The lantibiotic mersacidin inhibits peptidoglycan biosynthesis by binding to the peptidoglycan precursor lipid II. Mersacidin contains an unsaturated thioether bridge, which is proposed to be synthesized by posttranslational modifications of threonine residue ؉15 and the COOH-terminal cysteine residue of the mersacidin precursor peptide MrsA. We show that the flavoprotein MrsD catalyzes the oxidative decarboxylation of the COOH-terminal cysteine residue of MrsA to an aminoenethiol residue. MrsD belongs to the recently described family of homo-oligomeric flavin-containing Cys decarboxylases (i.e., the HFCD protein family). Members of this protein family include the bacterial Dfp proteins (which are involved in coenzyme A biosynthesis), eukaryotic salt tolerance proteins, and further oxidative decarboxylases such as EpiD. In contrast to EpiD and Dfp, MrsD is a FAD and not an FMN-dependent flavoprotein. HFCD enzymes are characterized by a conserved His residue which is part of the active site. Exchange of this His residue for Asn led to inactivation of MrsD. The lantibiotic-synthesizing enzymes EpiD and MrsD have different substrate specificities.Lantibiotics (for "lanthionine-containing antibiotics") are a group of ribosomally synthesized and posttranslationally modified antibiotic peptides containing the thioether amino acid lanthionine as a characteristic building block (33). In the last few years, several lantibiotics with unsaturated thioether bridges at the COOH terminus have been characterized. (2), gallidermin (12), cypemycin (27), and some of the mutacins (28-30, 34), whereas mersacidin contains a COOH-terminal S-[(Z)-2-aminovinyl]-D-cysteine is present in epidermin S-[(Z)-2-aminovinyl]-3-methyl-D-cysteine residue (15, 32).Among these lantibiotics, mersacidin, which is produced by a Bacillus strain (9), is of particular interest because it is active against methicillin-resistant Staphylococcus aureus strains (8). Brötz et al. (6,7) were able to show that mersacidin inhibits the transglycosylation reaction in peptidoglycan biosynthesis by binding to the cell wall precursor lipid II [undecaprenyldiphosphoryl-N-acetylmuramic acid-(pentapeptide)-N-acetylglucosamine]. The complex formed by mersacidin and lipid II differed from the well-known vancomycin-lipid II complex, making mersacidin to a new lead structure for the development of antimicrobial compounds. The biosynthesis of the S-[(Z)-2-aminovinyl]-D-cysteineresidue of epidermin depends on the flavoenzyme EpiD. EpiD catalyzes the oxidative decarboxylation of the COOH-terminal cysteine residue of the epidermin precursor peptide EpiA to a (Z)-enethiol structure (see Fig. 7) (13,17,20,21). The unsaturated thioether bridge is formed by addition of the enethiol group to a didehydroalanine residue, produced by dehydration of Ser at position ϩ19 of EpiA (residues within the COOHterminal propeptides of lantibiotic precursor peptides are labeled with "ϩ"). Recently, molecular characterization of EpiD helped to identify homologous enzymes catalyzing the decarboxylati...

The microbial flavoenzyme EpiD catalyzes the oxidative decarboxylation of peptidyl-cysteines to peptidyl-aminoenethiols. These unusual C-terminally modified peptides are intermediates in the biosynthesis of the tetracyclic peptide antibiotic epidermin, which belongs to the lantibiotics family. The peptide SFNSYCC represents the C-terminal partial sequence of the natural precursor peptide EpiA. EpiA is posttranslationally modified to form finally the lantibiotic epidermin. The substrate specificity of EpiD was investigated using high-resolution mass spectrometry and the heptapeptide library SFNSXCC. The enzymatic conversion of particular peptides can be observed by a mass loss of m/z 46. In contrast to the previously used triple quadrupole instrument, electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) was able to resolve and detect all precursor and converted peptides with identical nominal masses in a single measurement, avoiding the necessity to investigate single peptides. Furthermore, a new substrate SFNSCCC of the enzyme EpiD was detected within the reaction mixture.