Biosynthesis and Structure-Activity Relationship Studies of Okaramines That Target Insect Glutamate-Gated Chloride Channels.

Prenylated indole alkaloid okaramines selectively target insect glutamate-gated chloride channels (GluCls). Because of their highly complex structures, including azocine and azetidine rings, total synthesis of okaramine A or B has not been achieved, preventing evaluation of the biological activities of okaramines. Biosynthetic approaches provide alternatives to accessing structurally diverse derivatives and enabling the elucidation of structure-activity relationships. To explore the biosynthetic potential of okaramines, gene knockout experiments of an okaramine-producer fungus were performed. The deletion mutants of the oxygenase genes okaB, okaD, okaE, and okaG provided analogues that were unlikely to be accumulated in the normal biosynthetic process of the wild-type strain. Analysis of the structure-activity relationships of okaramines collected from the fungal cultures revealed that 1,4-dihydroazocine and N-aliphatic group attached to the indole were crucial for GluCl-activating activity. This provided insights into further derivatization of the complex structure of okaramines in order to facilitate the development of new insecticides.