide, succinyl-Ala-Ala-Pro-Phe p-nitroanilide and acetyl-Trp methyl ester. It has been shown that the dependence of the kinetic constants (kcat and Km) on the water content of the system, on wo (= [H2O]/[AOT]), is different for the different substrates. This indicates that activity-wo profiles for alpha-chymotrypsin in reverse micelles not only reflect an intrinsic feature of the enzyme alone. For the p-nitroanilides it was found that the lower kcat (and the higher Km) in aqueous solution, the higher kcat as well as Km in reverse micelles. "Superactivity" of alpha-chymotrypsin could only be found with the ester substrate and with relatively "poor" p-nitroanilides. The presence of a negative charge in the substrate molecule is not a prerequisite for alpha-chymotrypsin to show "superactivity".

Product inhibition of alpha-chymotrypsin in reverse micelles

The alpha-chymotrypsin-catalyzed hydrolysis of succinyl-L-alanyl-L-alanyl- L-prolyl-L-phenylalanyl p-nitroanilide has been studied in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in isooctane. It has been found that alpha-chymotrypsin is strongly inhibited competitively by the acidic peptide product which is formed during the course of the reaction. It has also been shown that the application of the integrated form of the Michaelis-Menten equation can be useful to detect possible inhibition effects and abnormal kinetic behavior of enzymes in reverse micelles. Furthermore, it has been shown that the turnover number (kcat) at low water content is lower than in water and increases as the water content in the system is lower than in water and increases as the water content in the system (wo = [H2O]/[AOT]) increases, kcat reaching the value in water at high wo. If however, initial velocity data, as obtained under conditions where the enzyme is not saturated with substrate, are plotted against wo, the curves are bell-shaped, with a maximum around wo = 15.

Trypsin-SBTI interaction in reverse micelles. A slow intermicellar exchange-dependent binding

Solubilisate exchange between reverse micelles must take place before any reaction inside reverse micelles occurs if the reactants are confined to the aqueous micellar core. When the interacting species are 2 small molecules or one small molecule and one macromolecule, it has been shown that the exchange is faster than the typical turnover of an enzymatic reaction. The study of the interaction between 2 macromolecules (trypsin and soybean trypsin inhibitor) in reverse micelles carried out in this work reveals that the exchange between these macromolecule-containing reverse micelles slows down by a thousand times and the limiting-step in the exchange, the fusion, by 10(6) times. Both reverse micellar size (omega 0 = [water]/[surfactant]) and temperature affected the rate of the fusion process. A hypothesis for the proposed active role of macromolecules in the exchange process is also given.

The effect of substrate partitioning on the kinetics of enzymes acting in reverse micelles

A theoretical model for the expression of enzymic activity in reverse micelles previously developed [Bru. Sánchez-Ferrer & García-Carmona (1989) Biochem. J. 259, 355-361] was extended in the present work. The substrate concentration in each reverse-micelle phase (free water, bound water and surfactant apolar tails) and the organic solvent was expressed as a function of the total substrate concentration, taking into account its partition coefficients, that is, partitioning of the substrate in a multiphasic system. In each phase the enzyme expresses a catalytic constant and a Km. Thus the whole reaction rate is the addition of the particular rates expressed in each domain. This model was compared with that developed for a biphasic system [Levashov, Klyachko, Pantin, Khmelnitski & Martinek (1980) Bioorg. Khim. 6, 929-943] by fitting the experimental results obtained with mushroom tyrosinase (working on both 4-t-butylcatechol and 4-methylcatechol) to the two models. The parameters which characterize reverse micelles, omega 0 (water/surfactant molar ratio) and theta (fraction of water) were investigated. The omega 0 profile was shown to be hyperbolic for both substrates. Activity towards 4-t-butylcatechol decreases as theta increases, this observation being attributable to a dilution of the substrate. A Km of 7.8 M for 4-t-butylcatechol could be calculated on the basis of the biphasic model, whereas it was 13.5 mM when calculating on the basis of our model. A new parameter, rho (= [substrate]/theta), was defined to characterize those substrates that mainly solubilize in the reverse micelle ('micellar substrates').

Fluorescence detection of enzymatically formed hydrogen peroxide in aqueous solution and in reversed micelles

A sensitive enzymatic assay for oxidase reactions both in aqueous solution and in hexadecyltrimethylammoniumbromide (CTAB) reversed micelles has been developed. The assay is based on the fluorescence detection of dichlorofluorescein, which is formed by hydrogen peroxide oxidation of the nonfluorescent precursor dichlorofluorescin. Hydrogen peroxide as product of the reaction catalyzed by glucose oxidase served to select the reaction conditions. The reaction rate is distinctly enhanced in CTAB reversed micelles as compared to the rate in aqueous solution. This effect, combined with the high sensitivity owing to the strong fluorescence of dichlorofluorescein, makes the assay attractive for the detection of low enzyme, substrate, or peroxide concentrations.