Semina T K, Petushkova E V
Biokhimiia. 1977 May;42(5):934-9.
Study of pH-dependence of Ca-ATPase activity of heavy meromyosin (HMM) at low and high ionic strength showed essential differences in the modifying effect of two sulfhydryl reagents, p-CMB and silver. Silver ions in conditions studied independently on pH and KCl concentration produce an inhibition of ATP hydrolysis by myosin and HMM, the shape of the pH-dependence curve remaining similar to that of the native enzyme up to 40% of blocking free sulfhydryl groups. At the same degree of binding of sulfhydryl groups with p-CMB at 0,5 M KCl the pH-dependence curve due to activation at neutral pH changes it's shape and becomes similar to that for dissociation of two ionizable groups (at neutral and alkaline regions). In contrast to this, a low or zero concentrations of KCl no activation was observed for the enzyme with 40-50% of SH-Groups modified by p-CMB and Ca-ATPase in this case seemed to be independent of pH. The data obtained suggest that SH-Groups are not included into the active site of myosin, and the activating effect observed for some sulfhydryl reagents, is due to conformational changes and it can be the result of the penetrance of the organic part of the reagent molecule into hydrophobic region of the protein.
在低离子强度和高离子强度下对重酶解肌球蛋白（HMM）的钙 - ATP酶活性的pH依赖性研究表明，两种巯基试剂对氯汞苯甲酸（p - CMB）和银的修饰作用存在本质差异。在独立于pH值和氯化钾浓度的研究条件下，银离子会抑制肌球蛋白和HMM的ATP水解，在高达40%的游离巯基被封闭之前，pH依赖性曲线的形状仍与天然酶相似。在0.5 M氯化钾条件下，当巯基与对氯汞苯甲酸的结合程度相同时，由于在中性pH下的激活作用，pH依赖性曲线改变了其形状，并变得类似于两个可电离基团解离时的曲线（在中性和碱性区域）。与此相反，在低或零浓度的氯化钾条件下，对于40 - 50%的巯基被对氯汞苯甲酸修饰的酶，未观察到激活作用，此时钙 - ATP酶似乎与pH无关。所获得的数据表明，巯基不包含在肌球蛋白的活性位点中，一些巯基试剂观察到的激活作用是由于构象变化，并且这可能是试剂分子的有机部分渗透到蛋白质疏水区域的结果。
