pH dependence of kinetic parameters for oxalacetate decarboxylation and pyruvate reduction reactions catalyzed by malic enzyme.
pH dependence of kinetic parameters for oxalacetate decarboxylation and pyruvate reduction reactions catalyzed by malic enzyme.
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苹果酸酶催化的草乙酸脱羧和丙酮酸还原反应动力学参数的 pH 依赖性。
DOI：
10.1021/bi00361a004
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发表时间：
1986
期刊：
影响因子：
2.9
通讯作者：
Cook,PF
中科院分区：
文献类型：
--
作者：
Park,SH;Harris,BG;Cook,PF
生物化学系，北德克萨斯州立大学/德克萨斯骨病医学院，丹顿，德克萨斯76203接收于1985年12月5日;修订的Mandarin pt接收于1986年2月28日摘要：鸡肝NADP-苹果酸酶和猪蛔虫NADP-苹果酸酶都催化乙酸脱氢酶的金属依赖性脱羧作用。两种酶在存在或不存在二核苷酸的情况下催化反应。二核苷酸的存在增加了乙酰胆碱酯酶对鸡肝NADP-苹果酸酶的亲和力，但这一信息在A.由于游离酶对NAD的亲和力较低，在pH值低于5.0时，鸡肝NADP-苹果酸酶催化乙酸脱羧反应的动力学机制为先加NADP的平衡有序反应。pH值低于5.0时，每pH值单位，NADP的K值增加10倍.酶残基需要被质子化以进行β-乙酸脱羧（通过两种酶）和丙酮酸还原（通过NAD-苹果酸酶），但β-乙酸的β-羧基必须未被质子化以进行反应（通过两种酶）。鸡肝NADP-苹果酸酶的酶残基的pK从不存在NADP时的6.4的值降低到存在NADP时的约5.5与Mg 2+和4.8与Mn 2+。A.丙酮酸脱羧或丙酮酸还原所需的酶残基pK值。猪的NAD-苹果酸酶约为5.5-6.0。虽然NAD乙酸与质子化和非质子化形式的NADP酶结合得同样好，但NAD酶需要NAD乙酸或丙酮酸选择性地与质子化形式的酶结合。这两种酶都更喜欢Mn 2+而不是Mg 2+用于乙酸脱氢酶脱羧。NAD-苹果酸酶也偏好Mn 2+用于丙酮酸还原，并且该反应的速率是苹果酸的氧化脱羧的速率（使用Mg 2+作为二价金属离子）。丙酮酸还原反应的动力学机制为先加入NADH的快速平衡机制。L-苹果酸的氧化脱羧和乙酸脱氢酶的脱羧的Kmax的绝对值的比较表明，乙酸脱氢酶中间体的脱羧是苹果酸酶反应中的主要速率决定步骤。由于A. suum催化金属依赖性脱羧乙酸脱氢酶和还原丙酮酸，目前对这种酶的分类（EC 1.1. 1.39）应改为EC 1.1。1.38.
Department of Biochemistry, North Texas State University/Texas College of Osteopathic Medicine, Denton, Texas 76203 Received December 5, 1985; Revised Manuscript Received February 28, 1986 abstract: Both chicken liver NADP-malic enzyme and Ascaris suum NAD-malic enzyme catalyze the metal-dependent decarboxylation of oxalacetate. Both enzymes catalyze the reaction either in the presence or in the absence of dinucleotide. The presence of dinucleotide increases the affinity of oxalacetate for the chicken liver NADP-malic enzyme, but thisinformation could not be obtained in the case of A. suum NAD-malic enzyme because of the low affinity of free enzyme for NAD. The kinetic mechanism for oxalacetate decarboxylation by the chicken liver NADP-malic enzyme is equilibrium ordered at pH values below 5.0 with NADP adding to enzyme first. The K¡ for NADP increases by a factor of 10 per pH unit below pH 5.0. An enzyme residue is required protonated for oxalacetate decarboxylation (by both enzymes) and pyruvate reduction (by the NAD-malic enzyme), but the/3-carboxyl of oxalacetate must be unprotonated for reaction (by both enzymes). The pK of the enzyme residue of the chicken liver NADP-malic enzyme decreases from a value of 6.4 in the absence of NADP to about 5.5 with Mg2+ and 4.8 with Mn2+ in the presence of NADP. The pK value of the enzyme residue required protonatedfor either oxalacetate de-carboxylation or pyruvate reduction for the A. suum NAD-malic enzyme is about 5.5-6.0. Although oxalacetate binds equally well to protonated and unprotonated forms of the NADP-enzyme, the NAD-enzyme requires that oxalacetate or pyruvate selectively bind to the protonated form of the enzyme. Both enzymes prefer Mn2+ over Mg2+ for oxalacetate decarboxylation. The NAD-malic enzyme also prefers Mn2+ for pyruvate reduction, and the rate of this reaction is Veootii the rate of the oxidative decarboxylation of malate (using Mg2+ as the divalent metal ion). The kinetic mechanism for pyruvate reduction is rapid equilibrium ordered with NADH adding first. A comparison of the absolute values of Kmax for the oxidative decarboxylation of L-malate and the decarboxylation of oxalacetate suggests that decarboxylation of the oxalacetate intermediate is the main rate-determining step in the malic enzyme reaction. Since the NAD-malic enzyme from A. suum catalyzes the metal-dependent decarboxylation of oxalacetate and reduction of pyruvate, the present classification for this enzyme (EC 1.1. 1.39) should be changed to EC 1.1. 1.38.
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