Sutherland, E.; Harding, C. J.; Monceau De Bergendal, T. M. Y. D.; Florence, G. J.; Ackermann, K.; Bode, B. E.; Synowksy, S.; Sundaramoorthy, R.; Czekster, C. M.

Cyclic dipeptides are produced by organisms in all domains of life. Many possess biological activities as anticancer and antimicrobial compounds. Oxidations are frequently present in these biologically active peptides. C-C bond oxidation is catalysed by tailoring enzymes, including cyclodipeptide oxidases. These flavin-dependent enzymes are underexplored due to their complex three-dimensional arrangement involving multiple copies of two distinct small subunits and unclear mechanistic features underlying substrate selection and catalysis. Here, we determined the structure and mechanism of the cyclodipeptide oxidase from the halophile Nocardiopsis dassonvillei (NdasCDO), part of the biosynthetic pathway for nocazine natural products. We show NdasCDO forms filaments in solution with a covalently bound FMN cofactor in the interface between three distinct subunits. The enzyme is promiscuous, using many cyclic dipeptides as substrates in a distributive manner. The reaction is optimal at high pH, involving the formation of a radical intermediate. Pre-steady state kinetics, a sizeable solvent kinetic isotope effect and lack of viscosity effects support that a step coupled to FMN regeneration determines the rate of the reaction. Our work dissects the complex mechanistic and structural features of this dehydrogenation reaction. This sets the stage to utilizing NdasCDO as a biocatalyst and expands the FMN-dependent oxidase family to include enzyme filaments.