Enzyme mimetics

Bakirci, H., Koner, A.L., Schwarzlose, T. and Nau, W.M. (2006) Analysis of host-assisted guest protonation exemplified for p-sulfonatocalix[4]arene — Towards enzyme-mimetic pKa shifts. Chem.—Eur. J., 12 (18), 4799 807. 

D. Enders, Enzyme-mimetic C-C-and C-N-Bond Formations in Stereoselective Synthesis (E. Ottow,... 

Usually mimetics of enzymes are not as sophisticated and their catalytic efficiency is taken into account as a main objective. The important feature of enzyme mimetics is an initial binding interaction between substrate and the catalyst, thus giving rise to Michaelis-Menten kinetics [35], Therefore for a model system to be called an artificial enzyme it should combine more than one of the key features identified for enzymes [35], For some systems it is difficult to decide if they can be considered as artificial photoenzymes or if they are just photocatalysts (described in Chapters 6, 7,10, and 21). It is not the purpose of this book to judge whether model systems can be called artificial photoenzymes or should be considered as just chemical photocatalysts. To get some feeling which type of model systems have been investigated we describe a few model systems that have claimed to have enzyme-like activity. 

Ultimately the distinction between biological and chemical catalysis may become less distinct as technologies for modified enzymes, enzyme mimetics, and chemoenzymatic catalysis advance. In the meantime, however, it is clear that further application of enzyme technology within industry will greatly benefit society in the twenty-first century. 

Potential development of industrially robust enzyme-mimetic systems to allow for specific biotransformations. These mimics would be catalytically active but not be dependent upon the amino acid backbone of existing enzymes. 

With the addition of a catalytic centre, an imprinted recognition site can be transformed into an enzyme mimetic material offering substrate selective catalysis. Over the past decade a few research groups have studied catalytic effects of imprinted metal oxides in esterification reactions. In this section a brief overview will be given on their efforts to understand the catalytic selectivity and structure of the active site. For additional reading an excellent review can be found by Davis et al. [46]. 

Typically reported topics are solvolytic reactions, oxidations, reductions, and C-C coupling reactions. The saponification of activated esters in aqueous micelles is a typical model for an enzyme mimetic reaction. The influence of the micellar medium on the reaction rate has been investigated, as well as the alteration of the stereoselectivity. Models of metalloenzymes were developed with the ligands 1-3 [8]. 

Synthesis and characterization of zeolite encaged enzyme-mimetic copper histidine complexes... 

The core of a micelle and the bilayer of a vesicle are comparable with a liquid-crystalline phase and can influence the stereoregularity of asymmetrically catalyzed reactions. Self-organization and the neighborhood of hydrophilic and hydrophobic regions are close to those of natural systems and we designate this as membrane mimetic or enzyme mimetic chemistry [45]. The large field of artificial enzymes was recently reviewed by Murakami et al. [46]. 

Selected examples of such oxidations of straight-chain aliphatic compounds are listed in Table 1. The remarkable monohalogenation and co-1 selectivities in these reactions have been attributed to polar, steric, and conformational effects. The yields of monohalogenated derivatives, based on converted (60-80"/)) substrate, are generally quantitative. Comparison of these selectivities with a number of oxidations using bromine, chlorine or rerf-butoxy radical chain carriers shows that aminium radical-mediated oxidation is far superior to others for synthetic, indeed industrial, applications [28]. The high selectivity and clean monooxidation displayed by the aminium radical chain process has been referred as an enzyme-mimetic reaction ... 

More elaborate enzyme mimetics have used acid-base pairs on the rim and in the center of cyclodextrin matrices with similar results. 

Besides the superoxide dismutation mechanism, the reactivity of metal centers, in particular manganese complexes, toward NO is very much dependent on the possibility for binding a substrate molecule. As it will be shown later, the possibility that MnSOD enzymes and some mimetics can react with NO has been wrongly excluded in the literature, simply based on the known redox potential for the (substrate) free enzymes, mimetics, and NO, respectively. Therefore, the general fact that, upon coordination, redox potentials of both the metal center and a coordinated species are changed should be considered in the case of any inner-sphere electron-transfer process as a possible reaction mechanism. 

Reversed micelles formed with various surfactants in apolar solvents in the presence of small amounts of water were extensively studied on their characteristic features of cores in enzyme-mimetic reactions. In the spatially restricted microenvironment of reversed micelles, a unique catalytic behavior analogous to enzymatic reac-iions was observed. 

Albin, V. and F. Bedioui (2003). First electrochemical evidence of existence of an oxomanganese(V) porphyrin intermediate in the reaction of manganese(III) porphyrin and hydrogen peroxide as an intermediate in the reaction of manganese(III) porphyrin and hydrogen peroxide as a model of enzyme mimetics. Electrochem. Commun. 5, 129-132. 

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