A new family of StnC-like pseudo-FMN-preferred reductase components in two-component flavoprotein monooxygenases

The reductase components of two-component flavoprotein monooxygenases (FPMOs) can reduce flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) for their monooxygenase partners. Typically, the type of flavin cofactor bound by a flavoenzyme dictates the cofactor applied in its catalytic reactions. Here, we report the discovery of StnC, a previously unknown reductase family member involved in the biosynthesis of streptonigrin, a potent antimicrobial and antitumor compound. StnC exhibits a paradoxical performance in binding ability and catalytic activity toward flavins. Specifically, it binds FMN 147-fold stronger than FAD, but reduces FAD six times faster than FMN, enabling it to supply reduced FAD to its FAD-preferred monooxygenase partner StnD efficiently. Crystallographic, computational, and structural comparative analyses identified key residues and distinct structural features in StnC-like reductases, including extended clamp-like loops, that enable tight FMN binding and efficient FAD reduction. These features define a family of FAD-preferred reductases under physiological conditions. Our findings significantly broaden the understanding of the sequence-structure-function relationships in FPMO reductase components, uncover a structurally unique family of FAD-preferred reductases, and provide mechanistic insights into their coordination with monooxygenase partners.