Mechanistic studies of 3-deoxy-D-manno-2-octulosonate-8-phosphate synthase from Escherichia coli.

Abstract

The anomeric specificity and the steady-state kinetic mechanism of homogeneous 3-deoxy-D-manno-2-octulosonate-8-phosphate (KDO8P) synthase were investigated. The open-chain 4-deoxy analogue of arabinose-5-phosphate (Ara5P), which is structurally prohibited from undergoing ring closure, was synthesized and tested as a substrate for the synthase. It was found that the analogue functions as a substrate with a similar kcat value to that of the original substrate. The kcat/Km value for the natural substrate is seven-times greater than that of the 4-deoxy analogue. However, taking into account the 9.5% and approximately 1% concentrations of the aldehyde forms of the 4-deoxy analogue and Ara5P in solution, then the 'true' Km values must be in the range 31.5 microM and 0.26 microM, respectively, requiring about a 3 kcal/mol contribution to the binding energy by the 4-hydroxyl group of Ara5P. The data provides evidence that the enzyme acts upon the acyclic form of the natural substrate. The steady-state kinetic study of KDO8P synthase was analyzed via inhibition using the products KDO8P and inorganic phosphate, and D-ribose-5-phosphate as a dead-end inhibitor. First, intersecting lines in double-reciprocal plots of initial-velocity data at substrate concentrations in the micromolar range suggest a sequential mechanism for the enzyme-catalyzed reaction. The inhibition by D-ribose-5-phosphate is competitive for Ara5P and uncompetitive for phosphoenolpyruvate (P-pyruvate). These inhibition patterns are consistent with the model wherein P-pyruvate binding precedes that of Ara5P binding. Furthermore, this order of substrate binding was supported by the observations that KDO8P is a competitive inhibitor for P-pyruvate binding, supporting the concept that KDO8P and P-pyruvate bind to the same enzyme form, and noncompetitively with respect to Ara5P. In addition, the inhibition by inorganic phosphate is noncompetitive with respect to both P-pyruvate and Ara5P, suggesting an apparent ordered release of products such that Pi first, followed by KDO8P. In conclusion, these data suggest a steady-state kinetic mechanism for KDO8P synthase where P-pyruvate binding precedes that of Ara5P, followed by the ordered release of inorganic phosphate and KDO8P.