Enantioselectivity substrate engineering

Subtle modification of the substrate in order to change the enantioselectivity of a reaction (substrate engineering) is one of the simplest methods for obtaining changes in optical purity. For the oxidative KR of sec-alcohols using Rhodococcus ruher DSM 44541, it was found that the introduction of C=C bonds into the side chain improved the enantioselectivity of the reaction [29]. For example,... 

I., and Remaud-Simeon, M. (2009) Control of lipase enantioselectivity by engineering the substrate binding site and access channel. ChemBioChem, 10 (17), 2760-2771. 

Brerma, E Fronza, G., Fuganti, C., Gatti, F.G., Manffedi, A., Parmeggiani, F and Ronchia, P. (2012) On the stereochemistry of the baker s yeast-mediated reduction of regioisomeric unsaturated aldehydes examples of enantioselectivity switch promoted by substrate-engineering./. Mol. Catal. B, 84, 94-101. 

As each BVMO is limited in substrate specificity, it is crucial to have a large collection of these oxidative biocatalysts available. Except for expanding the scope of possible reactions, a large toolbox of BVMOs would also increase the chance of being able to perform any wanted specific chemo-, regio- and/or enantioselective reaction. This contrasts with the present situation as only a relatively small number of BVMOs can be exploited for biocatalytic purposes. Therefore, it is still crucial to discover or engineer BVMOs with novel biocatalytic properties. 

Genetic engineering. The X-ray structures are known for many hydrolases, allowing for modeling of the substrate in the active site as well as structurally based, random or rational protein mutation to magnify or invert enantioselectivity. An example of the latter is provided by the rational design of a mutant of Candida antarctica lipase (CALB), which, instead of the wild-type R-selectivity, displayed... 

To evolve useful enzymes, genetic engineering technology has been applied increasingly to improve stability of enzymes, enantioselectivity, extension of substrate specificity for kinetic resolution of racemic compounds. Novel enzymes created by this technique will be available in large quantities and varieties within a next few years. In the near future, a lot of useful enzymes will be on the market and expanding number of chemists can use enzymes more freely than present due to the improvement in the simplification of experimental procedures. 

Biological enzymes are well known to carry out epoxidation. For example, MMO is an efficient and selective catalyst for epoxidation of small terminal olefins such as ethylene, propylene, and 1-butene [246,247]. Lipases have been used to generate peroxoacids which in turn are used for epoxidation reactions [248,249]. The subject has been reviewed [250]. Biocatalytic systems are of interest not only because they can carry out enantioselective epoxidation of substrates, but also because they offer the exciting possibility of being engineered for specific transformations of nonnatural reagents. 

Enzymes have high potential in organic synthesis. Applications have been until now mainly based on kinetic resolution, but many opportunities exist to use enzymatic catalysis for enantioselective syntheses. Recently, it was shown by Reetz et al. that a combination of genetic engineering and mutagenesis can easily provide modified enzymes of greatly improved stereoselectivity for the transformation of a given substrate [114]. This concept should find wide applications in catalyzed enantioselective reactions. 

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