Synonymy of the three apparent isoenzymes of 3‐deoxy‐D‐arabino‐heptulosonate 7‐phosphate synthase in Pisum sativum L. with 3‐deoxy‐ d ‐manno‐octulosonate 8‐phosphate synthase and the DS‐Co/DS‐Mn isoenzyme pair

summary Three enzymes, capable of condensing phosphoenolpyruvate with erythrose 4‐phosphate, were separated by DEAE‐cellulose column chromatography of extracts from young leaves of Pisum sativum. One enzyme was identified as 3‐deoxy‐d‐manno‐octulosonate 8‐phosphate (KDOP) synthase (EC 4.1 .2.16). This novel plant enzyme mimicks 3‐deoxy‐D‐arabino‐heptulosonate 7‐phosphate synthase (EC 4.1 .2.15) in vitro by virtue of its substrate ambiguity. KDOP synthase used erythrose 4‐phosphate 28% as well as arabinose 5‐phosphate at high substrate concentrations. The pH optimum was 6.5 and divalent cations were neither required nor stimulatory. The remaining two enzymes possessed the distinctive properties of the DAHP synthase (DS) isoenzymes, DS‐Mn and DS‐Co, previously established in tobacco and spinach to be chloroplast‐ and cytosol‐localized isoenzymes, respectively. DS‐Mn was highly specific for its substrates, required dithiothreitol (a hysteretic activator) for activity, was stimulated 3.6‐fold by 0.5 mM Mn2+, and exhibited a pH optimum of 7.5. DS‐Co, by contrast, showed an extreme degree of substrate ambiguity (giving greater rates with glycolaldehyde and glyceraldehyde 3‐phosphate than with erythrose 4‐phosphate at saturating substrate concentrations), had an absolute requirement for a divalent metal, and exhibited a high pH optimum of 9.0 for catalysis.