Structural and Mechanistic Insights into 4'-Dehydrogenation Catalyzed by AprD5 in Apramycin Biosynthesis.

AprD5 is a member of the NAD+-dependent short-chain dehydrogenase/reductase (SDR) superfamily. It participates in the apramycin biosynthetic pathway by catalyzing 4'-oxidation of UDP-β-d-glucose (UBG) to initiate the assembly of UDP-4'-amino-4'-deoxy-β-d-glucose in partnership with the aminotransferase AprL. Here, we report a 1.9 Å crystal structure of AprD5 in complex with NAD+ and its substrate UBG, which reveals a well-ordered ternary complex poised for catalysis. The cofactor and substrate are precisely aligned for hydride transfer by a conserved Ser-Tyr-Lys catalytic triad, while the nucleotide-sugar is constrained in a single reactive conformation through an extensive hydrogen-bonding network and a tight electrostatic clamp on the diphosphate group. Although 4'-oxidation is a common initiating step in SDR-catalyzed 4'-epimerization and 4',6'-dehydration, the structural features that direct these closely related enzymes toward distinct reaction outcomes remain poorly understood. Comparative structural analysis reveals that AprD5 lacks the catalytic architecture required for dehydration while a rigid phosphate clamp and extensive substrate-anchoring network restrict the conformational rearrangements necessary for epimerization. Together, these features suggest how a conserved SDR scaffold is specialized for dedicated 4'-dehydrogenation and highlight general structural principles that may govern catalytic divergence among nucleotidyl-sugar SDR enzymes.