Dimerization of MilM is essential for catalyzing the pyridoxal-5'-phosphate (PLP)-dependent Cγ-hydroxylation of L-arginine during mildiomycin biosynthesis

MilM from the mildiomycin biosynthetic pathway is a PLP-dependent enzyme, previously annotated as an aminotransferase, but has recently been demonstrated as L-arginine oxidase cum C-hydroxylase. Here, we report detailed biochemical, biophysical, structural modeling, and molecular dynamics simulation-based investigations of MilM from Streptoverticillium rimofaciens B-98891 to elucidate the mechanisms of substrate binding, catalysis, and the role of the active site residues involved in these processes. Our experimental findings confirmed that MilM functions as a stable homodimer, requiring the PLP cofactor and molecular oxygen to transform the L-arginine substrate into 5-guanidino-4-hydroxy-2-oxovaleric acid and 5-guanidino-2-oxovaleric acid through the intermediacy of a possible superoxide radical anion species, while generating H2O2 and NH3 as reaction by-products. Our labeling studies also established that the hydroxyl group in the product is obtained from the solvent, water. The structure-based three-dimensional modeling and simulation of MilM coupled with site-directed mutagenesis further confirmed that, in addition to the catalytic residues Lys232 and His31, active site residues from both the protomers are crucial for stabilization of the PLP cofactor (Ser92, Phe116, Asn164, Asp195, Lys240 from chain A and Tyr89 from chain B) and the substrate (Thr14, Glu17, Asn118, and Arg364 from chain A and Thr259, Ser260 from chain B). Moreover, the molecular dynamics simulation uncovered a dimer-mediated alternating lid mechanism in which large-scale, concerted motions of the dimer interface helices reciprocally expose and occlude the two active sites. This see-saw-like dynamics controls the substrate entry and product release through transiently formed tunnels, while preserving a catalytically protective environment, a critical phenomenon previously left unnoticed in similar PLP-dependent oxidases/hydroxylases. Overall, these findings provide new insights into the substrate/cofactor stabilization and the catalytic mechanism of MilM, a recent member of an emerging family of remarkable PLP-dependent oxidases and help us decode a key puzzle in the mildiomycin biosynthetic pathway.