Three Dimensional Structure of the Carbohydrate Moiety of a Lipopolysaccharide. Computer Calculations

Lipopolysaccharide, Carbohydrate Moiety, Structure The cell wall of the mutant R 595 of Salmonella minnesola contains a lipopolysaccharide in which only a trisaccharide consisting of three KDO * residues is linked to a lipid termed lipid A. Considering the sterical requirements of fourteen rotation angles, we have calculated allowed conformation of this trisyccharide unit and its linkage to lipid A. The calculations have been based on averaged crystallographic data of pyranose rings, ester-, N-acetyl-, phosphateand carboxylgroups. Because of considerable sterical hindrance, there are unique positions for the rotation around the axes 0 3C 3 of the glucosamine residue and O j -Q of KDO residue 1 as well as for the rota­ tion of the N-acetylgroup at the C2-atom of glucosamine. Similarly, the rotation angle of the carboxylgroup on KDO residue 1 and the rotation angles of the phosphategroup linked to glucos­ amine are highly restricted, while a large range of angles is allowed for the bond of the ester group to glucosamine. Chemical sequence analysis yields two possibilities for the linkage between the KDO residues 1 and 2. Linkage of 0 4 of KDO 1 in equatorial position is restricted to a narrow range of angles, whereas the linkage to 0 4 in axial and to Oa in axial and equatorial position is unfavourable. Furtheron chemical sequence analysis suggests two ways how to link KDO residue 1 and 3. The linkage to the oxygen atom on C7 of KDO residue 1 can be described by four, the linkage to the oxygen on C6 with three rotation angles. In either case two of the rotations are highly restricted. In the first case the two remaining angles have large rotational freedom, while the second case is sterically unfavorable. The feasibility of the computer calculations has been demonstrated by the construction of a three-dimensional atomic model.