Structure of the Escherichia coli Heptosyltransferase WaaC: Binary Complexes with ADP AND ADP-2-deoxy-2-fluoro Heptose

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Lipopolysaccharides constitute the outer leaflet of the outer membrane of Gram-negative bacteria and are therefore essential for cell growth and viability. The heptosyltransferase WaaC is a glycosyltransferase (GT) involved in the synthesis of the inner core region of LPS. It catalyzes the addition of the first l- glycero- d- manno-heptose (heptose) molecule to one 3-deoxy- d- manno-oct-2-ulosonic acid (Kdo) residue of the Kdo 2-lipid A molecule. Heptose is an essential component of the LPS core domain; its absence results in a truncated lipopolysaccharide associated with the deep-rough phenotype causing a greater susceptibility to antibiotic and an attenuated virulence for pathogenic Gram-negative bacteria. Thus, WaaC represents a promising target in antibacterial drug design. Here, we report the structure of WaaC from the Escherichia coli pathogenic strain RS218 alone at 1.9 Å resolution, and in complex with either ADP or the non-cleavable analog ADP-2-deoxy-2-fluoro-heptose of the sugar donor at 2.4 Å resolution. WaaC adopts the GT-B fold in two domains, characteristic of one glycosyltransferase structural superfamily. The comparison of the three different structures shows that WaaC does not undergo a domain rotation, characteristic of the GT-B family, upon substrate binding, but allows the substrate analog and the reaction product to adopt remarkably distinct conformations inside the active site. In addition, both binary complexes offer a close view of the donor subsite and, together with results from site-directed mutagenesis studies, provide evidence for a model of the catalytic mechanism.