White H D, Taylor E W
Biochemistry. 1976 Dec 28;15(26):5818-26. doi: 10.1021/bi00671a020.
Rate constants were determined for the reaction of actin with subfragment 1 (S1), S1-product complex, heavy meromyosin (HMM), and HMM-products complex for a range of temperatures, pH's, and ionic strengths. For actin concentrations up to 10 muM, the rate of reassociation of the product intermediate was equal to the rate of actomyosin subfragment 1 (acto-S1) or acto-HMM adenosine triphosphatase (ATPase). Therefore, under these conditions, the only important pathway for adenosine triphosphate hydrolysis is through the dissociation and recombination of S1 or HMM. The apparent rate constants for the association of S1 and S1-product with actin showed a similar large ionic strength dependence. The S1-product reaction had a large temperature dependence paralleling the rate of acto-S1 ATPase, while the reaction with S1 had a much smaller variation with temperature. The low value of the rate constant for the S1-product reaction and its relationship to the s1 areaction suggests that the apparent rate constant does not measure a simple second-order reaction. A plausible mechanism is a rapid equilibrium for the binding step, followed by a transition (product release) which increases the association constant. A refractory state could also reduce the apparent rate constant of recombination. An approximate assignment of equilibrium constants for the acto-S1 ATPase reaction was made based on the interpretation of the present evidence and equilibrium constnats for the S1 ATPase.
在一系列温度、pH值和离子强度条件下，测定了肌动蛋白与亚片段1（S1）、S1 - 产物复合物、重酶解肌球蛋白（HMM）以及HMM - 产物复合物反应的速率常数。对于高达10μM的肌动蛋白浓度，产物中间体的重新结合速率等于肌动球蛋白亚片段1（肌动蛋白 - S1）或肌动球蛋白 - HMM腺苷三磷酸酶（ATP酶）的速率。因此，在这些条件下，三磷酸腺苷水解的唯一重要途径是通过S1或HMM的解离和重组。S1和S1 - 产物与肌动蛋白结合的表观速率常数显示出类似的对离子强度的强烈依赖性。S1 - 产物反应对温度有很大的依赖性，与肌动蛋白 - S1 ATP酶的速率平行，而与S1的反应随温度的变化要小得多。S1 - 产物反应速率常数的值较低及其与S1反应的关系表明，表观速率常数并非测量一个简单的二级反应。一个合理的机制是结合步骤的快速平衡，随后是一个增加缔合常数的转变（产物释放）。一个不应期状态也可能降低重组的表观速率常数。基于对现有证据的解释以及S1 ATP酶的平衡常数，对肌动蛋白 - S1 ATP酶反应的平衡常数进行了近似赋值。
