Microemulsions enzyme structure

Activity and stability are often comparable to values in aqueous media. Many substrates which cannot be made to react in water or in pure organic solvents such as hexane owing to lack of solubility can be brought to reaction in microemulsions. Whereas enzyme structure and mechanism do not seem to change upon transition from water to the microemulsion phase (Bommarius, 1995), partitioning effects often are very important. Besides an enhanced or diminished concentration of substrates in the vidnity of microemulsion droplets and thus of enzyme molecules, the effective pH values in the water pool of the droplets can be shifted in the presence of charged surfactants. Frequently, observed acceleration or deceleration effects on enzyme reactions can be explained with such partitioning effects (Jobe, 1989). 

Besides biocatalytic applications, it is interesting to note that nanostructures within a microemulsion are recognized as models of biological structures that facilitate the investigation of protein/enzyme structure—activity relationship under conditions that mimic biological environments in optically transparent solutions. 

As a result of the micellar environment, enzymes and proteins acquire novel conformational and/or dynamic properties, which has led to an interesting research perspective from both the biophysical and the biotechnological points of view [173-175], From the comparison of some properties of catalase and horseradish peroxidase solubilized in wa-ter/AOT/n-heptane microemulsions with those in an aqueous solution of AOT it was ascertained that the secondary structure of catalase significantly changes in the presence of an aqueous micellar solution of AOT, whereas in AOT/n-heptane reverse micelles it does not change. On the other hand, AOT has no effect on horseradish peroxidase in aqueous solution, whereas slight changes in the secondary structure of horseradish peroxidase in AOT/n-heptane reverse micelles occur [176],... 

For a typical biomolecule-containing w/o-ME system, only a fraction of the w/o-ME population (e.g., 0.1-1%) will contain proteins. Because of their small concentration and the short (microsecond-scale) lifetime of any given w/o-ME droplet, due to the rapid collision and exchange rate for w/o-ME systems, isolation of the protein-containing, or filled w/o-ME populations is difficult to achieve. Various techniques have demonstrated that encapsulated enzymes can alter the structural properties and behavior of the w/o-MEs, and that filled, w/o-MEs may differ in properties from the empty w/o-MEs in a given microemulsion system [46-51]. However, a clear understanding of the structural and dimensional differences between filled and empty w/o-MEs has yet to be achieved. 

Tween 85 is used extensively for RME [84]. Russell and coworkers [234] used Tween 85/isopropanol microemulsions in hexane to solubilize proteins and not only showed >80% solubilization of cytochrome C at optimum conditions, but also proved that Tween 85 does not have a detrimental effect on the structure, function, and stability of subtilisin and cytochrome C. There are other reports available on the extraction and purification of proteins using Tween 85-RMs and also on the stability of protein activity in these systems [234]. It has also been shown that Tween 85-RMs can solubilize larger amounts of protein and water than AOT. Tween 85 has an HLB of 11, which indicates that it is soluble in organic solvents. In addition, it is biodegradable and can be successfully used as an additive in fertihzers [235,236]. Pfammatter et al. [35] have demonstrated that RMs made of Tween 85 and Span 80 can be successfully used for the solubilization and growth of whole cells. Recently, Hossain et al. [84] showed an enhanced enzymatic activity of Chromobacterium viscosum Hpase in AOT/Tween 85 mixed reverse micellar systems when compared to that in classical AOT-RMs. This is due to the modification of the interface in AOT-RMs caused by the co-adsorption of Tween 85, and increased availability of the oHve oil molecules (substrate) to the enzyme. 

Microemulsions with different structures, like micelles, reverse micelles or bicontinuous networks, can be used for several inorganic, organic [72] or catalytic reactions which require a large contact area between oil and water. Besides enzyme catalysis, this can be the formation of nanoparticles [54, 73, 74], hydro-formylation reactions [75] or polymerisations [76-78]. 

Enzyme activity is highly dependent on the composition, and as a consequence on different microstructures of the microemulsion. Up to the present, no suitable theory exists to explain the correlation between the reaction media properties and their effects on enzymatic reactions in microemulsions. All experimental results on enzymatic reactions show that the activity is greatly affected by the structure of the microemulsion. 

