P450 Fusion Protein Expressed in E. coli for Regioselective Hydroxylation of Flavonoids

Plant cytochrome P450 monooxygenases (CYPs) are valuable biocatalysts for the regioselective hydroxylation of aromatic compounds. However, their expression in bacterial hosts is hampered by poor solubility, membrane anchoring and the requirement for redox partners. In this work, we report the design and characterization of modular expression systems that enable the functional production of SbCYP82D1.1 from Scutellaria baicalensis (SbF6H) in Escherichia coli. Both independent expression and synthetic fusion systems were evaluated by combining a CYP with a compatible reductase (ATR2_tr from Arabidopsis thaliana) to catalyze the conversion of chrysin into baicalein. A combinatorial library of N-terminal variants, host strains, media, and induction strategies was constructed and screened. Among the tested host, E. coli DH 10-beta provided the highest product titers, particularly when cultures were supplemented with 5-aminolevulinic acid. Truncation of the native transmembrane anchor significantly improved catalytic performance, whereas the addition of the heterologous MALLLAVF tag decreased activity. Fusion systems outperformed separate expression formats, showing approximately two-fold higher activity, with the flexible glycine-serine linker (L_GS) supporting the highest hydroxylation product formation. The corresponding fusion construct showed an apparent conversion of 0.1 mM chrysin to baicalein of up to 90% under the applied whole-cell reaction and analytical conditions, although this value should be interpreted with caution due to the concurrent instability of baicalein observed in all reactions and culture conditions. This result nevertheless indicates a marked improvement in whole-cell baicalein formation compared with previously reported bacterial systems. Together, these results demonstrate that rational N-terminal engineering combined with fusion protein design can enable efficient bacterial expression of plant CYPs, representing a promising step toward scalable production of hydroxylated flavonoids.