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The outer core (OC) region of Yersinia enterocoliticaserotype O:3 lipopolysaccharide is a hexasaccharide essential for the integrity of the outer membrane. It is involved in resistance against cationic antimicrobial peptides and plays a role in virulence during early phases of infection. We show here that the proximal residue of the OC hexasaccharide is a rarely encountered 4‐keto‐hexosamine, 2‐acetamido‐2,6‐dideoxy‐D‐xylo‐hex‐4‐ulopyranose (Sugp) and that WbcP is a UDP‐GlcNAc‐4,6‐dehydratase enzyme responsible for the biosynthesis of the nucleotide‐activated form of this rare sugar converting UDP‐2‐acetamido‐2‐deoxy‐D‐glucopyranose (UDP‐D‐GlcpNAc) to UDP‐2‐acetamido‐2,6‐dideoxy‐D‐xylo‐hex‐4‐ulopyranose (UDP‐ Sugp). In an aqueous environment, the 4‐keto group of this sugar was present in the 4‐dihydroxy form, due to hydration. Furthermore, evidence is provided that the axial 4‐hydroxy group of this dihydroxy function was crucial for the biological role of the OC, that is, in the bacteriophage and enterocoliticin receptor structure and in the epitope of a monoclonal antibody.