Enzyme catalysis micelles

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

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]. 

The surfactant mass fraction in a microemulsion defines the size of the interfacial area between the water and oil. The reaction rate of organic reactions in microemulsions can be dramatically enhanced by increasing the specific interfacial area [95]. Enzyme catalysis in microemulsions is usually not influenced by the size of the interfacial area because only a small fraction of the reverse micelles are hosting a bio-molecule. Most investigations published so far were made with low enzyme concentrations resulting in a low population of enzymes per reverse micelle. 

Enzymes and micelles resemble each other with respect to both structure (e.g., globular proteins and spherical aggregates) and catalytic activity. Probably the most common form of enzyme catalysis follows the mechanism known in biochemistry as Michaelis-Menton kinetics. In this the rate of the reaction increases with increasing substrate concentration, eventually leveling off. According to this mechanism, enzyme E and substrate A first react reversibly to form a complex EA, which then dissociates to form product P and regenerate the enzyme ... 

The solubilization phenomenon, which refers to the dissolution of normally insoluble or only slightly soluble compounds in water caused by the addition of surfactants, is one of the most striking effects encountered for surfactant systems. Solubilization is of considerable physico-chemical interst, such as in discussion of the structure and dynamics of micelles and of the mechanism of enzyme catalysis, and has numerous practical applications, such as in detergency, in pharmaceutical preparations and in micellar catalysis. In biology, solubilization phenomena are most significant, e.g., cholesterol solubilization in phospholipid bilayers and fat solubilization in fat digestion and transport. 

Reactions in micelles and in the double layers of vesicles are analogous to natural membrane systems and in special cases are comparable with the function of an enzyme (cf. Section 3.2.1). In contrast to macromolecular enzymes, amphiphile aggregates with linked catalytic centers have a less rigid supramolecular structure. The preorganization of an enzyme is highly selective and as a consequence enzyme catalysis is much more effective than micellar catalysis, a type of artifical mimic [6]. 

Any survey of the literature dealing with studies of the catalytic properties of solutions containing polymers or micelles will quickly show that the most powerful motivation for such work was provided by the hope that such systems would exhibit some analogy to the behavior of enzymes. It seems, therefore, appropriate to touch on a few characteristics of enzymic catalysis before considering the subject of this review. 

Since a number of the studies we shall review were concerned with the effect of synthetic chain molecules or micelles on the hydrolysis rate of nitrophenyl acetate and similar esters, it will be useful to consider briefly some characteristics of the enzymic catalysis of this process. A particularly detailed study has been carried out on the enzyme chymo-trypsin (14) and a great deal of evidence shows that the catalytic site of this enzyme contains a serine residue with an unusually reactive hydroxyl group. Denoting the chymotrypsin by Ch—OH, the interaction with the ester involves first acyl transfer to the enzyme and this is followed by acyl enzyme hydrolysis to regenerate Ch—OH ... 

The study of catalytic and inhibitory effects in solutions of flexible chain polymers and micelles is of sufficient intrinsic interest, so that no special justification should be required for investigations of this tyj)e. Nevertheless, many of the workers active in this field insist on emphasizing the utility of such systems as enzyme models and we should, therefore, try to answer two crucial questions. What has been learned so far from these studies about the nature of enzymic catalysis What is the probability that studies of this type will contribute to the clarification of the enzyme problem in the future ... 

Surfactant aggregates (microemulsions, micelles, monolayers, vesicles, and liquid crystals) are recently the subject of extensive basic and applied research, because of their inherently interesting chemistry, as well as their diverse technical applications in such fields as petroleum, agriculture, pharmaceuticals, and detergents. Some of the important systems which these aggregates may model are enzyme catalysis, membrane transport, and drug delivery. More practical uses for them are enhanced tertiary oil recovery, emulsion polymerization, and solubilization and detoxification of pesticides and other toxic organic chemicals. 

The quantitative treatment of kinetic data is based on the pseudophase separation approach, i.e. the assumption that the aggregate constitutes a (pseudo)phase separated from the bulk solution where it is dispersed. Some of the equations below are reminiscent of the well-known Michaelis- Menten equation of enzyme kinetics [101]. This formal similarity has led many authors to draw a parallel between micelle and enzyme catalysis. However, the analogy is limited because most enzymatic reactions are studied with the substrate in a large excess over the enzyme. Even for systems showing a real catalytic behavior of micelles and/or vesicles, the above assumption of the aggregate as a pseudophase does not allow operation with excess substrate. The condition... 

Micelles are used in many applications. Their largest industrial use is in emulsion polymerization, as detailed in Section 5.9 below. On the other hand, micelles made of ionic surfactants can trap hydrocarbon wastes in polluted water, since these hydrocarbon molecules prefer to be in the hydrocarbon interior of the micelle in an aqueous environment. In addition, ionic wastes dissolved in water adsorb onto the polar heads of these micelles. The resulting waste-filled micelles may be removed by simple ultrafiltration. As an example of another application, micelles can affect the rate of several chemical reactions and are used in micellar catalysis, similar to enzyme catalysis, in biochemistry. The rate of the chemical reaction increases with increasing micelle concentration, eventually leveling off. Nevertheless, micellar catalysts are less specific than enzymes. 

III. KINETIC CHARACTERISTICS OF ENZYMIC CATALYSIS IN SYSTEMS OF REVERSE MICELLES AND THEIR REGULATION... 

The fundamental principles controlling activity in nonaqueous systems are the same as those for aqueous solutions, except that the specificity of the micellar core for the solubilization of polar substrates is much greater than for the aqueous situation. The popularity of reversed micelles as models for enzyme catalysis stems from the fact that the micellar core is capable of binding substrates in concentrations and orientations that can be very specific to certain functionalities, much as an enzyme would do. As a result, reaction rate enhancements can be obtained comparable (with luck) to those of the natural systems, and far in excess of what can be explained on the basis of partitioning or availability of substrate. 

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