Changing Fates of the Substrate Radicals Generated in the Active Sites of the B₁₂-Dependent Radical SAM Enzymes OxsB and AlsB

OxsB is a B12-dependent radical SAM enzyme that catalyzes the oxidative ring contraction of 2′-deoxyadenosine 5′-phosphate to the dehydrogenated, oxetane containing precursor of oxetanocin A phosphate. AlsB is a homologue of OxsB that participates in a similar reaction during the biosynthesis of albucidin. Herein, OxsB and AlsB are shown to also catalyze radical mediated, stereoselective C2′-methylation of 2′-deoxyadenosine monophosphate. This reaction proceeds with inversion of configuration such that the resulting product also possesses a C2′ hydrogen atom available for abstraction. However, in contrast to methylation, subsequent rounds of catalysis result in C–C dehydrogenation of the newly added methyl group to yield a 2′-methylidene followed by radical addition of a 5′-deoxyadenosyl moiety to produce a heterodimer. These observations expand the scope of reactions catalyzed by B12-dependent radical SAM enzymes and emphasize the susceptibility of radical intermediates to bifurcation along different reaction pathways even within the highly organized active site of an enzyme

Targeting Tumor Associated Phosphatidylserine with New Zinc Dipicolylamine-Based Drug Conjugates

A series of zinc­(II) dipicolylamine (ZnDPA)-based drug conjugates have been synthesized to probe the potential of phosphatidylserine (PS) as a new antigen for small molecule drug conjugate (SMDC) development. Using <i>in vitro</i> cytotoxicity and plasma stability studies, PS-binding assay, <i>in vivo</i> pharmacokinetic studies, and maximum tolerated dose profiles, we provided a roadmap and the key parameters required for the development of the ZnDPA based drug conjugate. In particular, conjugate <b>24</b> induced tumor regression in the COLO 205 xenograft model and exhibited a more potent antitumor effect with a 70% reduction of cytotoxic payload compared to that of the marketed irinotecan when dosed at the same regimen. In addition to the validation of PS as an effective pharmacodelivery target for SMDC, our work also provided the foundation that, if applicable, a variety of therapeutic agents could be conjugated in the same manner to treat other PS-associated diseases