Bagshaw C R, Trentham D R
Biochem J. 1973 Jun;133(2):323-8. doi: 10.1042/bj1330323.
For the simplest kinetic model the reverse rate constants (k(-1) and k(-2)) associated with ATP binding and cleavage on purified heavy meromyosin and heavy meromyosin subfragment 1 from rabbit skeletal muscle in the presence of 5mm-MgCl(2), 50mm-KCl and 20mm-Tris-HCl buffer at pH8.0 and 22 degrees C are: k(-1)<0.02s(-1) and k(-1)=16s(-1). Apparently, higher values of k(-1) and k(-2) are found with less-purified protein preparations. The values of k(-1) and k(-2) satisfy conditions required by previous (18)O-incorporation studies of H(2) (18)O into the P(i) moiety on ATP hydrolysis and suggest that the cleavage step does involve hydrolysis of ATP or formation of an adduct between ATP and water. The equilibrium constant for the cleavage step at the myosin active site is 9. If the cycle of events during muscle contraction is described by the model proposed by Lymn & Taylor (1971), the fact that there is only a small negative standard free-energy change for the cleavage step is advantageous for efficient chemical to mechanical energy exchange during muscle contraction.
对于最简单的动力学模型，在含有5mM - MgCl₂、50mM - KCl和20mM - Tris - HCl缓冲液（pH8.0，22℃）的条件下，与兔骨骼肌纯化的重酶解肌球蛋白及重酶解肌球蛋白亚片段1上ATP结合和裂解相关的逆向速率常数（k(-1)和k(-2)）为：k(-1)<0.02s⁻¹且k(-2)=16s⁻¹。显然，对于纯度较低的蛋白质制剂，会发现k(-1)和k(-2)有更高的值。k(-1)和k(-2)的值满足先前关于H₂¹⁸O掺入ATP水解时Pi部分的¹⁸O掺入研究所需的条件，并表明裂解步骤确实涉及ATP的水解或ATP与水之间加合物的形成。肌球蛋白活性位点处裂解步骤的平衡常数为9。如果用Lymn和Taylor（1971）提出的模型来描述肌肉收缩过程中的一系列事件，那么裂解步骤只有很小的负标准自由能变化这一事实有利于肌肉收缩过程中高效地进行化学能到机械能的转换。
