Mechanistic insights into polycycle formation by reductive cyclization in ikarugamycin biosynthesis

Area of Science:

• Biochemistry
• Natural Product Biosynthesis
• Enzymology

Background:

• Ikarugamycin belongs to the polycyclic tetramate macrolactams (PTMs), a class of natural products with significant biological activities.
• The biosynthetic pathways for constructing the complex polycyclic frameworks of PTMs are not well understood.
• Understanding these mechanisms is crucial for the discovery and engineering of novel bioactive compounds.

Purpose of the Study:

• To elucidate the enzymatic mechanism responsible for the formation of the inner five-membered ring in ikarugamycin.
• To identify the key enzymes and reactions involved in this crucial cyclization step.
• To provide biochemical insights into polycyclic ring formation within the PTM family.

Main Methods:

• Genetic analysis of the ikaABC gene cluster from marine-derived Streptomyces sp. ZJ306.
• Heterologous expression of the ikaABC cassette to confer ikarugamycin production.
• Biochemical characterization of IkaC enzyme activity, focusing on the cyclization reaction.

Main Results:

• The three-gene cassette ikaABC was found to be sufficient for ikarugamycin production in a heterologous host.
• The enzyme IkaC was identified as the catalyst for a reductive cyclization reaction.
• This reaction forms the characteristic inner five-membered ring via a Michael addition-like mechanism.

Conclusions:

• This study presents the first biochemical evidence for polycyclic ring formation in PTMs.
• A novel reductive cyclization strategy for PTM biosynthesis has been proposed.
• This mechanism may be applicable to the synthesis of other PTMs, offering potential for future drug discovery.