Identification of the Early Steps in Herbicidin Biosynthesis Reveals an Atypical Mechanism of C-Glycosylation

Abstract

Herbicidins are adenosine-derived nucleoside antibiotics with an unusual tricyclic core structure. Deletion of the genes responsible for formation of the tricyclic skeleton in Streptomyces sp. L-9-10 reveals the in vivo importance of Her4, Her5, and Her6 in the early stages of herbicidin biosynthesis. In vitro characterization of Her4 and Her5 demonstrates their involvement in an initial, two-stage C–C coupling reaction that results in net C5′-glycosylation of ADP/ATP by UDP/TDP–glucuronic acid. Biochemical analyses and intermediate trapping experiments imply a noncanonical mechanism of C-glycosylation reminiscent of NAD-dependent S-adenosylhomocysteine (SAH)-hydrolase catalysis. Structural characterization of the isolated metabolites suggests possible reactions catalyzed by Her6 and Her7. An overall herbicidin biosynthetic pathway is proposed based on these observations.

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

“…When OxsB was incubated for 15 h with SAM, DTT, MV, MgCl 2 , and HO-Cbl under the above conditions with 0.2 mM substrate analogues possessing either a 3′-O-acetyl (12) or a 3′-Omethyl (13) substituent, LCMS analysis revealed enzymedependent formation of the corresponding methylated products, dehydrogenation products, and heterodimers. In contrast, the use of substrate analogues either having a 3′-F ( 14), 3′-epi-OH (15), or 3′-NH 2 (16) substituent (Figure 3B) only led to observable formation of the respective methylation products and dehydrogenation products.…”

“…A subgroup of radical SAM enzymes also binds cobalamin, which in most cases studied to date mediates net transfer of a methyl group from a second molecule of SAM to the substrate radical or anion via an intermediary methylcobalamin donor. ,− However, this class of enzymes does not appear to be strictly limited to catalyzing methylation reactions, as proposed during the biosynthesis of ladderanes, hopanoids, bacteriochlorophyll, and herbicidins (Figure S1). Moreover, an increasing number of cobalamin-dependent radical SAM enzymes have been shown experimentally to catalyze reactions other than methylation in vitro . Examples include OxsB, which binds a single [Fe 4 S 4 ] cluster in addition to cobalamin and operates together with its partner enzyme OxsA to catalyze the formation of intermediate ( 4 ) from 2′-deoxyadenosine monophosphate (2′-dAMP, 3 ) during the biosynthesis of oxetanocin A ( 5 ) .…”

Copper/photoredox catalysis enables desulfonylative radical N-glycosylation

“…For example, nucleosides bearing heteroaromatic nitrogen motifs are the key building blocks of nucleic acids, and nucleoside analogs are commonly found in natural products (e.g. Herbicidin) [9] and widely used in drug development (e.g. Ribavirin) [10] (Figure 1a).…”