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

Biochemical Characterization of an Arabinoside Monophosphate Specific 5'-Nucleotidase-like Enzyme from Streptomyces antibioticus

To investigate the function of the gene penF in the pentostatin and vidarabine (Ara-A) biosynthetic gene cluster in Streptomyces antibioticus NRRL 3238, PenF was recombinantly expressed and characterized. Enzymatic characterization of the enzyme demonstrated that PenF exhibited metal-dependent nucleoside 5'-monophosphatase activity, showing a substrate preference for arabinose nucleoside 5'-monophosphate over 2'-deoxyribonucleoside 5'-monophosphate and ribonucleoside 5'-monophosphate. Metal ions such as Mg2+ and Mn2+ significantly enhanced enzyme activity, whereas Zn2+, Cu2+, and Ca2+ inhibited it. For vidarabine 5'-monophosphate, the Km and kcat values were determined to be 71.5 μM and 33.9 min-1, respectively. The kcat/Km value was 474.1 mM-1·min-1 for vidarabine 5-monophosphate and was 68-fold higher than that for 2'-deoxyadenosine 5'-monophosphate. Comparative sequence alignment and structural studies suggested that residues outside the primary substrate-binding site are responsible for this substrate specificity. In conclusion, PenF's activity toward vidarabine 5'-monophosphate likely plays a role in the dephosphorylation of precursors during Ara-A biosynthesis.

Advances in the understanding and exploitation of carbohydrate-active enzymes

Carbohydrate-active enzymes (CAZymes) are responsible for the biosynthesis, modification and degradation of all glycans in Nature. Advances in genomic and metagenomic methodologies, in conjunction with lower cost gene synthesis, have provided access to a steady stream of new CAZymes with both well-established and novel mechanisms. At the same time, increasing access to cryo-EM has resulted in exciting new structures, particularly of transmembrane glycosyltransferases of various sorts. This improved understanding has resulted in widespread progress in applications of CAZymes across diverse fields, including therapeutics, organ transplantation, foods, and biofuels. Herein, we highlight a few of the many important advances that have recently been made in the understanding and applications of CAZymes.

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.