Iron-sulfur cluster-binding SPASM domain of an uncharacterized group of radical SAM proteins
Members of this group are radical S-adenosylmethionine (SAM) enzymes with a SPASM domain, named after the biochemically characterized members, AlbA, PqqE, anSME, and MftC, which are involved in Subtilosin A, Pyrroloquinoline quinone, Anaerobic Sulfatase, and Mycofactocin maturation, respectively. 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 enzymes with a C-terminal SPASM domain contain at least one other iron-sulfur cluster. This group appears to contain one auxillary Fe-S cluster, similar to the auxillary 4Fe-4S cluster in Bacillus circulans butirosin biosynthetic enzyme BtrN.