The kinetics of an enzyme catalysed reactions in a w/o-microemulsions is dependent on several parameters. For example, the substrates and enzymes distribute within the different parts of a one-phase microemulsion with different concentrations. The enzymes are located in the water and hydrophobic substrates are mainly dissolved in the oil. Additionally, the choice of oil and surfactant, the water concentration, and the structure of the interfacial layer can influence the activity and stability of biocatalysts. The influences of the main parameters on the kinetics will be discussed in this chapter. 

If the objective is to keep the enzyme active and stable in an aqueous phase but otherwise to use as much organic phase as possible, microemulsions are an option as a reaction medium. In contrast to ordinary emulsions they are thermodynamically stable and, at a particle diameter of 1-20 nm, accommodate most often only one enzyme molecule (Figure 12.5). The microemulsion droplets communicate rapidly and exchange their contents through elastic collisions. The boundary between microemulsions and reversed micelles is not clearly delineated, and the two notions are often used interchangeably. Enzyme of almost all classes and structures have been solubilized in microemulsion systems and used for reactions (Shield, 1986). 

The structure and dynamics of inverse (water in oil) micellar solutions and microemulsions are of interest because of the unique properties of the water core, the view that such micelles may serve as models of enzyme active sites, and the potential use of inverse micelles as hosts for enzymatic reactions (80-82). 

This may be quite important in processes in which the ionic strength is determinant such as sol-gel transitions or chemical reactions in microemulsions [137-141]. Double-tailed surfactants such as dioctyl sulfosuccinate or diallQ lmethylammonium salts are likely to produce either vesicles (with excess water) or inverse W/O microemulsions with a polar core [142,143] that is used as a nanoreactor for a score of processes such as esterification or hydrolysis [144] in which enzymes are immobilized in an organogel [145]. Organogels can be made so that their structure depends on the composition of the microemulsion [146-148]. 

The fluorescence emission spectrum of ANS depends strongly on the polarity of the environment, with a red shift of the emission maximum in more polar media. Therefore this probe has been used to obtain information on the shell-like structuring in the interior of microemulsion water droplets [73]. Fluorescence spectroscopy can often be used to obtain information on the location of additives in microemulsions [70]. A change in the emission intensity or in the wavelength of the emission maximum indicates interactions of the probe with the interfacial layer, either direct or indirect. From a comparative analysis of the fluorescence spectra of labeled enzyme it was concluded that hydrophobically modified probes were forced toward the interface [77]. 

Candida rugosa and Pseudomonas sp. are considerably different in reverse micelles from those in aqueous solution, indicating that both enzymes lose their native structure in the microemulsion environment [51]. 

NMR self-diffusion measurements indicated that all microemulsions consisted of closed water droplets and that the structure did not change much during the course of reaction. Hydrolysis was fast in microemulsions based on branched-chain anionic and nonionic surfactants but very slow when a branched cationic or a linear nonionic surfactant was employed (Fig. 11). The cationic surfactant was found to form aggregates with the enzyme. No such interactions were detected with the other surfactants. The straight-chain, but not the branched-chain, alcohol ethoxylate was a substrate for the enzyme. A slow rate of triglyceride hydrolysis for a Ci2E4-based microemulsion compared with formulations based on the anionic surfactant AOT [61,63] and the cationic surfactant cetyltrimethylammonium bromide (CTAB) [63] was observed in other cases also. Evidently, this type of lipase-catalyzed reaction should preferably be performed in a microemulsion based on an anionic or branched nonionic surfactant. Nonlipolytic enzymes such as cholesterol oxidase seem to function well in microemulsions based on straight-chain nonionic surfactants, however [64]. CTAB was reported to cause slow inactivation of different types of enzymes [62,64,65] and also, in the case of Chromobacterium viscosum lipase [66], to provide excellent stability. 

HLADH was used as redox catalyst in a coupled substrate-coenzyme regenerating cycle, and the enzymatic activity was studied as a function ofoil fraction of the microemulsion [118]. The oil fraction was varied while the surfactant (AOT) concentration was kept constant, leading to a change in the microstructure of the solution from an O/W to a W/O microemulsion via a bicontinuous structure, as determined by self-diffusion NMR. The enzyme exhibited good stability in the various types of structures. Variation of the initial reaction rate could be described by modifying the rate equation, valid in pure buffer, taking into account partitioning of the substrate between the water and hydrocarbon domains. 

