Battistuzzi G, Borsari M, Ranieri A, Sola M
Department of Chemistry, University of Modena and Reggio Emilia, Italy.
Arch Biochem Biophys. 2001 Feb 1;386(1):117-22. doi: 10.1006/abbi.2000.2183.
The thermodynamic parameters of the alkaline transition of beef heart ferricytochrome c have been measured through direct electrochemistry experiments carried out at variable pH and temperature in the presence of different sulfate concentrations. Sulfate is known to bind specifically to cytochrome c in a sequential manner at two surface sites. The effects of such a specific binding reflect on the thermodynamics of the transition and can be satisfactorily interpreted within the frame of the Debye-Hückel theory with simple electrostatic considerations. In particular, the increase in the thermodynamic pKa values (extrapolated to I = 0) upon sulfate binding turns out to be a fully enthalpic effect which can be accounted for by considering the coulombic effects of the formation of ionic couple(s) on the protein surface. This study also shows that the apparent pKa values at finite ionic strength are only moderately affected by the nature of the anion in solution, and differences tend to vanish at high ionic strength.
通过在不同硫酸盐浓度下，于可变pH和温度条件下进行的直接电化学实验，测定了牛心铁细胞色素c碱性转变的热力学参数。已知硫酸盐会以顺序方式在两个表面位点特异性结合到细胞色素c上。这种特异性结合的影响反映在转变的热力学上，并且可以在德拜-休克尔理论的框架内，通过简单的静电考虑得到令人满意的解释。特别是，硫酸盐结合后热力学pKa值（外推至I = 0）的增加结果是一种完全的焓效应，这可以通过考虑蛋白质表面离子对形成的库仑效应来解释。该研究还表明，在有限离子强度下的表观pKa值仅受到溶液中阴离子性质的适度影响，并且在高离子强度下差异趋于消失。
