Biochemical characterization and substrate specificity of the gene cluster for biosyntheses of K-252a and its analogs by in vitro heterologous expression system of Escherichia coli

Abstract

The indolocarbazole family of natural products has attracted great attention because of their unique structural features and potential therapeutic applications. Structurally distinct in the family, K-252a is characterized by an unusual dihydrostreptose moiety cross-bridged to K-252caglycone with two C–N linkages. K-252a has served as a valuable lead for treatments of various cancers and neurodegenerative disorders. Recent cloning of the nokgene cluster for biosyntheses of K-252a and its analogs from Nocardiopsis sp. K-252 (NRRL15532) has revealed the nokABCDgenes indispensible for K-252cbiosynthesis and the key gene (nokL) coding for N-glycosylation. Herein, we report the first, successful demonstration of in vitrosugartransferase activity of indolocarbazole N-glycosyltransferase (NokL) by use of soluble protein expressed from Escherichia coli. Notably, NokL was found to exhibit peculiar mode of substrate promiscuity. Moreover, NokA and NokB reactions were biochemically characterized thoroughly by natual and alternative (e.g.fluoro-) substrates and by ammonium hydroxide (NH₄OH). Interestingly, the in vitro expression of NokA revealed high substrate stereoselectivity, giving several indole-3-pyruvic acid-derived compounds, including indol-3-carboxaldehyde (ICA) and indole-3-acetic acid. The use of NH₄OH successfully dissected the in vitro NokA/NokB coupled reaction, revealing mechanistic insight into the enzymes and their cross-talking relationship. Also, a simple, useful method to synthesize K-252d, ICA and chromopyrrolic acid (the NokB product) was developed by the E. coli expression systems of NokL, NokA and NokA/NokB, respectively. Together with NokA and NokB, NokL may serve as a useful tool for combinatorial engineering of K-252a and its analogs for improved therapeutic values.