Lantibiotics are ribosomally synthesized and post-translationally modified antimicrobial peptides containing thioether rings. In addition to these crosslinks, the clinical candidate lantibiotic NAI-107 also possesses a C-terminal S-[(Z)-2-aminovinyl]-D-cysteine (AviCys) and a unique 5-chloro-L-tryptophan (ClTrp) moiety linked to its potent bioactivity. Bioinformatic and genetic analyses on the NAI-107 biosynthetic gene cluster identified mibH and mibD encoding flavoenzymes responsible for the formation of ClTrp and AviCys, respectively. The biochemical basis for the installation of these modifications on NAI-107 and the substrate specificity of either enzyme is currently unknown. Using a combination of mass spectrometry, liquid chromatography, and bioinformatic analyses we demonstrate that MibD is an FAD-dependent Cys decarboxylase and that MibH is an FADH2-dependent Trp halogenase. Most FADH2-dependent Trp halogenases halogenate free Trp, but MibH was only active when Trp was embedded within its cognate peptide substrate deschloro NAI-107. Structural comparison of the 1.85-Å resolution crystal structure of MibH with other flavin-dependent Trp halogenases revealed that subtle amino acid differences within the MibH substrate binding site generates a solvent exposed crevice presumably involved in determining its unusual substrate specificity.
Lanthipeptides are a class of ribosomally-produced and post-translationally modified peptides (RiPPs) that possess a variety of biological activities, but typically act as antimicrobial agents (lantibiotics). Haloduracin is a lantibiotic that is composed of two post-translationally modified peptides, Halα and Halβ, which are biosynthesized from the precursor peptides HalA1 and HalA2 by their cognate lanthipeptide synthetases, HalM1 and HalM2, respectively. Co-expression studies of HalM1 and HalM2 with chimeric peptides consisting of the leader peptide of HalA1 and the core peptide of HalA2 (or vice versa) showed that the synthetases require both the cognate leader and core peptides for efficient processing. Investigation of the affinity in vitro showed that binding of the N-terminal leader peptide by HalM2 increases its affinity for the C-terminal core peptide. Thus, the two segments of the precursor peptide HalA2 synergistically bind to HalM2.
Cytolysin, a two-component lanthipeptide comprising cytolysin S (CylLS″) and cytolysin L (CylLL″), is the only family member to exhibit lytic activity against mammalian cells, in addition to synergistic antimicrobial activity. A subset of the thioether crosslinks of CylLS″ and CylLL″ have LL-stereochemistry instead of the canonical DL-stereochemistry in all previously characterized lanthipeptides. The synthesis of a CylLS″ variant with DL stereochemistry is reported. Its antimicrobial activity was found to be decreased but not its lytic activity against red blood cells. Hence, the unusual LL-stereochemistry is not responsible for the lytic activity.
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