Structure of an antibiotic-synthesizing UDP-glucuronate 4-epimerase MoeE5 in complex with substrate

The epimerase MoeE5 from Streptomyces viridosporus converts UDP-glucuronic acid (UDP-GIcA) to UDP-galacturonic acid (UDP-GaIA) to provide the first sugar in synthesizing moenomycin, a potent inhibitor against bacterial peptidoglycan glycosyltransferases. The enzyme belongs to the UDP-hexose 4-epimerase family, and uses NAD(+) as its cofactor. Here we present the complex crystal structures of MoeE5/NAD(+)/UDP-GIcA and MoeE5/NAD(+)/UDP-glucose, determined at 1.48 angstrom and 1.66 angstrom resolution. The cofactor NAD(+) is bound to the N-terminal Rossmann-fold domain and the substrate is bound to the smaller C-terminal domain. In both crystals the C4 atom of the sugar moiety of the substrate is in close proximity to the C4 atom of the nicotinamide of NAD(+), and the O4 atom of the sugar is also hydrogen bonded to the side chain of Tyr154, suggesting a productive binding mode. As the first complex structure of this protein family with a bound UDP-GIcA in the active site, it shows an extensive hydrogen-bond network between the enzyme and the substrate. We further built a model with the product UDP-GalA, and found that the unique Arg192 of MoeE5 might play an important role in the catalytic pathway. Consequently, MoeE5 is likely a specific epimerase for UDP-GIcA to UDP-GaIA conversion, rather than a promiscuous enzyme as some other family members. (C) 2019 Elsevier Inc. All rights reserved.