Inverted enantiopreference

Additionally, the same researchers studied the enantiopreference of GALA and CALB across a variety of racemic ethyl 3-hydroxy-3-(furyl/thienyl)propanoates. The separation of enantiomers at the maximum theoretical yield was achievable because of the high enantioselectivity displayed ( >200). An interesting inverted enantiopreference of GALA was reported despite its low enantioselectivity (E = 14-54) the behaviour of GALA and CALB depends on the substrate structure [12]. 

In all the reported examples, the enzyme selectivity was affected by the solvent used, but the stereochemical preference remained the same. However, in some specific cases it was found that it was also possible to invert the hydrolases enantioselectivity. The first report was again from iQibanov s group, which described the transesterification of the model compound (13) with n-propanol. As shown in Table 1.6, the enantiopreference of an Aspergillus oryzae protease shifted from the (l)- to the (D)-enantiomer by moving from acetonitrile to CCI4 [30]. Similar observations on the inversion of enantioselectivity by switching from one solvent to another were later reported by other authors [31]. 

Computational enzyme-substrate docking studies carried out on a CRED from Sporoholomyces salmonicolor showed that residues Met 242 and Gin 245 were in close proximity to the para-substituent of acetophenones in the substrate binding site. Site-saturation mutagenesis of Met 242 or Gin 245 (replacing these residues with all 19 other possible amino acids) and double mutation of both Met 242 and Gin 245 were carried out to enhance the enzyme s enantioselectivity toward the reduction of para-substituted acetophenones. The enantiopreference was inverted from R)- to (S)-configuration for three Gin 245 mutants that were obtained. Preference for the formation of (S)-enantiomeric alcohols was shown for four Met 242 mutant enzymes when compared to the wild-type enzyme Gin 245 and Met 242/Gln 245 double variations inverted the enantiopreference and affected the enantiomeric purity of the product alcohols. Residues 242 and 245 also exerted an effect on the catalytic activity of this GRED [23]. 

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