Rates of elementary steps catalyzed by rat liver cytosolic and mitochondrial inorganic pyrophosphatases in both directions.
Rates of elementary steps catalyzed by rat liver cytosolic and mitochondrial inorganic pyrophosphatases in both directions.
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大鼠肝胞质和线粒体无机焦磷酸酶双向催化的基本步骤的速率。
DOI：
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发表时间：
1995
影响因子：
3.9
通讯作者：
A. Baykov
中科院分区：
文献类型：
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作者：
I. Smirnova;V. Kasho;S. Volk;A. Ivanov;A. Baykov
我们已经研究了动力学焦磷酸盐合成和磷酸-水氧交换催化的大鼠肝细胞质和线粒体焦磷酸酶的存在下，镁作为辅因子。一个共同的动力学模型，这些反应意味着它们涉及酶结合焦磷酸的形成，并通过两个平行的途径进行：途径I，利用两个磷酸镁分子，和途径II，利用磷酸镁和游离磷酸盐。与溶液相比，焦磷酸盐的形成在两种焦磷酸酶的活性位点中大大促进（[E.PPi]/[E. 2 Pi] = 0.11-0.24）。对于途径I，列举了PPi结合/释放、结合PPi水解/合成以及由胞质和线粒体焦磷酸酶催化的两个Pi结合/释放步骤的速率常数。两种酶在任一方向上的途径I没有独特的限速步骤。磷酸镁对氧交换的调节作用与胞质焦磷酸酶，可解释在一个变构磷酸结合位点或随机顺序释放的两个磷酸分子从活性位点。与微生物对应物相比，这些哺乳动物焦磷酸酶的一个显着特征是它们在焦磷酸盐合成中的高效性。在合成方向的营业额分别为14和9.3 s-1的胞质和线粒体酶，分别（9和16%，相对于水解营业额）。结果表明，酶催化合成焦磷酸盐，最简单的高能多磷酸盐，可以进行在没有外部能量输入，如质子动力在膜系统中提供的情况下，在高速率。
We have investigated kinetics of pyrophosphate synthesis and phosphate-water oxygen exchange catalyzed by rat liver cytosolic and mitochondrial pyrophosphatases in the presence of Mg2+ as cofactor. A common kinetic model derived for these reactions implies that they involve formation of enzyme-bound pyrophosphate and proceed through two parallel pathways: pathway I, utilizing two magnesium phosphate molecules, and pathway II, utilizing both magnesium phosphate and free phosphate. Pyrophosphate formation is greatly facilitated in the active sites of both pyrophosphatases ([E.PPi]/[E.2Pi] = 0.11-0.24) compared to solution. The rate constants for PPi binding/release, bound PPi hydrolysis/synthesis, and two Pi binding/release steps catalyzed by cytosolic and mitochondrial pyrophosphatases were enumerated for pathway I. There is no unique rate-limiting step for pathway I for both enzymes in either direction. A modulating effect of magnesium phosphate on the oxygen exchange is observed with the cytosolic pyrophosphatase, explicable in terms of an allosteric phosphate-binding site or random-order release of two phosphate molecules from the active site. A remarkable feature of these mammalian pyrophosphatases versus their microbial counterparts is their high efficiency in pyrophosphate synthesis. The turnover numbers in the direction of synthesis are 14 and 9.3 s-1 for the cytosolic and mitochondrial enzymes, respectively (9 and 16% relative to hydrolysis turnover numbers). The results demonstrate that the enzyme-catalyzed synthesis of pyrophosphate, the simplest high-energy polyphosphate, can proceed at a high rate in the absence of an external energy input, such as that provided by protonmotive force in membrane systems.