Altering the Regioselectivity of Cytochrome P450 BM3 Variant M13 toward Genistein through Protein Engineering and Variation of Reaction Conditions

The biocatalysts responsible for the enzymatic synthesis of hydroxygenisteins, derivatives of genistein with multiple activities, usually show regioselective promiscuity, hydroxylating genistein to form a mixture of multiple products, which, in turn, results in a cumbersome separation and purification. Hence, it is highly desired to explore the underlying mechanism regulating the regioselectivity of hydroxylases. M13 is a variant of cytochrome P450 BM3 with oxidant activity toward genistein. Herein, genistein was demonstrated to be hydroxylated by M13 to form a mixture of 3'-hydroxygenistein (3'-OHG) and 8-hydroxygenistein (8-OHG), each giving 4% conversion with a ratio of 1:1. Protein engineering toward M13 was thus performed to improve its regioselectivity. When isoleucine at position 86 was mutated into cysteine, the resultant variant M13I86C displayed improved regioselectivity toward 3'-OHG with an increased conversion of 8.5%. The double mutation M13I86CP18W further boosted the conversion of 3'-OHG to 9.6%, and the ratio of 3'-OHG to 8-OHG increased to 12:1. Conversely, both CoCl₂ and glucose 6-phosphate (G6P) could lead to more 8-OHG. When Co²⁺ reached 37.5 mM, M13I86CP18W could give an 8-OHG conversion of 22.4%. The maximal ratio of 8-OHG to 3'-OHG reached 130 when 62.5 mM Co²⁺ was included in the reaction mixture. With the increase of G6P from 10 to 40 mM, the conversion of M13I86CP18W to 8-OHG gradually increased to 22.6%, while the conversion to 3'-OHG decreased to 6%. Thus, both intrinsic residues and external reaction conditions can affect the regiospecificity of M13, which laid the foundation for the selection of suitable biocatalysts for the hydroxylation of genistein.