(a) Overlay of binding sites of S25-2 in complex with Kdo-(2→4)-5,6-dehydro-Kdo (yellow; interactions shown in green) and Kdo-(2→4)-Kdo (white; interactions shown in red). In Kdo-(2→4)-Kdo the 5-OH group of the second Kdo residue forms a hydrogen bond to Asn53 (heavy chain) orienting the Kdo to form a salt bridge with Arg27f (light chain). The lack of the 5-OH group in the second Kdo of Kdo-(2→4)-5,6-dehydro-Kdo results in different interactions in the binding site. (b) Stereoview of Kdo-(2→4)-5,6-dehydro-Kdo (yellow) and Kdo-(2→4)-Kdo (white), which exhibit different orientations in the binding site. Amino-acid residues in the structure are numbered according to the Kabat numbering scheme for antibodies (Kabat et al., 1983); (H) refers to residues on the heavy chain and (L) refers to residues on the light chain.