Iron-sulfur cluster-binding SPASM domain of antilisterial bacteriocin subtilosin biosynthesis protein AlbA and similar proteins
Bacillus subtilis antilisterial bacteriocin subtilosin biosynthesis protein AlbA is a radical S-adenosylmethionine (SAM) enzyme that catalyzes the formation of three thioether bonds in the post-translational modification of a linear peptide into the cyclic peptide subtilosin A. The thioether bonds formed are between the sulfur of three cysteine residues and the alpha-carbons of two phenylalanines and one threonine to produce a rigid cyclic peptide. 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. AlbA appears to contain one auxillary Fe-S cluster, similar to the auxillary 4Fe-4S cluster in Bacillus circulans butirosin biosynthetic enzyme BtrN.