Abstract
Ranking among the most effective anticancer drugs, anthracyclines represent an important family of aromatic polyketides generated by type II polyketide synthases (PKSs). After formation of polyketide cores, the post-PKS tailoring modifications endow the scaffold with various structural diversities and biological activities. Here we demonstrate an unprecedented four-enzyme-participated hydroxyl regioisomerization process involved in the biosynthesis of kosinostatin. First, KstA15 and KstA16 function together to catalyze a cryptic hydroxylation of the 4-hydroxyl-anthraquinone core, yielding a 1,4-dihydroxyl product, which undergoes a chemically challenging asymmetric reduction-dearomatization subsequently acted by KstA11; then, KstA10 catalyzes a region-specific reduction concomitant with dehydration to afford the 1-hydroxyl anthraquinone. Remarkably, the shunt product identifications of both hydroxylation and reduction-dehydration reactions, the crystal structure of KstA11 with bound substrate and cofactor, and isotope incorporation experiments reveal mechanistic insights into the redox dearomatization and rearomatization steps. These findings provide a distinguished tailoring paradigm for type II PKS engineering.