An approach based on Density Functional Theory (DFT) calculations to assess the Candida antarctica lipase B selectivity in rutin, isoquercitrin and quercetin acetylation

摘要

Flavonoids are naturally occurring polyphenolic antioxidants of increasing biotechnological interest. Through lipase-catalyzed bioprocesses, they can be selectively converted into acylated derivatives displaying improved stabilities and bioavailabilites within nutritional or cosmetic formulations. In order to better understand the molecular basis for the selectivity in these bioconversions, we previously described the application of an enzyme-substrate docking protocol, which identified the most favourable orientations of the flavonoid glycosides isoquercitrin and rutin and their aglycon analogue quercetin within the catalytic cavity of the Candida antarctica lipase B (CALB). In the present work, we further applied quantum chemical calculations, based on the Density Functional Theory (DFT), in these enzyme-substrates complexes. The goal is to verify if the flavonoids OH groups reaching the CALB catalytic residues Ser105 and His224 are expected to form the ester bond that leads to the formation of the tetrahedral intermediate. According to the DFT results, after the transfer of the OH proton to the His224, an ester bond with the carbonyl carbon of the Serl05-bound acetate is expected to be formed for the glucose 6"-O of isoquercitrin and for the rhamnose 4"'-O for rutin. On the contrary, this ester bond is not expected to be formed with the B-ring 3'-O of quercetin. These theoretical results agree with available experimental data concerning the CALB-catalyzed acetylation of these three flavonoids and show that the reaction selectivity is influenced not only by the structural accessibility of the flavonoids OH groups to the catalytic residues, but also by the intrinsic chemical reactivity these OH groups.