UC San Diego

Pyrroloquinoline alkaloids have been extensively isolated in the past three decades due to their broad-spectrum bioactivities. Although many total synthesis reports have described the unique chemistry of these tricyclic compounds, the biosynthesis of this well-studied natural product class remains elusive despite their simple structures. In this work, cryptic genes of the previously validated ammosamide biosynthetic gene cluster were subjects of in vitro biochemical investigation. Amm3, the putative FADH2-dependent tryptophan halogenase, was successfully expressed, purified, and screened for optimal crystallization conditions. Amm3 does not appear to bind flavin, nor does it halogenate a free-tryptophan substrate. Amid these experiments, it was reported by Ortega and colleagues that MibH, the Microbispora tryptophan-5-halogenase that is the closest homologue to Amm3, set the first precedent of acting on a peptide substrate. In this vein, the amm6 peptide was found to successfully express under quick and aggressive growth conditions in Escherichia coli, and appears to be a promising substrate candidate for Amm3 halogenation.