Green production of indigo and indirubin by an engineered Baeyer–Villiger monooxygenase

Abstract

Indigoids are well-known dye molecules of widespread industrial applications in textiles as well as more recently in pharmaceuticals. Historically they have been predominantly produced using chemical reactions that generate undesired by-products. The use of flavin-containing monooxygenases as biocatalysts is an environmentally-friendly alternative to the oxidation of indole leading to the production of indigoids such as indigo and indirubin. In this work, one such enzyme, an active site mutant of the Baeyer–Villiger monooxygenase from Acinetobacter radioresistens (Ar-BVMO), has been used for the transformation of indole into indigo or indirubin. Under optimal conditions, E. coli whole-cells expressing this mutant are capable of producing up to 154 mg/L of the blue indigo dye. Supplementation with 5 mM cysteine switches the production to the more medically relevant red dye, indirubin, where 138 mg/L can be obtained. In silico docking experiments performed with the 3D molecular model of this BVMO demonstrates that the enlarged active site of the mutant enzyme not only facilitates indole access but also its productive binding. Taken together, this work represents one of the first characterizations of a green process driven by a BVMO as a biocatalyst for the production of indigoids, specially the medically relevant indirubin.