Tracy J W, Kohlhaw G B
Proc Natl Acad Sci U S A. 1975 May;72(5):1802-6. doi: 10.1073/pnas.72.5.1802.
alpha-Isopropylmalate synthase [3-hydroxy-4-methyl-3-carboxyvalerate 2-oxo-3-methylbutyrate-lyase (CoA-acetylating); EC 4.1.3.12], the enzyme catalyzing the first committed step in leucine biosynthesis, and homocitrate synthase [3-hydroxy-3-carboxyadipate 2-oxoglutarate-lyase (CoA-acetylating); EC 4.1.3.21], the first enzyme in lysine biosynthesis in yeast, are rapidly inactivated in the presence of low concentrations of coenzyme A, a product of both reactions. Closely related compounds like 3-dephospho-coenzyme A or oxidized coenzyme A are almost without effect, as are other sulfhydryl compounds. Citrate (si)-synthase [citrate oxaloacetate-lyase (pro-3S-CH2-COO-minus leads to acetyl-CoA); EC 4.1.3.7] appears to be completely resistant against inactivation by coenzyme A. Inactivated alpha-isopropylmalate and homocitrate synthases can be reactivated by dialysis, but not by adding excess substrate. Protection against coenzyme-A-mediated inactivation is provided by relatively high concentrations of the alpha-ketoacid substrate or the specific end product inhibitor of each of the two enzymes. The coenzyme-A-mediated inactivation of alpha-isopropylmalate synthase has been more closely investigated. It requires the presence of divalent metal ions, with Zn++being most effective. The inactivation does not require molecular oxygen. It occurs in the presence of low concentrations of substrates and is observed in toluene-treated cells. These results, together with evidence that alpha-isopropylmalate synthase and homocitrate synthase are located in the mitochondria, suggest a mechanism by which increasing intra-mitochondrial coenzyme A concentrations might serve as a signal of decreasing acetyl-coenzyme A levels, triggering a temporary inactivation of biosynthetic acetyl-coenzyme A-consuming reactions in order to channel the available acetyl-coenzyme A into the citrate cycle.
α-异丙基苹果酸合酶[3-羟基-4-甲基-3-羧基戊酸 2-氧代-3-甲基丁酸裂解酶（辅酶A乙酰化）；EC 4.1.3.12]，催化亮氨酸生物合成中第一个关键步骤的酶，以及同柠檬酸合酶[3-羟基-3-羧基己二酸 2-氧代戊二酸裂解酶（辅酶A乙酰化）；EC 4.1.3.21]，酵母中赖氨酸生物合成的第一个酶，在低浓度辅酶A（这两个反应的产物）存在下会迅速失活。3-去磷酸辅酶A或氧化型辅酶A等密切相关的化合物几乎没有影响，其他巯基化合物也是如此。柠檬酸（顺式）合酶[柠檬酸草酰乙酸裂解酶（pro-3S-CH2-COO- 生成乙酰辅酶A）；EC 4.1.3.7]似乎完全抵抗辅酶A介导的失活。失活的α-异丙基苹果酸合酶和同柠檬酸合酶可通过透析重新激活，但不能通过添加过量底物来激活。相对高浓度的α-酮酸底物或这两种酶各自的特异性终产物抑制剂可提供针对辅酶A介导失活的保护作用。对α-异丙基苹果酸合酶的辅酶A介导失活进行了更深入的研究。它需要二价金属离子的存在，其中Zn++最为有效。失活不需要分子氧。它在低浓度底物存在下发生，并且在经甲苯处理的细胞中也能观察到。这些结果，连同α-异丙基苹果酸合酶和同柠檬酸合酶位于线粒体中的证据，提示了一种机制，即线粒体中辅酶A浓度的增加可能作为乙酰辅酶A水平降低的信号，触发生物合成中消耗乙酰辅酶A的反应暂时失活，以便将可用的乙酰辅酶A引导至柠檬酸循环中。
