Characterization and Mechanistic Studies of Desll: A Radical S-Adenosyl-L-methionine Enzyme Involved in the Biosynthesis of TDP-D-Desosamine

D-Desosamine (1) is a 3-(N,N-dimethylamino)-3,4,6-trideoxyhexose found in a number of macrolide antibiotics including methymycin (2), neomethymycin (3), pikromycin (4), and narbomycin (5) produced by Streptomyces venezuelae. It plays an essential role in conferring biological activities to its parent aglycones. Previous genetic and biochemical studies of the biosynthesis of desosamine in S. venezuelae showed that the conversion of TDP-4-amino-4,6-dideoxy-D-glucose (8) to TDP-3-keto-4,6-dideoxy-D-glucose (9) is catalyzed by Desll, which is a member of the radical S-adenosyl-L-methionine (SAM) enzyme superfamily. Here, we report the purification and reconstitution of His(6)-tagged Desll, characterization of its [4Fe-4S] cluster using UV-vis and EPR spectroscopies, and the capability of flavodoxin, flavodoxin reductase, and NADPH to reduce the [4Fe-4S](2+) cluster. Also included are a steady-state kinetic analysis of Desll-catalyzed reaction and an investigation of the substrate flexibility of Desll. Studies of deuterium incorporation into SAM using TDP-[3-H-2]-4-amino-4,6-dideoxy-D-glucose as the substrate provides strong evidence for direct hydrogen atom transfer to a 5'-deoxyadenosyl radical in the catalytic cycle. The fact that hydrogen atom abstraction occurs at C-3 also sheds light on the mechanism of this intriguing deamination reaction.