Bagshaw C R, Trentham D R, Wolcott R G, Boyer P D
Proc Natl Acad Sci U S A. 1975 Jul;72(7):2592-6. doi: 10.1073/pnas.72.7.2592.
When ATP binds to myosin in the presence of Mg2+ there follows a rapid cleavage reaction to yield a myosin-product complex whose breakdown is rate-limiting in the overall adenosine triphosphatase reaction at 21 degrees and pH 8.0. Recent kinetic studies on this system have led to the proposal that the cleavage of ATP bound to myosin is reversible. This conclusion is based in part on the observation that when ATP is mixed with an excess of myosin active sites a small amount of tightly bound ATP exists whose life-time coincides with that of the myosin-product complex and implies these two species are in equilibrium during their decay. Previous oxygen exchange studies have shown that phosphate released as free product contains more than one oxygen atom from water. A rapid equilibration between myosin-bound ATP and a myosin-products complex can account for the extra water oxygen incorporation of the product phosphate. Such a model requires that the gamma-phosphoryl group of the bound ATP also exchanges its oxygen atoms with water. Results presented in this paper show that protein-bound ATP labeled in the three terminal oxygen atoms of the gamma-phosphoryl group with 18O exchanges about 75% of its label within 2 s of binding to the active site of myosin. This result provides chemical evidence for a model in which bound ATP undergoes a reversible reaction with water. Incomplete exchange may arise from kinetic and/or structural restraints on the mechanism and plausible models are discussed.
当ATP在Mg2+存在的情况下与肌球蛋白结合时，会发生快速的裂解反应，生成肌球蛋白 - 产物复合物，在21摄氏度和pH 8.0的条件下，该复合物的分解是整个三磷酸腺苷酶反应的限速步骤。最近对该系统的动力学研究提出，与肌球蛋白结合的ATP的裂解是可逆的。这一结论部分基于以下观察结果：当ATP与过量的肌球蛋白活性位点混合时，存在少量紧密结合的ATP，其寿命与肌球蛋白 - 产物复合物的寿命一致，这意味着这两种物质在衰变过程中处于平衡状态。先前的氧交换研究表明，作为游离产物释放的磷酸盐含有来自水的不止一个氧原子。肌球蛋白结合的ATP与肌球蛋白 - 产物复合物之间的快速平衡可以解释产物磷酸盐中额外的水氧掺入。这样的模型要求结合的ATP的γ - 磷酸基团也与水交换其氧原子。本文给出的结果表明，在γ - 磷酸基团的三个末端氧原子用18O标记的蛋白质结合的ATP，在与肌球蛋白活性位点结合的2秒内，约75%的标记发生交换。这一结果为结合的ATP与水发生可逆反应的模型提供了化学证据。不完全交换可能源于该机制的动力学和/或结构限制，并对合理的模型进行了讨论。
