Twitch Radical S ‐adenosyl‐ l ‐methionine Alcohol Dehydrogenases

Abstract Radical S ‐adenosyl‐ l ‐methionine (SAM) enzymes are currently the largest known group of metalloenzymes. These enzymes catalyze an extraordinarily broad range of chemical reactions via radical‐mediated catalytic cycles that begin with the reductive cleavage of SAM by an active site [Fe 4 S 4 ] cluster. The twitch subgroup of radical SAM enzymes also binds a second, auxiliary [Fe 4 S 4 ] cluster, which is believed to participate in the reaction though its precise catalytic role remains uncertain. Like their parent group, the twitch radical SAM enzymes are also known to catalyze several different types of reactions acting as epimerases, dehydrogenases, and lyases. Currently, two twitch radical SAM alcohol dehydrogenases have been characterized in vitro . The first is BtrN, which catalyzes the dehydrogenation of 2‐deoxy‐ scyllo ‐inosamine to yield 3‐amino‐2,3‐dideoxy‐ scyllo ‐inosose during the biosynthesis of the aminoglycoside antibiotics butirosin A and B. The second is SpeY, which catalyzes the dehydrogenation of (2′ R ,3′ S )‐tetrahydrospectinomycin and thereby facilitates formation of the dioxane bridge in spectinomycin, which is another aminoglycoside natural product. Moreover, SpeY is related to HygY from the hygromycin B biosynthetic pathway that normally operates as a twitch radical SAM epimerase on a substrate structurally similar to that of SpeY. The activities of these homologs can be interchanged via the mutation of a single homologous cysteine/serine residue thereby converting the HygY mutant into a twitch alcohol dehydrogenase as well. Much work has been done to characterize the structures and mechanisms of these enzymes and is summarized here; however, many questions still remain for future investigation.