Mass spectroscopic analysis of a novel enzymatic reaction. Oxidative decarboxylation of the lantibiotic precursor peptide EpiA catalyzed by the flavoprotein EpiD

The epidermin biosynthetic reaction between the flavoprotein EpiD and the precursor peptide EpiA was investigated by reversed-phase chromatography and ion spray mass spectrometry. Several products with molecular masses 46 and 104 Da less than that of EpiA were observed; these results were confirmed by using an MBP-EpiD fusion protein as enzyme and the mutant peptides EpiAR-1Q and K-EpiA as substrates. The reaction was inhibited by Zn2+ ions. Modifications were localized in the C-terminal fragment of EpiA as shown by factor Xa cleavage of the products followed by mass spectrometry analysis. In addition, EpiD reacted with the precursor peptides and with proepidermin, indicating that the leader peptide is not necessary for the recognition of EpiA by EpiD. Sequence analysis of modified proepidermin revealed that at least the amino acids Ile(+1)-Tyr+20 are unmodified. The observed decrease in mass of 46 Da and the modification at the C terminus of EpiA is in agreement with the proposed enzymatic function of EpiD, the oxidative decarboxylation of the precursor peptide. In addition, the increased absorbance at 260 nm of the modified peptides indicates the presence of a thioenol group in the C-terminal proepidermin.