Preliminary X-ray analysis of a new crystal form of the Escherichia coli KDO8P synthase

3-Deoxy-D-mnnno-octulosonate 8-phosphate (KDO8P) synthase catalyzes the bio synthesis of an essential component of the Lipopolysaccharide of all Gram-n egative bacteria. The structure and mechanism of KDO8P synthase are being a ctively studied as this enzyme represents an important target for antibioti c therapy. The structure of the Escherichia coli KDO8P synthase in cubic cr ystals (space group I23) has recently been determined and the enzyme shown to be a tetramer of identical subunits. However, this information is challe nged by biochemical studies, which suggest that the enzyme behaves in solut ion as a homotrimer. Here, the preparation and preliminary X-ray analysis o f monoclinic crystals of KDO8P synthase are reported. The crystals belong t o space group P2(1), with unit-cell parameters a similar or equal to 50, b similar or equal to 140, c similar or equal to 74 Angstrom, beta similar or equal to 105 degrees. The structure of KDO8P synthase in the monoclinic cr ystal form was determined by molecular replacement, using as a search model one of the subunits of the enzyme in the cubic crystals. A tetramer of KDO 8P synthase with 222 local symmetry is also present in the asymmetric unit of the P2(1) crystals, with a solvent content of 43%. The observation that the same quaternary structure of KDO8P synthase is observed in two differen t crystal forms belonging to distinct crystal systems (monoclinic and cubic ) suggests that a tetramer is the native form of the enzyme.