CALÀ B (Lipase B from Candida antarctica) catalyzed aza-Michael addition of a set of aniline compounds with acrylates under mild conditions was described. A systematic study allowed to determine the appropriate solvent, enzyme loading, reaction temperature and time. In order to speculate and verify its mechanism, the wild-type and three mutants (S105 A, H224 A and I189 A) of CALÀ B were expressed. Some control experiments demonstrated the active site was responsible for the enzymatic process, in which Ser105 and His224 played a crucial role. Besides, the mutation of Ile189 also affected its activity a lot. Based on these results, a docking experiment was performed to speculate the mechanism: the oxyanion hole (Thr40 and Gln106) of the active site activated the acrylates and stabilized the transition states. The Ile189 residues, as an important part of active cavity, could form a strong hydrophobic interaction with substrates. And the Ser105 and His224 residues were responsible for proton transfer during the catalytic process. This would help to understand the promiscuity of CALÀ B, and provide ideas to design novel enzyme to improve the efficiency of its promiscuity.[a] B.
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