Isolation and characterization of dihydrodipicolinate synthase from maize.

Dihydrodipicolinate synthase (EC 4.2.1.52), the first enzyme specific to lysine biosynthesis in plants, was purified from maize ( Zea mays L.) cell suspension cultures and leaves. The subunit molecular weight of maize dihydrodipicolinate synthase was estimated to be 38,000 based on SDS-PAGE. The condensation of l-aspartate semialdehyde and pyruvate by highly purified dihydrodipicolinate synthase exhibited kinetics characteristic of a Ping Pong Bi Bi ordered reaction in which pyruvate binds first to the enzyme. Substrate inhibition evident at higher concentrations of l-aspartate semialdehyde was partially alleviated by increasing concentrations of pyruvate. Pyruvate binding exhibited cooperativity with an apparent number of 2 and 1.86 millimolar concentration required for 50% of maximal activity. The K m for aspartate semialdehyde was estimated to be 0.6 millimolar concentration. Lysine was an allosteric cooperative inhibitor of dihydrodipicolinate synthase with an estimated Hill number of 4 and 23 micromolar concentration required for 50% inhibition. The physical and kinetic data are consistent with a homotetramer model for the native enzyme.