HIGHER-PLANTS EXPRESS 3-DEOXY-D-MANNO-OCTULOSONATE 8-PHOSPHATE SYNTHASE

The enzymatic activity of 3-deoxy-D-manno-octulosonate 8-phosphate (KDOP) synthase was detected in eight diverse plant species, thus providing enzymological data consistent with recent reports of the presence of 3-deoxy-D-manno-octulosonate in plant cell walls. KDOP synthase from spinach was partially purified and characterized. It possessed weak activity as 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase. In the presence of phosphoenolpyruvate, which conferred dramatic thermostability, KDOP synthase had a catalytic temperature optimum of about 53-degrees-C. The pH optimum was 6.2, and divalent cations were neither stimulatory nor required for activity. The Km values for arabinose 5-P and phosphoenolpyruvate were 0.27 mol m-3 and about 35 mmol m-3, respectively. The kinetics of periodate oxidation of KDOP formed by spinach KDOP synthase indicate that the same stereochemical configuration exists as with bacterial KDOP. The possibility that an unregulated species of DAHP synthase found in some bacteria might in fact be a KDOP synthase exhibiting substrate ambiguity of the type seen in higher plants was examined. However, the DAHP synthase isozyme, DS-O, from Acinetobacter calcoaceticus was found to be specific for erythrose 4-P. The KDOP synthase of Acinetobacter calcoaceticus was also found to be specific for arabinose 5-P.