Wong K F, Dennis D T
Department of Biology, Queen's University, Kingston, Ontario, Canada.
Plant Physiol. 1973 Feb;51(2):322-6. doi: 10.1104/pp.51.2.322.
Aspartokinase has been isolated from wheat germ and a preliminary survey made of its properties in a partially purified extract. The enzyme has an absolute requirement for ATP and a divalent metal ion. The phosphate donor can be either ATP or GTP, but other nucleotides are ineffective. Both magnesium and manganese will activate the enzyme, whereas calcium shows a trace amount of activity. The enzyme has a Km of 16.7 mm for aspartate, 1.2 mm for ATP, and 3.3 mm for MgCl(2). Lysine inhibits the reaction at fairly low concentrations, and threonine inhibits at high concentrations. Other amino acids which are derived from aspartate (methionine, homoserine, threonine, and isoleucine) have little effect. When lysine and threonine are added together, they show a concerted inhibition of the reaction. The enzyme is also stabilized against heat inactivation by lysine and threonine together but not by either when added separately. It is suggested that aspartokinase from plants is a regulatory enzyme and exhibits a concerted feedback mechanism.
天冬氨酸激酶已从小麦胚芽中分离出来，并在部分纯化的提取物中对其性质进行了初步研究。该酶对ATP和二价金属离子有绝对需求。磷酸供体可以是ATP或GTP，但其他核苷酸无效。镁和锰都能激活该酶，而钙仅显示出微量活性。该酶对天冬氨酸的Km值为16.7 mM，对ATP为1.2 mM，对MgCl₂为3.3 mM。赖氨酸在相当低的浓度下抑制反应，苏氨酸在高浓度下抑制反应。其他从天冬氨酸衍生而来的氨基酸（蛋氨酸、高丝氨酸、苏氨酸和异亮氨酸）影响很小。当赖氨酸和苏氨酸一起添加时，它们对反应表现出协同抑制作用。赖氨酸和苏氨酸一起还能使该酶稳定，防止热失活，但单独添加时则无此作用。有人提出，来自植物的天冬氨酸激酶是一种调节酶，并表现出协同反馈机制。
