Zhang_2019_Mol.Catal_466_146

The present study investigates the kinetic model of the enzymatic Michael addition of butylamine to 2-methyl-1,4-benzoquinone to form 2-methyl-3-n-butylaminoyl-1-hydro-4-quinone in citrate buffer solution (pH 7.0). The yield of the product of 98% was achieved, mainly due to the excellent regioselectivity of immobilized lipase from T. laibacchii. The immobilized preparation used here was obtained by a method of purification and in situ immobilization. Through the purification using a PEG 4000/ K2HPO4 aqueous two-phase system (ATPS), the T. laibacchii lipase was partitioned predominantly in the PEG-rich top phase where diatomite was added to achieve in situ immobilization via interfacial activation on the hydrophobic support. A proposed reaction mechanism of the Michael addition involves (1) the oxyanion hole polarizes the alpha,beta-unsaturated carbonyl of 2-methyl-1,4 -benzoquinone, increasing its electrophilic ability, (2) the catalytic histidine deprotonates the nucleophile n-butyl amine. A modified sequential mechanism including ordered and random sequential bi-bi was proposed for the first, and it is beneficial to add these modification mechanisms to the family of enzyme complex reaction mechanism because the mechanism is partly expanded. The kinetic parameters were directly obtained by combining the numerical integration toolbox ode45 to solve differential equations and the nonlinear optimization toolbox fmincon for error minimizing objective function. A very satisfactory agreement between experimental data and model results was obtained based on the modified random bi-bi mechanism, implying that the enzymatic Michael addition may follow the modified random bi-bi mechanism. The mass transfer limitations were investigated, and it is found that both internal and external mass transfer limitations could be ignored.