A molecular modelling study to rationalize the regioselectivity in acylation of flavonoid glycosides catalyzed by Candida antarctica lipase B

The regioselective behaviour of the Candida antarctica   lipase B (CALB) towards two flavonoid glycosides, rutin and isoquercitrin, in the acetylation reaction was investigated through molecular modelling. A protocol constituted by a Monte Carlo-based docking procedure and classical force fields calculations was applied to find probable binding modes of the substrates inside the catalytic cavity and optimize the corresponding complexes. The analysis of these complexes allowed identifying productive ones (that means, those able to lead to the formation of the ester product) according to three parameters: (1) protein distortion; (2) stability of hydrogen bond interactions with the oxyanion hole residues; (3) localization of hydroxyl groups with regard to the region comprised between the catalytic histidine and serine residues. Results showed that the aglycon part of both rutin and isoquercitrin was localized at the entrance of the binding pocket, stabilized by hydrogen bond and hydrophobic interactions. The sugar part of the flavonoids was placed close to the pocket bottom. In particular, only the primary 6″-OH of the isoquercitrin glucose and the secondary 4‴4‴-OH of the rutin rhamnose were expected to be acetylated, as they were the only ones to stabilize simultaneously near to the catalytic histidine and the acetate bound to the catalytic serine. These findings are in accordance with experimental data and give a suitable explanation, at an atomic level, of the regioselectivity of CALB in the flavonoid glycosides acetylation.