BEHAVIOUR OF HORSERADISH PEROXIDASE IN AOT REVERSED MICELLES

Influence of nanoreactor environment and substrate location on the activity of horseradish peroxidase in olive oil based water-in-oil microemulsions

Oxidative enzymatic reactions using horseradish peroxidase (HRP) were carried out in water-in-oil (w/o) microemulsions composed of olive oil/lecithin/1-propanol/water, a model biomimetic system. The substrates used (gallic acid, octyl gallate and 2,2'-azino-bis[3-ethylbenzo-thiazoline-6-sulfonic acid] (ABTS)) have different hydrophobicities and possible locations in the microemulsion system. HRP reactivity with reference to substrate hydrophobicity and structural characteristics of the microemulsions is discussed. The nature of the enzyme microenvironments was examined using dynamic light scattering (DLS), differential scanning calorimetry (DSC) and diffusion NMR (DOSY) methodologies while the location of various enzymatic substrates in the microemulsion phase was assessed by solubility measurements and by taking pressure-area isotherms of mixed monolayers of the substrates with dipalmitoyl-phosphatidylcholine (DPPC), which is a major constituent of lecithin. In contrast to the bulk aqueous phase, in the severely restricted environment of the polar domains of the microemulsion HRP reacted faster with octyl gallate, a substrate that is solubilized at the lipid interfaces. HRP was deactivated in the olive oil microemulsions within a few hours, a phenomenon that has also been observed in other microemulsion systems.

Molecular assembly-assisted biocatalytic reactions in ionic liquids

Room temperature ionic liquids (RTILs), having no measurable vapor pressure, represent an interesting class of tunable designer solvents. Due to their many unique properties, ILs have been used as attractive alternatives to environmentally harmful ordinary organic solvents in a wide range of applications including enzymatic biotransformation. Compared to conventional organic solvents, ILs offer many advantages for biocatalysis such as enhanced conversion rates, high enantioselectivity, better enzyme stability, and improved catalyst recoverability and recyclability. However, biocatalysis in ILs has not yet fully achieved its potential because many biocatalysts are insoluble in most ILs. This limitation could be overcome by the formation of nano/micrometer-sized aqueous microemulsion droplets in an IL continuous phase (referred to as water-in-IL microemulsions) stabilized by a layer of surfactants. Enzymes can be dissolved in such water droplets and protected from the unfavorable effect of ILs by the surfactant layer. In this chapter, a simple and effective method for the development of aqueous microemulsion droplets in a hydrophobic IL comprising an anionic surfactant sodium bis(2-ethyl-1-hexyl) sulfosuccinate (AOT) is presented. For this approach, we have synthesized a hydrophobic IL [C(8)mim][Tf(2)N] (1-octyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl) amide) containing a long pendant hydrocarbon chain to facilitate the dissolution of AOT molecules. A detailed description of the procedure for the potential use of this newly developed water-in-IL reverse microemulsion for biocatalysis is also included.

Imidazolium bromide-based ionic liquid assisted improved activity of trypsin in cationic reverse micelles

The present work reports the imidazolium-based ionic liquids (ILs) assisted enhancement in activity of water-pool solubilized enzyme trypsin in cationic reverse micelles of CTAB. A set of imidazolium ILs (1-alkyl-3-methyl imidazolium bromides) were prepared with varying lengths of their side arm which results in the differential location of these organic salts in the reverse micelles. The different ILs offered varied activating effects on the biocatalyst. The activity of trypsin improved approximately 30-300% in the presence of 0.1-10 mM of different ILs in reverse micelles of CTAB. Trypsin showed approximately 300% (4-fold) increment in its activity in the presence of IL 2 (1-ethyl-3-methyl imidazolium bromide, EMIMBr) compared to that observed in the absence of IL in CTAB reverse micelles. The imidazolium moiety of the IL, resembling the histidine amino acid component of the catalytic triad of hydrolases and its Br(-) counterion, presumably increases the nucleophilicity of water in the vicinity of the enzyme by forming a hydrogen bond that facilitates the enzyme-catalyzed hydrolysis of the ester. However, the ILs with increasing amphiphilic character had little to no effect on the activity of trypsin due to their increased distance from the biocatalyst, as they tend to get localized toward the interfacial region of the aggregates. Dynamic light scattering experimentation was carried out in the presence of ILs to find a possible correlation between the trypsin activity and the size of the aggregates. In concurrence with the observed highest activity in the presence of IL 2, the circular dichroism (CD) spectrum of trypsin in CTAB reverse micelles doped with IL 2 exhibited the lowest mean residue ellipticity (MRE), which is closest to that of the native protein in aqueous buffer.

Superior activity of structurally deprived enzyme-carbon nanotube hybrids in cationic reverse micelles

In the present work, we report the superior activity of hydrophobically adsorbed enzymes onto single-walled carbon nanotubes (SWNTs) in the reverse micelles of cationic surfactants. Horseradish peroxidase and soybean peroxidase adsorbed onto SWNTs endure a notable loss in secondary structure and catalytic activity. This structurally and functionally deformed enzyme-SWNT when confined in CTAB reverse micelles showed approximately 7-9-fold enhancement in activity compared to that was in water and also importantly approximately 1500-3500 times higher activity than that of the enzymes in aqueous-organic biphasic mixtures. The activation observed for this nanobiocomposite is due to the (i) possible localization of enzyme-SWNT hybrid at the micellar interface; (ii) facile transport of substrates across the microscopic interface of reverse micelles; and (iii) greater local concentration of substrates at the augmented interfacial space in the presence of SWNT. This interfacial localization of the SWNT-protein hybrid was tested using FITC-tagged protein (BSA) by fluorescence spectroscopy. FTIR and CD spectroscopy established that the enzyme notably loses its native structure as it gets adsorbed onto the CNTs. However, this loss in the secondary structure is neither aggravated nor recovered when the enzyme-SWNT resides at the reverse micellar interface. So, localization of the surface-active peroxidase-CNT hybrids at the interface is the main reason for significant enzyme activation. The generality of the activation of the enzyme-CNT hybrid by reverse micelles was tested using amphiphiles with varying headgroup sizes, where an overall enhancement in activity was observed with an increase in headgroup size. Activation of this nanobiocomposite would find utmost importance in material science as the activity of structurally deprived enzyme in reverse micelles surpassed (approximately 1.7-fold) even the activity of the native enzyme in water.

