Regioselectivity, enzyme

Substrate B does not change the regioselectivity of substrate A The regioselectivity of the enzyme is determined by the interactions between the substrate and the active site. Since the substrate saturation curve is defined by the Km of the enzyme, regioselectivity cannot be a function of substrate or inhibitor concentration [I],... 

Enzyme regioselectivity also enables the conversion of multifunctional monomers (functionality >3) to linear or nearly linear homo- and copolymers. In 1991, Dordick and co-workers [60] reported that, by using the protease Proleather, condensation polymerizations (45 °C, 5 days) performed in pyridine between sucrose and bis(2,2,2-trifluoroethyl) sebacate proceed with high regioselectivity giving sucrose oligoesters (DP 11) in 20% yield (see also Chapter 1). This inspired subsequent work by others that demonstrated such copolymerizations with polar multifunctional polyols could be performed under bulk reaction conditions without activation of carboxylic acids (see below). 

To date (ca 1996) many potentially usefiil sucrose derivatives have been synthesized. However, the economics and complexities of sucrochemical syntheses and the avadabiLity of cheaper substitutes have limited their acceptance hence, only a few of them are in commercial use. A change in the price and availability of petroleum feedstocks could reverse this trend. Additional impetus may come from regioselective, site-specific modifications of sucrose to produce derivatives to facilitate and improve the economics of sucrochemical syntheses. For example, the microbe yigwbacterium tumifaciens selectively oxidizes sucrose to a three-keto derivative, a precursor of alkylated sucroses for detergent use (50). Similarly, enzymes have been used for selective synthesis of specific sucrose derivatives (21). 

The reversibility of halohydrin dehalogenase-catalyzed reactions has been used for the regioselective epoxide-opening with nonnatural nucleophiles (an example is given in Scheme 10.34) [133]. The stereoselectivity of the enzyme results in the resolution of the racemic substrate. At the same time, the regioselectivity imposed by the active site geometry yields the anti-Markovnikov product. [128]... 

It is very likely that a similar Type I polyketide synthase constructs the naphthoate fragment of azinomycin B. This will be a very interesting enzyme to study, since it will need to perform an unprecedented three regioselective reduction reactions, as well as controlling the polyketide chain length and directing its cycliza-tion. 

The term medium engineering , that is the possibility to affect enzyme selectivity simply by changing the solvent in which the reaction is carried out, was coined by Klibanov, who indicated it as an alternative or an integration to protein engineering [5aj. Indeed, several authors have confirmed that the enantio-, prochiral-, and even regioselectivity of enzymes can be influenced, sometimes very remarkably, by the nature of the organic solvent used. 

The alkane hydroxylase from Pseudomonas oleovorans is particularly suitable for the epoxidation of terminal aliphatic double bonds and enables rapid access to the (3-blocker metoprolol (Scheme 9.14) [113,116]. Complementing this regioselectivity, chloroperoxidases are particularly suitable biocatalysts for the epoxidation of (ds substituted) subterminal olefins [112,117]. This enzyme also accepts terminal olefins and is utilized for the effident synthesis of P-mevalono-ladone [118]. 

It has been shown that it is possible to compel regiospecific para substitution by enclosing the substrate molecules in a cavity from which only the para position projects. Anisole was chlorinated in solutions containing a cyclodextrin, a molecule in which the anisole is almost entirely enclosed (see Fig. 3.4). With a high enough concentration of cyclodextrin, it was possible to achieve a para/ortho ratio of 21.6 (in the absence of the cyclodextrin the ratio was only 1.48). This behavior is a model for the regioselectivity found in the action of enzymes. 

However, the particular usefulness of enzymes stems from their selectivity. There are three types of selectivity exerted by enzymes chemoselectivity, regioselectivity, and stereoselectivity. 

It is not only the activity that can be altered by incorporation of noncoded amino acids. Introduction of structures possessing certain chemical functions leads to the possibility of highly regioselective modification of enzymes. For example, selective enzymatic modification of cystein residues with compounds containing azide groups has led to the preparation of enzymes that could be selectively immobilized using click chemistry methods [99]. 

Soffers AEMF, Ploeman JHTM, Moonen MJH, Wobbes T, van Ommen B, Vervoort J, et al. Regioselectivity and quantitative structure-activity relationships for the conjugation of a series of fluoronitrobenzenes by purified glutathione S -transferase enzymes from rat and man. Chem Res Toxicol 1996 9 638-46. 

Recently, the distribution of 2,3-dihydroxybenzoate decarboxylase has been found in a variety of fungal strains (unpubhshed data), and the carboxylation activity for catechol is confirmed by the reaction using resting cells (or cell-free extract) in the presence of 3M KHCO3. The detailed comparative studies of enzyme structures and catalytic properties between 2,3-dihydroxybenzoate decarboxylase and 3,4-dihyroxybenzoate decarboxylase might explain how the decarboxylases catalyze the regioselective carboxylation of catechol. 

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