肌浆网 Ca(2+)-ATP 酶催化位点的功能研究:Ca(2+)缺失时 ATP 的结合和水解

A Lax, F Soler, F Fernandez Belda
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摘要

分离的肌质网囊泡在有 Mg(2+)和无 Ca(2+)的条件下仍具有显著的 ATP 水解活性,这种活性可归因于 Ca(2+)-ATP 酶蛋白。在中性 pH 值和 5 mMMg(2+)存在的条件下,水解速率对线性 ATP 浓度标度的依赖性可以用一个双曲线函数来拟合。MgATP 的水解受到游离 Mg(2+) 或游离 ATP 的抑制。ATP 的水解速率不受钒酸盐的干扰,而对硝基苯磷酸的水解速率则不受不可水解的 ATP 类似物的影响。在中性pH和不含 Ca(2+)的介质中,Mg(2+)会增加 ATP 的结合亲和力,但当 Mg(2+)存在时,香草酸盐会降低 ATP 的结合亲和力。这表明,在没有 Ca(2+)的情况下,MgATP 的水解需要对酶的细胞质结构域进行某种优化调整。在细胞质 Ca(2+) 的基础水平或 Ca(2+) 结合转换受阻时,Ca(2+) 依赖性活性发挥作用。
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Functional approach to the catalytic site of the sarcoplasmic reticulum Ca(2+)-ATPase: binding and hydrolysis of ATP in the absence of Ca(2+)
Isolated sarcoplasmic reticulum vesicles in the presence of Mg(2+) and absence of Ca(2+) retain significant ATP hydrolytic activity that can be attributed to the Ca(2+)-ATPase protein. At neutral pH and the presence of 5 mM Mg(2+), the dependence of the hydrolysis rate on a linear ATP concentration scale can be fitted by a single hyperbolic function. MgATP hydrolysis is inhibited by either free Mg(2+) or free ATP. The rate of ATP hydrolysis is not perturbed by vanadate, whereas the rate of p-nitrophenyl phosphate hydrolysis is not altered by a nonhydrolyzable ATP analog. ATP binding affinity at neutral pH and in a Ca(2+)-free medium is increased by Mg(2+) but decreased by vanadate when Mg(2+) is present. It is suggested that MgATP hydrolysis in the absence of Ca(2+) requires some optimal adjustment of the enzyme cytoplasmic domains. The Ca(2+)-independent activity is operative at basal levels of cytoplasmic Ca(2+) or when the Ca(2+) binding transition is impeded.
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