Purification and Characterization of Dihydrodipicolinate Synthase from Wheat Suspension Cultures

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Abstract
Dihydrodipicolinate synthase, the first enzyme unique to lysine biosynthesis in higher plants, was purified about 5100-fold from suspension-cultured cells of wheat (Triticum aestivum var Chinese Spring). The synthase has an average molecular weight of 123,000 as determined by gel filtration and exhibited maximum activity at pH 8.0. The kinetics of the condensation reaction are compatible with a “Ping Pong” mechanism in which pyruvate reacts first with the enzyme to form a Schiff base. Pyruvate and l-aspartic-β-semialdehyde (ASA) have respective Km values of 11.76 and 0.80 millimolar. Allosteric inhibition was observed with increasing concentrations of l-lysine and its structural analogs, including threo-4-hydroxy-l-lysine and S-(2-aminoethyl)-l-cysteine, with respective I0.5 values of 51, 141, and 288 micromolar. These amino acids were competitive inhibitors with respect to ASA and noncompetitive inhibitors with respect to pyruvate. We propose that the binding site for lysine overlaps with the ASA binding site, possibly by an attachment of the common alanyl moiety. The wheat enzyme was inhibited by Zn2+, Cd2+, and Hg2+ and also by sulfhydryl inhibitors, p-(hydroxymercuri)benzoic acid and p-chloromercuribenzenesulfonic acid.