Spectroscopic and Electrochemical Characterization of the Mycofactocin Biosynthetic Protein, MftC, Provides Insight into Its Redox Flipping Mechanism

Mycofactocin is a putative redox cofactor and is classified as a ribosomally synthesized and post-translationally modified peptide (RiPP). Some RiPP natural products, including mycofactocin, rely on a radical S-adenosylmethionine (RS, SAM) protein to modify the precursor peptide. Mycofactocin maturase, MftC, is a unique RS protein that catalyzes the oxidative decarboxylation and C–C bond formation on the precursor peptide MftA. However, the number, chemical nature, and catalytic roles for the MftC [Fe–S] clusters remain unknown. Here, we report that MftC binds a RS [4Fe–4S] cluster and two auxiliary [4Fe–4S] clusters that are required for MftA modification. Furthermore, electron paramagnetic resonance spectra of MftC suggest that SAM and MftA affect the environments of the RS and Aux I cluster, whereas the Aux II cluster is unaffected by the substrates. Lastly, reduction potential assignments of individual [4Fe–4S] clusters by protein film voltammetry show that their potentials are within 100 mV of each other.