Proteomics Guided Discovery of Flavopeptins: Anti-proliferative Aldehydes Synthesized by a Reductase Domain-Containing Non-ribosomal Peptide Synthetase

Due to the importance of proteases
in regulating cellular processes,
the development of protease inhibitors has garnered great attention.
Peptide-based aldehydes are a class of compounds that exhibit inhibitory
activities against various proteases and proteasomes in the context
of anti-proliferative treatments for cancer and other diseases. More
than a dozen peptide-based natural products containing aldehydes have
been discovered such as chymostatin, leupeptin, and fellutamide; however,
the biosynthetic origin of the aldehyde functionality has yet to be
elucidated. Herein we describe the discovery of a new group of lipopeptide
aldehydes, the flavopeptins, and the corresponding biosynthetic pathway
arising from an orphan gene cluster in Streptomyces sp. NRRL-F6652, a close relative of Streptomyces flavogriseus ATCC 33331. This research was initiated using a proteomics approach
that screens for expressed enzymes involved in secondary metabolism
in microorganisms. Flavopeptins are synthesized through a non-ribosomal
peptide synthetase containing a terminal NAD­(P)­H-dependent reductase
domain likely for the reductive release of the peptide with a C-terminal
aldehyde. Solid-phase peptide synthesis of several flavopeptin species
and derivatives enabled structural verification and subsequent screening
of biological activity. Flavopeptins exhibit sub-micromolar inhibition
activities against cysteine proteases such as papain and calpain as
well as the human 20S proteasome. They also show anti-proliferative
activities against multiple myeloma and lymphoma cell lines.