Mueggler P A, Wolfe R G
Biochemistry. 1978 Oct 31;17(22):4615-20. doi: 10.1021/bi00615a006.
At pH 8.0 in 0.05 M Tris-acetate buffer at 25 degrees C, homogeneous supernatant malate dehydrogenase exhibits substrate activation by L-malate. The turnover number, Michaelis constant for L-malate, and Michaelis constant for NAD are: 0.46 X 10(4) min(-1), 0.036 mM, and 0.14 mM, respectively, for nonactivated enzyme and 1.1 X 10(4) min(-1), 0.2mM, and 0.047 mM for the same series of constants in activated enzyme. Nonactivating behavior is observed at concentrations between 0.02 and 0.15 mM L-malate and activating behavior is observed between 0.15 and 0.5 mM L-malate. L-Malate activation is compared with similar activation of mitochondrial malate dehydrogenase. While it is not possible to exclude unequivocally all mechanisms, the data seem to be consistent with the occurrence of a fundamentally ordered bi bi mechanism, possibly involving activation through the allosteric binding of L-malate. It is concluded that the data are consistent with a form of the "reciprocating compulsory order mechanism" in which nonactivated enzyme reflects catalysis by one subunit and activated catalysis expresses the coordinated activity of two subunits. The allosteric interaction and the "reciprocating mechanism/ are not mutually exclusive.
在25℃下，于0.05M Tris - 乙酸盐缓冲液中，pH值为8.0时，均一的上清液苹果酸脱氢酶表现出被L - 苹果酸激活底物的特性。对于未激活的酶，其转换数、对L - 苹果酸的米氏常数以及对NAD的米氏常数分别为：0.46×10⁴ min⁻¹、0.036 mM和0.14 mM；对于激活的酶，相同系列常数分别为1.1×10⁴ min⁻¹、0.2 mM和0.047 mM。在L - 苹果酸浓度为0.02至0.15 mM之间观察到非激活行为，而在0.15至0.5 mM之间观察到激活行为。将L - 苹果酸的激活作用与线粒体苹果酸脱氢酶的类似激活作用进行了比较。虽然不可能明确排除所有机制，但数据似乎与一种基本有序的双底物双产物机制的发生相一致，可能涉及通过L - 苹果酸的变构结合而激活。得出的结论是，数据与“往复强制顺序机制”的一种形式相一致，其中未激活的酶反映一个亚基的催化作用，而激活的催化作用则表现出两个亚基的协同活性。变构相互作用和“往复机制”并非相互排斥。
