Sleep J A, Hutton R L
Biochemistry. 1980 Apr 1;19(7):1276-83. doi: 10.1021/bi00548a002.
The ATP in equilibrium with Pi exchange of actin-activated myosin subfragment 1 was investigated as a function of actin concentration. The maximum rate of exchange was 1.7 x 10(-3) s-1 at a Pi concentration of 2 mM, and this rate was independent of medium ADP concentration. The observed rate is 50 times faster than expected if reversal starts from the AM.ADP state that can be formed from medium ADP. The existence of a preceding higher energy AM'.ADP state which exists in significant concentration only during the steady-state hydrolysis of ATP accounts for this discrepancy and also for the independence of the rate of in equilibrium with Pi exchange on medium ADP concentration. Addition of ADP and Pi to myosin results in the formation of an equilibrium amount of enzyme-bound ATP. The inhibitory effect of actin on the formation of enzyme-bound ATP has been investigated and a value of 4 x 10(6) M-1 has been determined for the binding constant of actin to M*.ADP and an upper limit of 2 x 10(4) M-1 has been determined for the binding constant of actin to M*.ATP.
研究了与肌动蛋白激活的肌球蛋白亚片段1的磷酸交换处于平衡状态的ATP作为肌动蛋白浓度的函数。在磷酸浓度为2 mM时，交换的最大速率为1.7×10⁻³ s⁻¹，且该速率与介质中ADP浓度无关。如果逆转从可由介质ADP形成的AM·ADP状态开始，观察到的速率比预期快50倍。仅在ATP的稳态水解过程中以显著浓度存在的先前更高能量的AM'·ADP状态的存在解释了这种差异，也解释了与磷酸交换处于平衡状态的速率对介质ADP浓度的独立性。向肌球蛋白中添加ADP和磷酸会导致形成平衡量的酶结合ATP。研究了肌动蛋白对酶结合ATP形成的抑制作用，确定肌动蛋白与M*·ADP的结合常数为4×10⁶ M⁻¹，肌动蛋白与M*·ATP的结合常数上限为2×10⁴ M⁻¹。
