Rosenfeld S S, Taylor E W
J Biol Chem. 1987 Jul 25;262(21):9994-9.
The effective rate of dissociation of 1-N6-ethenoadenosine diphosphate (epsilon ADP) from the regulated actin X subfragment 1 X epsilon ADP complex of rabbit skeletal muscle is approximately 10-15 times smaller in the absence of calcium ion compared to the presence of calcium ion. The decrease in fluorescence emission with dissociation of the bound epsilon ADP fitted two exponential terms. The evidence is consistent with a kinetic scheme in which two first-order transitions precede the dissociation step: (Formula: see text) where D is epsilon ADP, A is regulated actin, M is subfragment 1, the asterisks refer to the degree of fluorescence enhancement, and AM(D) is a collision complex in equilibrium with free epsilon ADP. Both rate constants k-2 and k-1 were reduced approximately 15-fold in the absence of calcium ion. The rate constants for the dissociation of epsilon ATP, epsilon ADP X Pi, formed in the enzyme cycle, and epsilon ADP are all reduced in the absence of calcium ion; consequently, the primary effect in calcium regulation of the actin-subfragment 1 ATPase is on the rate constant of a transition (or transitions) between actomyosin-nucleoside phosphate complexes.
与存在钙离子的情况相比，在不存在钙离子时，1-N6-乙烯基腺苷二磷酸（ε-ADP）从兔骨骼肌调节型肌动蛋白X亚片段1 X ε-ADP复合物上解离的有效速率约小10 - 15倍。随着结合的ε-ADP解离，荧光发射的降低符合两个指数项。证据与一种动力学机制一致，即在解离步骤之前有两个一级转变：（公式：见原文）其中D为ε-ADP，A为调节型肌动蛋白，M为亚片段1，星号表示荧光增强程度，AM(D)是与游离ε-ADP处于平衡的碰撞复合物。在不存在钙离子时，速率常数k-2和k-1均降低了约15倍。在酶循环中形成的ε-ATP、ε-ADP X Pi以及ε-ADP的解离速率常数在不存在钙离子时均降低；因此，钙离子对肌动蛋白-亚片段1 ATP酶调节的主要作用在于肌动球蛋白-核苷磷酸复合物之间一个（或多个）转变的速率常数。