Biocatalysis in water-in-ionic liquid microemulsions: a case study with horseradish peroxidase

In this article we report the first results on the enzymatic activity of horseradish peroxidase (HRP) microencapsulated in water-in-ionic liquid (w/IL) microemulsions using pyrogallol as the substrate. Toward this goal, the system used in this study was composed of anionic surfactant AOT (sodium bis(2-ethyl-1-hexyl)sulfosuccinate)/hydrophobic IL [C(8)mim][Tf(2)N] (1-octyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide)/water/1-hexanol. In this system, the catalytic activity of HRP was measured as a function of substrate concentrations, W(0) (molar ratio of water to surfactant), pH, and 1-hexanol content. The curve of the activity-W(0) profile was found to be hyperbolic for the new microemulsion. The apparent Michaelis-Menten kinetic parameters (k(cat) and K(m)) were estimated and compared to those obtained from a conventional microemulsion. Apparently, it was found that HRP-catalyzed oxidation of pyrogallol by hydrogen peroxide in IL microemulsuions is much more effective than in a conventional AOT/water/isooctane microemulsion. The stability of HRP solubilized in the newly developed w/IL microemulsions was examined, and it was found that HRP retained almost 70% of its initial activity after incubation at 28 degrees C for 30 h.

Nonionic surfactants: a key to enhance the enzyme activity at cationic reverse micellar interface

The primary objective of the present study is to understand how the different nonionic surfactants modify the anisotropic interface of cationic water-in-oil (W/O) microemulsions and thus influences the catalytic efficiency of surface-active enzymes. Activity of Chromobacterium viscosum lipase (CV-lipase) was estimated in several mixed reverse micelles prepared from CTAB and four different nonionic surfactants, Brij-30, Brij-92, Tween-20, and Tween-80/water/isooctane/n-hexanol at different z ([cosurfactant]/[surfactants]) values, pH 6 (20 mM phosphate), 25 degrees C across a varying range of W0 ([water]/[surfactants]) using p-nitrophenyl-n-octanoate as the substrate. Lipase activity in mixed reverse micelles improved maximum up to approximately 200% with increasing content of non-ionic surfactants compared to that in CTAB probably due to the reduced positive charge density as well as plummeted n-hexanol (competitive inhibitor of lipase) content at the interfacial region of cationic W/O microemulsions. The highest activity of lipase was observed in CTAB (10 mM) + Brij-30 (40 mM)/isooctane/n-hexanol)/water system, k2 = 913 +/- 5 cm3 g-1 s-1. Interestingly, this observed activity is even higher than that obtained in sodium bis (2-ethyl-1-hexyl) sulfosuccinate (AOT)/n-heptane reverse micelles, the most popular W/O microemulsion in micellar enzymology. To ascertain the influence of non-ionic surfactants in improving the activity of surface-active enzymes is not limited to lipase only, we have also investigated the catalytic activity of Horseradish peroxidase (HRP) in different mixed W/O microemulsions. Here also following the similar trend as observed for lipase, HRP activity enhanced up to 2.5 fold with increasing concentration of nonionic surfactants. Finally, the enzyme activity was correlated with the change in the microenvironment of mixed reverse micelles by steady-state fluorescence study using 8-anilino-1-napthalenesulphonic acid (ANS) as probe.

Tailoring of horseradish peroxidase activity in cationic water-in-oil microemulsions

Horseradish peroxidase (HRP) in cationic water-in-oil (W/O) microemulsions has always been ignored in reverse micellar enzymology, mainly because cationic surfactants are inhibitors of enzyme peroxidase. In the present study, for the first time, we have successfully introduced the cationic W/O microemulsion as an attractive host for efficient HRP activity. To this notion, much improved activity of HRP was observed in the W/O microemulsion of cetyltrimethylammonium bromide (CTAB) with an increase in n-hexanol concentration and W0 ([water]/[surfactant]), presumably due to the increased interfacial area of the microemulsions. In support of our above observation, six surfactants were synthesized with an increased headgroup size where the methyl groups of CTAB were subsequently replaced by the n-propyl and 2-hydroxyethyl groups, respectively, to prepare mono-, di-, and tripropylated/hydroxyethylated n-hexadecylammonium bromide. The peroxidase activity enhanced with headgroup size and also followed an overall trend similar to that found in the case of CTAB. Possibly, the reduced positive charge density at the augmented interfacial area by means of increase, either in headgroup size, cosurfactant concentration, and/or W0, is not capable of inactivating HRP. Also, the larger space at the interface may facilitate easier solubilization of the enzyme and increase the local concentration of enzyme and substrate, leading to the higher activity of HRP. The best activity was obtained with surfactant N-hexadecyl-N,N,N-tripropylammonium bromide, the highest ever found in any cationic W/O microemulsions, being almost 3 times higher than that found in water. Strikingly, this observed highest activity is comparable with that observed in an anionic bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT)-based system, the best W/O microemulsions used for HRP.