A New Enzyme Family Catalyzing Methyllanthionine Sulfoxide Formation in Anti-phage Lanthipeptides

Lanthipeptides are one of the largest classes of ribosomally synthesized and post-translationally modified peptides (RiPPs). The coi biosynthetic gene cluster (BGC) from Streptomyces coelicolor A3(2) encodes a canonical class I lanthipeptide dehydratase (CoiB) and cyclase (CoiC), a bifunctional enzyme (CoiSA) with an O-methyltransferase (MT) and glutamyl lyase (GL) domain, and a protein of unknown function (CoiH). The product of the coi BGC was recently shown to impart anti-phage activity, but its structure is still unresolved. Previous work investigated the regioselectivity of the GL domains in CoiB and CoiSA and the stereochemistry of the cyclized precursor peptide, but the function of CoiH was not addressed. In this study, co-expression of the peptide CoiA1 with CoiBCSAH resulted in a +16 Da addition on the cyclized peptide compared to when CoiH was omitted. LC-MS/MS analysis indicated that this modification occurred in the first thioether ring. A combination of site-directed mutagenesis, comparison of linear and cyclized peptide substrates, hydrogen peroxide (H₂O₂) treatment, and collision-induced dissociation (CID) mass spectrometric analysis suggested that the sulfur atom in the first methyllanthionine was oxidized to a sulfoxide group by CoiH. This hypothesis was confirmed by NMR analysis. CoiH represents a previously uncharacterized oxygenase family catalyzing sulfoxide formation. Structure prediction tools suggest a novel enzyme fold without obvious metal or cofactor binding sites, raising the possibility that CoiH is a cofactor independent oxidation enzyme.