Microemulsions have larger structures than micelles and a large oil drop centre for binding hydrophobic molecules. However, the photoionization behaviour of molecules solubilized in microemulsion aggregates is similar to that in micelles. The kinetics of enzyme-catalysed reactions solubilized in water pools in organic solvents by surfactants have been analysed. ... 

In recent years, W/O microemulsions have found numerous applications as microreactors for specific reactions (for comprehensive reviews, see Refs. 94 and 95). Thus, it has been shown that hydrophilic enzymes can be solubilized without loss of enzymatic activity and used to catalyze various chemical and photochemical reactions [96,97]. Other interesting applications involve the polymerization of solubilizates in microemulsions [98] and the preparation of micro-porous polymeric materials by polymerization of single-phase microemulsions [99]. Furthermore, microemulsions have been used as microreactors for the synthesis of nanosized particles for various applications [93,95] such as metal clusters (Pt, Pd, Rh, Au) for catalysis [100,101], semiconductor clusters [102-104] (ZnS, CdS, etc.), silver halides [105], calcium carbonates, and calcium fiuoride [106]. Recently it was shown [107,108] that it is possible to use W/O microemulsions for the control of polymorphism of water-soluble organic compounds. In most of these appUcations, one or more reactants are solubilized within a microemulsion and then a reaction is initiated. Depending on its molecular structure. 

Enzyme Kinetics as a Useful Probe for Micelle and Microemulsion Structure and Dynamics... 

On the other hand, once an enzyme and its affinity to the interface are well characterized, this enzyme can be a very sensitive probe for tiny changes in the composition of microemnlsion. There are not many other techniques that have such a high sensitivity. Thermodynamic experiments such as calorimetry are also very sensitive, bnt they do not give a detailed insight into structures. By contrast, scattering techniques and NMR yield more detailed pictures, but they are less sensitive. Therefore, enzyme reactions can be a useful complement for the investigation of microemulsion structures. This is the topic of this chapter. 

Horse liver alcohol dehydrogenase (HLAD) is a cheap and often used enzyme to study microemulsion systems [3]. Its structure is known in detail. It converts alcohols to aldehydes according to the following reaction scheme ... 

Despite this donble complexity, it is possible to use enzymes in surfactant solutions to get a deeper insight in their structure and dynamics. In this chapter, we showed that enzymes can yield useful information on surfactant hydration, interfacial film rigidity, and partitioning of cosurfactants. Enzymes are also useful as an independent check for pH in reverse microemulsions. We could also show that for some of the properties of microemulsions such as cosurfactant partition coefficients, semi-qnantitative results can be obtained. 

Reverse micelles formed in water-in-oil (w/o) microemulsions, structurally inverse analogues to normal micelles, are capable of hosting proteins/enzymes in their so-called water pool. The biomolecule can be entrapped in the water pools, avoiding direct contact with the organic solvent, potentially limiting their... 

In 1988, Walde and coworkers studied the kinetic and structural properties of another serine protease, namely trypsin, in two reverse micellar systems, AOT/ isooctane and CTAB/chloroform/isooctane, employing three different model substrates, an amide and two esters [71], The main aim of this work was to compare the behavior of trypsin in reverse micelles with that of a-chymotrypsin. In the case of trypsin, superactivity was not observed and in general no obvious similarities between the two enzymes were recorded. Some years later, reverse micelles formulated with biocompatible surfactants such as lecithin of variable chain lengths in isooctane/alcohol were studied in relation to their capacity to solubilize a-chymotrypsin and trypsin [72]. The hydrolytic behavior of the same serine proteases, namely a-chymotrypsin and trypsin, in both AOT and CTAB microemulsions was studied and related to the polarity of the reaction medium as expressed by the logP value and measured by the hydrophilic probe 1-methyl-8-oxyquinolinium betaine [39], In this study a remarkable superactivity of trypsin in reverse micelles formed with the cationic surfactant CTAB was reported. 

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