Greene L E
Biochemistry. 1981 Apr 14;20(8):2120-6. doi: 10.1021/bi00511a008.
The binding of heavy meromyosin (HMM) to F-actin was examined at varying ionic strengths and temperatures and in the presence of ADP and AMPPNP and then compared to the binding of subfragment 1 (S-1) under identical conditions. In the absence of nucleotide (mu = 0.12-0.43 M, 22 degrees C), HMM binds 100-1000-fold more strongly to F-actin than does S-1. This indicates that, in the absence of nucleotide, both heads of HMM bind to F-actin, with the second head making a significant contribution to the free energy of binding. On the other hand, in the presence of ADP (mu = 0.43 M, 22 degrees C) or AMPPNP (mu = 0.12 M, degrees C), the binding of HMM to F-actin is quite similar to the binding of S-1, indicating that here the second head of HMM does not make a strong contribution to the free energy of binding. In fact, in the presence of AMPPNP, HMM appears to bind to F-actin primarily with one head, while the detached head may be interfering with the binding of another HMM molecule at an adjacent actin site. With all of the different agents tested (ionic strength, temperature, and nucleotide), the effect of the agent on the binding of HMM to F-actin is approximately the square of its effect on the binding of S-1 to F-actin, results consistent with these various agents affecting the binding of each of the two HMM heads to the same extent as they affect the binding of an S-1 head.
在不同离子强度、温度以及存在二磷酸腺苷（ADP）和腺苷-5'-三磷酸-β,γ-亚氨酯（AMPPNP）的条件下，研究了重酶解肌球蛋白（HMM）与F-肌动蛋白的结合情况，然后将其与相同条件下1片段（S-1）的结合情况进行比较。在不存在核苷酸的情况下（μ = 0.12 - 0.43 M，22℃），HMM与F-肌动蛋白的结合比S-1强100 - 1000倍。这表明，在不存在核苷酸时，HMM的两个头部都与F-肌动蛋白结合，第二个头部对结合自由能有显著贡献。另一方面，在存在ADP（μ = 0.43 M，22℃）或AMPPNP（μ = 0.12 M，℃）的情况下，HMM与F-肌动蛋白的结合与S-1的结合非常相似，这表明此时HMM的第二个头部对结合自由能没有很大贡献。实际上，在存在AMPPNP的情况下，HMM似乎主要以一个头部与F-肌动蛋白结合，而分离的头部可能会干扰另一个HMM分子在相邻肌动蛋白位点的结合。在所有测试的不同因素（离子强度、温度和核苷酸）中，该因素对HMM与F-肌动蛋白结合的影响大约是其对S-1与F-肌动蛋白结合影响的平方，这些结果与这些不同因素对HMM两个头部结合的影响程度与它们对S-1头部结合的影响程度相同一致。
