Using peptide substrate analogs to characterize a radical intermediate in NosN catalysis

Area of Science:

• Biochemistry
• Organic Chemistry
• Microbiology

Background:

• Nosiheptide is a potent thiopeptide antibiotic effective against Gram-positive bacteria.
• Its complex structure includes a macrocycle with thiazole rings and unique modifications.
• A key modification involves a 3,4-dimethyl-2-indolic acid (DMIA) moiety, formed by the radical S-adenosylmethionine (SAM) enzyme NosN.

Purpose of the Study:

• To elucidate the mechanism of the NosN enzyme in nosiheptide biosynthesis.
• To investigate the formation of a crucial radical intermediate during DMIA moiety synthesis.
• To develop synthetic strategies for detecting this radical intermediate.

Main Methods:

• Utilized synthetic approaches to mimic the NosN enzymatic reaction.
• Employed electron paramagnetic resonance (EPR) spectroscopy for radical detection.
• Investigated the role of two SAM molecules in the radical generation pathway.

Main Results:

• Demonstrated that NosN transfers a methylene group from SAM to 3-methylindolic acid (MIA).
• Provided evidence for the formation of a highly electrophilic species and a subsequent radical intermediate on the MIA substrate.
• Successfully developed methods enabling the detection of this proposed radical intermediate via EPR spectroscopy.

Conclusions:

• The study clarifies the function of NosN in forming the DMIA side-ring system in nosiheptide.
• The findings support a mechanism involving radical intermediates generated by SAM cleavage and abstraction.
• The developed synthetic and spectroscopic methods offer a new tool for studying similar radical-mediated biosynthetic pathways.