Frisch D A, Gengenbach B G, Tommey A M, Sellner J M, Somers D A, Myers D E
Department of Agronomy and Plant Genetics and Plant Molecular Genetics Institute, University of Minnesota, St. Paul, Minnesota 55108.
Plant Physiol. 1991 Jun;96(2):444-52. doi: 10.1104/pp.96.2.444.
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.
二氢二吡啶甲酸合酶（EC 4.2.1.52）是植物赖氨酸生物合成中的首个特异性酶，从玉米（Zea mays L.）细胞悬浮培养物和叶片中纯化得到。基于SDS - PAGE，玉米二氢二吡啶甲酸合酶的亚基分子量估计为38,000。高度纯化的二氢二吡啶甲酸合酶催化L - 天冬氨酸半醛和丙酮酸的缩合反应，呈现乒乓双双有序反应的动力学特征，其中丙酮酸首先与酶结合。在较高浓度的L - 天冬氨酸半醛下明显的底物抑制作用，通过增加丙酮酸浓度得到部分缓解。丙酮酸结合表现出协同性，表观结合数为2，50%最大活性所需的丙酮酸浓度为1.86毫摩尔。L - 天冬氨酸半醛的K(m)估计为0.6毫摩尔浓度。赖氨酸是二氢二吡啶甲酸合酶的变构协同抑制剂，估计希尔系数为4，50%抑制所需的赖氨酸浓度为23微摩尔。这些物理和动力学数据与天然酶的同四聚体模型一致。
