The 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. © 2013 Elsevier Ltd.
Gao, X., Jiang, W., Jiménez-Osés, G., Choi, M. S., Houk, K. N., Tang, Y., & Walsh, C. T. (2013). An iterative, bimodular nonribosomal peptide synthetase that converts anthranilate and tryptophan into tetracyclic asperlicins. Chemistry and Biology, 20(7), 870–878. https://doi.org/10.1016/j.chembiol.2013.04.019