UC San Diego

Nature produces a plethora of bioactive secondary metabolites with diverse hydrocarbon scaffolds and bioactivities. These natural products serve as signaling and defense molecules in their physiological settings, and as inspiration for therapeutics in the clinic. Natural product scaffolds are elaborated with functional groups that influence their biological activities. One such chemical functionality found in thousands of natural product molecules is halogenation. Reflective of the structural diversity of halogenated natural products (halometabolites), biology has evolved an equally diverse set of enzyme catalysts using numerous strategies for activating halides for addition onto various oxidation states of carbon. The last two decades have witnessed a renaissance in the discovery of mechanistically diverse halogenating enzymes due to the unprecedented opportunity to study them in the context of halometabolite biosynthetic pathways afforded by advancements in nucleic acid sequencing technologies and computational biology. Despite the fact that the majority of halometabolites are brominated compounds derived from the marine environment, the majority of halogenting enzymes characterized to date are involved in chlorination of terrestrial microbial metabolites. Chapter 2 of this thesis describes the biosynthesis of the highly brominated pyrrole-phenol marine bacterial metabolite pentabromopseudilin by the brominated marine pyrrole/phenol (Bmp) biosynthetic pathway, which revealed the first two examples of brominating enzymes—pyrrole and phenol halogenases—from a confirmed biosynthetic context. Chapter 3 provides an in-depth investigation of regiopromiscuous pyrrole halogenase Bmp2 in contrast to canonical regioselective pyrrole chlorinases. In addition to enzymes involved in the addition of halogen atoms to the aromatic building blocks, the Bmp pathway was found to contain a dehalogenating enzyme (Bmp8) that partially undoes the work of the Bmp2 to allow a free bromopyrrole moiety to participate in the final step of pentabromopseudilin biosynthesis. Chapter 4 describes the activity and mechanism of Bmp8 as the first example of a dehalogenating tailoring enzyme from the confirmed context of a halometabolite biosynthetic pathway. Chapter 5 of this thesis concludes with an exploration of additional opportunities in the study of natural products containing halogenated pyrrole moieties, describing a strategy for the elucidation of the biosynthesis of the bioactive pyrrole-imidazole alkaloid class of sponge secondary metabolites.