Elucidation of the structure of SA‐FF22, a lanthionine‐containing antibacterial peptide produced by Streptococcus pyogenes strain FF22

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Abstract
The antibacterial peptide SA‐FF22, produced by the pathogen Streptococcus pyogenes strain FF22 was purified and features of its primary and secondary structure were characterised. Mass spectrometry demonstrated the pure peptide had a mass of 2794Da while, amino acid analysis revealed the presence of the unusual, thioether amino acids lanthionine and 3‐methyllanthionine; thus SA‐FF22 is a member of the group of antibacterial polypeptides termed lantibiotics. Furthermore, amino acid sequencing showed a unique sequence which was blocked at position 23 by a residue of the unsaturated amino acid 2,3‐didehydrobutyrine. Carboxypeptidase‐Y digestion could be used to demonstrate that serine occupies the C‐terminal position only after complete oxidation of the thioether amino acid bridges, suggesting that the three‐dimensional structure of the native peptide may prevent access of the enzyme to the C‐terminus. Fragmentation of the native peptide with a variety of proteolytic enzymes failed to yield a peptide containing less than all three of the cross‐linked lanthionine and methyllanthionine residues and demonstrated that all three thioether bridges overlapped. Analysis of the circular dichroism of SA‐FF22 in various concentrations of 2,2,2‐trifluoroethanol in water, SDS micelles and in the presence of artificial phospholipid vesicles suggested that there is significant change in its secondary structure from aqueous to lipophilic environments.