Natural products and natural product derivatives have been the leading source of pharmaceutical compounds since the initial application of modern medicine. To fight the increasing occurrence of drug resistance and to reach the ultimate goal of personalized drugs for treating complicated symptoms, novel natural products are demanded. Lanthipeptides are ribosomal peptides with post-translationally incorporated thioether crosslinks named lanthionine. This family of natural products has garnered substantial attention during the past few decades due to their favorable biological activities and the potential for engineering. This thesis focuses on a subclass of lanthipeptides, the class II compounds, for which the synthesis of dehydroamino acids and the formation of thioether linkages are carried out by a bifunctional lanthionine synthetase. Chapter 2 presents the structural characterization of the enterococcal cytolysin, a lanthipeptide tightly linked to Enterococcus faecalis virulence. The stereoselectivity of lanthionine synthesis is discussed in chapters 3 and 4. A non-canonical configuration of lanthionines was discovered in a few lanthipeptides and the selective synthesis of the unusual stereochemistry was found to be induced by the peptide sequence rather than the lanthionine synthetase. Such a substrate-controlled stereoselectivity is rarely identified in naturally occurring enzymatic processes. Although it has been more than ten years since the first in vitro reconstitution of a class II lanthionine synthetase, no structural information was available for this class of proteins before the study presented in chapter 5 with respect to the cytolysin synthetase CylM. Unexpectedly, the CylM dehydratase domain resembles the catalytic core of lipid kinases despite the absence of notable sequence homology. Mutagenesis study of CylM provides further insights into the mechanism of the modification process. The maturation of lanthipeptides typically requires a proteolytic step that removes the leader peptide from the modified precursor peptides. Characterization of two peptidases involved in the synthesis of lichenicidin and cytolysin, described in chapters 6 and 7, provides mechanistic insights into these subtilisin-like proteins and reveals their potential as sequence-specific proteases. In addition, structural elucidation of four prochlorosins, a set of lanthipeptides synthesized by a highly substrate-tolerant synthetase ProcM in marine bacteria, is included as chapter 8. The unveiled structural information of these lanthipeptides and the biochemical studies with respect to the biosynthetic process described in this thesis may assist the genome mining and synthetic biology efforts towards novel lanthipeptides for therapeutic purposes and other applications.
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