Malbrancheamide is a complex halogenated indole alkaloid with a dichlorinated indole ring that is imperative to its biological activity. Synthetic halogenation methods for complex molecules like malbrancheamide have posed a challenge due to the abundance of chemically equivalent sites. With a large number of natural products undergoing late-stage functionalization by tailoring enzymes, there is a unique opportunity to use biocatalysis to perform difficult chemical transformations.
In this work, a flavin-dependent halogenase called MalA was characterized and used to generate a variety of halogenated malbrancheamide analogues. Structural analysis and computational studies of MalA were utilized to elucidate the enzymatic mechanism of site-selective chlorination. A serine residue positioned nearest to the inherently less favorable C8-H was shown to facilitate chlorination at this site.
Based on this mechanism, engineering of the active site region led to site selective mutants of MalA. With a new understanding of the mechanism of MalA-mediated selective halogenation, future efforts will focus on rational engineering of MalA to modulate its regioselectivity, substrate scope, and iterative function.