Iron-sulfur cluster-binding SPASM domain of sactionine bond-forming enzyme CteB and similar proteins
Clostridium thermocellum sactionine bond-forming enzyme CteB is a radical S-adenosylmethionine (SAM) enzyme that catalyzes the formation of the requisite thioether bridge between a cysteine and the alpha-carbon of an opposing amino acid that is required in sactipeptide biosynthesis. Radical SAM enzymes are characterized by a conserved CxxxCxxC motif, which coordinates the conserved iron-sulfur cluster that is involved in the reductive cleavage of SAM and generates a 5'-deoxyadenosyl radical, which in turn abstracts a hydrogen from the appropriately positioned carbon atom of the substrate. Radical SAM (RS) enzymes with a C-terminal SPASM domain contain at least one other iron-sulfur cluster. CteB contains two auxillary 4Fe-4S clusters in its SPASM domain; the auxillary cluster nearest the RS site, called AuxI, exhibits an open coordination site in the absence of peptide substrate, which is coordinated by a peptidyl-cysteine residue in the bound state.