An Iterative, Bimodular Nonribosomal Peptide Synthetase that Converts Anthranilate and Tryptophan into Tetracyclic Asperlicins

•NRPS AspA is reconstituted to synthesize multicyclic fungal alkaloids asperlicin C and D•The first module of AspA iteratively activates two molecules of anthranilate•CT cyclizes Ant-Ant-Trp into a macrolactam that undergoes transannular cyclization•Kinetic factors determine the regiochemical outcome of the transannular cyclizations

SummaryThe bimodular 276 kDa nonribosomal peptide synthetase AspA from Aspergillus alliaceus, heterologously expressed in Saccharomyces cerevisiae, converts tryptophan and two molecules of the aromatic β-amino acid anthranilate (Ant) into a pair of tetracyclic peptidyl alkaloids asperlicin C and D in a ratio of 10:1. The first module of AspA activates and processes two molecules of Ant iteratively to generate a tethered Ant-Ant-Trp-S-enzyme intermediate on module two. Release is postulated to involve tandem cyclizations, in which the first step is the macrocyclization of the linear tripeptidyl-S-enzyme, by the terminal condensation (CT) domain to generate the regioisomeric tetracyclic asperlicin scaffolds. Computational analysis of the transannular cyclization of the 11-membered macrocyclic intermediate shows that asperlicin C is the kinetically favored product due to the high stability of a conformation resembling the transition state for cyclization, while asperlicin D is thermodynamically more stable.