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Alkaloid Biosynthetic Enzyme Generates Diastereomeric Pair via Two Distinct Mechanisms.

Ergopeptines constitute one of the representative classes of ergoline alkaloids and carry a tripeptide extension on the lysergic acid core. In the current study, we discovered and structurally characterized newly isolated ergopeptine-like compounds named lentopeptins from a filamentous fungus Aspergillus lentulus, a close relative of A. fumigatus. Interestingly, in lentopeptins, the common lysergic acid moiety of ergopeptines is replaced by a cinnamic acid moiety at the N-terminus of the peptide segment. Moreover, lentopeptins lack the C-terminal proline residue necessary for the spontaneous cyclization of the peptide extension. Herein, we report the atypical lentopeptin biosynthetic pathway identified through targeted deletion of the len cluster biosynthetic genes predicted from the genome sequence. Further in vitro characterizations of the thiolation-terminal condensation-like (T-CT) didomain of the nonribosomal peptide synthetase LenA and its site-specific mutants revealed the mechanism of peptide release via diketopiperazine formation, an activity previously unreported for CT domains. Most intriguingly, in vitro assays of the cytochrome P450 LenC illuminated the unique mechanisms to generate two diastereomeric products. Lentopeptin A forms via a stereospecific hydroxylation, followed by a spontaneous bicyclic lactam core formation, while lentopeptin B is produced through an initial dehydrogenation, followed by a bicyclic lactam core formation and stereospecific hydration. Our results showcase how nature exploits common biosynthetic enzymes to forge new complex natural products effectively (213/250).