Characterization of C3’ Epimerization of the Thioheptose Core in the Biosynthesis of Albomycin <i>δ</i> ₂ Catalyzed by the Radical <i>S</i> -Adenosylmethionine Enzyme AbmJ

Albomycin <i>δ</i> ₂ isolated from species of <i>Streptomyces</i> is an antibiotic notable for its oligopeptidyl thionucleoside structure. Its biosynthesis has garnered significant interest, particularly regarding the formation of its unique d-<i>xylo</i>-thioheptose core. The radical <i>S</i>-adenosylmethionine enzyme AbmJ has been shown to catalyze C3'-epimerization of the d-<i>ribo</i>-thiofuranose ring, thereby generating the d-<i>xylo</i>-thioheptose core. However, the catalytic mechanism underlying this bio-transformation remains uncharacterized. Hence, a systematic approach was taken to gain insight into AbmJ catalysis. The findings presented herein rule out a dehydrogenation route for epimerization and provide firm evidence supporting a unified mechanism for this class of enzymes. Namely, the reactive 5'-deoxyadenosyl radical, which is derived from the reductive cleavage of SAM, is shown to abstract the C3' hydrogen from the <i>pro</i>-SB-217452 substrate. The nascent substrate radical is subsequently quenched by a hydrogen-atom transfer to C3' from the opposite face, thereby completing epimerization. The highlights of this study include the identification of Cys362 as the hydrogen donor in the AbmJ reaction and the discovery that AbmJ can also catalyze epimerization when the C3'-hydroxyl group is replaced with fluorine. These results not only unravel a direct radical quenching mechanism for AbmJ-catalyzed epimerization but also demonstrate that radical SAM-catalyzed stereoinversion is not limited to carbinols.