3-Deoxy-D-manno-octulosonate 8-phosphate synthase (KDO8PS) is only expressed in plants and Gram-negative bacteria, where it is involved in the lipopolysaccharide biosynthesis pathway.¹ KDO8PS is closely related to the enzyme 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS), which catalyses the first step of the shikimate pathway.

 KDO8PS catalyses the condensation of arabinose 5-phosphate (A5P) and phosphoenolpyruvate (PEP) to form 3-deoxy-D-manno-octulosonate 8-phosphate (KDO8P).² In contrast DAH7PS catalyses the reaction between PEP and erythrose 4-phosphate (E4P).  Many of the key steps of these reactions have been previously determined.

 There are slight differences between the active-sites of KDO8PS and DAH7PS. In DAH7PS there is a conserved glutamate residue, which in KDO8PS is conserved as an aspartate residue. In DAH7PS, the catalytic glutamate residue has been suggested to take part in a putative proton relay chain critical for the catalytic mechanism.

 We have examined the role of the conserved active site aspartate in KDO8PS by examining variants where this residue and a neighbouring proline residue are substituted.  The functional properties of these variants and their structures shed light on both the catalytic mechanism of KDO8PS and the evolutionary relationship between KDO8PS and DAH7PS.