Chemo-Enzymatic Synthesis of Bioactive Carbazole Derivatives

Carbazoles are key scaffolds of either antimicrobial/antiviral alkaloid natural products or therapeutics. As such, access to structurally diverse indole-containing carbazoles has attracted considerable attention. In this report, a pilot study is described using biotransformation to provide carbazoles that contain various acyl substituents. The biotransformation system contains the thiamine-diphosphate (ThDP)-dependent enzyme NzsH, the FabH-like 3-ketoacyl-ACP synthase NzsJ, and the aromatase/cyclase NzsI, encoded in the biosynthetic gene cluster (nzs) of the bacterial carbazole alkaloid natural product named neocarazostatin A. The utilization of a range of acyl-SNACs (synthetic acyl-thioester analogues of the native substrate) together with indole-3-pyruvate and pyruvate in the designed biotransformation system allows production of carbazole derivatives. Our results demonstrate that this three-enzyme system displays a considerable substrate profile toward acyl donors for production of carbazoles with different acyl substituents. Finally, two more enzymes were included in the biotransformation system: the tryptophan synthase stand-alone β-subunit variant, PfTrpB, generated from directed evolution in the literature, and a commercially available L-amino acid oxidase (LAAO). The addition of these two enzymes allows the transformation to start with indole building blocks to provide carbazoles with modifications in the indole ring system.