Lobyshev V I, Vishnevskaia Z I
Biofizika. 1992 Sep-Oct;37(5):920-2.
The kinetic isotope effect of hydrolysis of ATP by myosin subfragment-I in the presence of K+, NH4+, Rb+ was measured. VH/VD was found to be 1.8; 1.3; 2.0, respectively. According to the thermodynamic isotope effect induced by hydration, the kinetic isotope effect must increase with the increase of cation size from K+ to Rb+. The size of ammonium ions is the intermediate between K+ and Rb+, but the observed isotope effect in the presence of ammonium is much lower than in the presence of K+ and Rb+. The results suggest that monovalent cations occur near the active center of the enzyme and contribute to some extent to the hydrolysis reaction but the specificity of ammonium ions seems not to be due to its ideal steric accordance. The obtained results support the viewpoint that NH4+ ions as donor of protons participate in the chemical stage of ATP hydrolysis by subfragment-I.
在存在K⁺、NH₄⁺、Rb⁺的情况下，测定了肌球蛋白亚片段-I对ATP水解的动力学同位素效应。发现VH/VD分别为1.8；1.3；2.0。根据水合作用引起的热力学同位素效应，动力学同位素效应应随着阳离子尺寸从K⁺到Rb⁺的增加而增加。铵离子的尺寸介于K⁺和Rb⁺之间，但在存在铵的情况下观察到的同位素效应远低于存在K⁺和Rb⁺时的情况。结果表明，单价阳离子出现在酶的活性中心附近，并在一定程度上促进水解反应，但铵离子的特异性似乎并非源于其理想的空间契合。所得结果支持了NH₄⁺离子作为质子供体参与亚片段-I对ATP水解的化学阶段的观点。
