Enzymatic Stereoselective Nucleophilic Cyclization Governs Atypical Spirotetronate Assembly in Lucensimycin A
Meng Yu Xi1, Bo Zhang1, Ting Peng1
1State Key Laboratory of Pharmaceutical Biotechnology, Department of Neurology, Nanjing Drum Tower Hospital, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Life Sciences, Nanjing University, Nanjing 210023, China.
None:
Lucensimycin A is a structurally unique spirotetronate polyketide featuring a rare spiro[tetronate-hydrophenanthrene] tetracyclic core, distinct from the classical spiro[tetronate-cyclohexene] scaffolds formed via intramolecular Diels-Alder (IMDA) cyclizations. Here, we identified and characterized the luc biosynthetic gene cluster from Streptomyces fagopyri NAX0062, revealing a divergent biosynthetic logic. The pathway begins with type I PKS assembly of a linear polyketide, followed by tetronate ring formation by a canonical tetronate cassette. A flavin-dependent Diels-Alderase (LucM) then catalyzes an IMDA reaction to form a decalin intermediate. Unusually, the Diels-Alderase homologue LucK catalyzes a stereoselective intramolecular nucleophilic cyclization─rather than a pericyclic reaction─to generate the spiro[tetronate-hydrophenanthrene] core, following acetylation by LucN. Oxidative cleavage of a terminal alkene (by LucO3) completes the pathway. Structural and mutational analysis of LucK revealed that Glu16 and Glu85 function as general acid/base catalysts to drive the nucleophilic cyclization reaction, highlighting LucK as a mechanistically distinct cyclase. This work uncovers a previously unrecognized enzymatic strategy for spirocyclic construction and expands the catalytic repertoire of β-barrel enzymes in polyketide biosynthesis.
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