Flavin reductases in two-component systems: Mechanistic insights, structural classification, and biotechnological...

Flavin reductases (FRs) are essential redox enzymes that supply reduced flavin cofactors (FMNH^-/FADH^-) to various monooxygenase partners in two-component flavin-dependent monooxygenase (TC-FDMO) systems. These enzymes play critical roles in numerous biological processes and industrial biocatalytic reactions, including hydroxylation, halogenation, and epoxidation. In this review, we provide a comprehensive analysis of the structural features, oligomeric states, kinetic mechanisms, and newly proposed classification strategies of FRs. We highlight the limitations of existing classification systems that rely solely on physiological function and propose a more informative framework based on amino acid sequences and domain architectures. Detailed mechanistic insights from transient kinetics, charge-transfer complex formation, and flavin transfer pathways are discussed, with emphasis on enzyme-specific features such as half-site reactivity and substrate-enhanced catalysis. Advances in protein engineering and fusion protein design aimed at improving FR stability, catalytic performance, and cofactor regeneration are also critically evaluated. In addition, we explore alternative strategies for supplying reduced flavin to monooxygenase partners, including non-enzymatic regeneration methods and the use of nicotinamide analogs. Finally, we outline key challenges and future directions for developing next-generation FRs with enhanced industrial applicability. This knowledge provides a foundation for engineering TC-FDMO systems for scalable, sustainable, and industrially relevant biocatalysis.