Fig. 5: Biosynthetic proposal leading to the formation of crocagins A (1) and B (2).
From: Unusual peptide-binding proteins guide pyrroloindoline alkaloid formation in crocagin biosynthesis
a, Proposed mechanism for installation of the tetracyclic crocagin core. b, CgnB, CgnC and CgnE install two β-hydroxylations and facilitate the two ring-closure reactions. CgnD cleaves the core scaffold off the leader peptide, which is subsequently methylated and carbamoylated. Crocagin B, the low abundance crocagin in extracts from the natural producer, appears to originate from oxidation before ring closure during the biosynthesis of 3a to yield 3b.
