Deciphering the δ-Lactam Formation and lron-Reducing Activity of Spinactins from Saccharopolyspora spinosa

The cyclic structure of non-ribosomal peptides (NRPs) is critical for enhancing their stability and bioactivity, which highlights the importance of exploring NRP cyclization enzymes for natural product discovery. Thioesterases (TEs) are crucial enzymes that catalyze the formation of various lactams, including macrolactams, β-lactams, and γ-lactams; however, their potential to produce other lactam types remains largely unexplored. In this study, we identified spinactin A (1) and novel derivatives, spinactin B-E (2-5), from Saccharopolyspora spinosa NRRL 18395 and characterized the biosynthetic enzymes involved, particularly a unique TE SncF, responsible for δ-lactam formation. Remarkably, compound 1 exhibited lower cytotoxicity and superior iron-reducing activity than United States Food and Drug Administration (FDA)-approved iron chelators deferiprone (DFP) and deferoxamine (DFO), indicating its potential for treating iron overload disorders, especially in beta-propeller protein-associated neurodegeneration (BPAN) cells. These findings highlight TEs' roles in expanding the repertoire of δ-lactam-containing NRPs with therapeutic potential.