Structure and mechanism of 3-deoxy-D-manno-octulosonate 8-phosphate synthase

3-Deoxy-D-manno-octulosonate 8-phosphate (KDO8P) synthase catalyzes the con densation of phosphoenolpyruvate (PEP) with arabinose B-phosphate (A5P) to form KDO8P and inorganic phosphate, KDO8P is the phosphorylated precursor o f 3-deoxy-D-manno-octulosonate, an essential sugar of the lipopolysaccharid e of Gram-negative bacteria. The crystal structure of the Escherichia coli KDO8P synthase has been determined by multiple wavelength anomalous diffrac tion and the model has been refined to 2.4 Angstrom (R-factor, 19.9%; R-fre e, 23.9%). KDO8P synthase is a homotetramer in which each monomer has the f old of a (beta/alpha)(8) barrel. On the basis of the features of the active site, PEP and A5P are predicted to bind with their phosphate moieties 13 A ngstrom apart such that KDO8P synthesis would proceed via a linear intermed iate. A reaction similar to KDO8P synthesis, the condensation of phosphoeno lpyruvate, and erythrose 4-phosphate to form 3-deoxy-D-arabino-heptulosonat e 7-phosphate (DAH7P), is catalyzed by DAH7P synthase, In the active site o f DAH7P synthase the two substrates PEP and erythrose 4-phosphate appear to bind in a configuration similar to that proposed for PEP and A5P in the ac tive site of KDO8P synthase, This observation suggests that KDO8P synthase and DAH7P synthase evolved from a common ancestor and that they adopt the s ame catalytic strategy.