In this work we study the coupling between ionization and conformational properties of two IDPs, histatin-5 and β-amyloid 42, in the presence of neutral and charged crowders. The latter is modeled to resemble bovine serum albumin (BSA). With this aim, semi-grand canonical Monte Carlo simulations are performed, so that the IDP charge is a dynamic property, undergoing protonation/deprotonation processes. Both ionization properties (global and specific amino acid charge and binding capacitance) and radius of gyration are analyzed in a large range of pH values and salt concentrations. Without crowder agents, the titration curve of histatin-5, a polycation, is salt-dependent while that of β-amyloid 42, a polyampholyte, is almost unaffected. The salt concentration is found to be particularly relevant at pH values where the protein binding capacitance (directly linked with charge fluctuation) is larger. Upon addition of neutral crowders, charge regulation is observed in histatin-5, while for β-amyloid 42 this effect is very small. The main mechanism for charge regulation is found to be the effective increase in the ionic strength due to the excluded volume. In the presence of charged crowders, a significant increase in the charge of both IDPs is observed in almost all the pH range. In this case, the IDP charge is altered not only by the increase in the effective ionic strength but also by its direct electrostatic interaction with the charged crowders.

中文翻译：

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大分子拥挤对内在无序蛋白电荷调节的影响

在这项工作中，我们研究了在存在中性和带电拥挤剂的情况下，两种IDP（组蛋白5和β-淀粉样蛋白42）的电离与构象性质之间的耦合。后者被建模为类似于牛血清白蛋白（BSA）。出于这个目的，执行了半盛大的经典蒙特卡洛模拟，以使IDP装料具有动力学特性，并经历了质子化/去质子化过程。在较大的pH值和盐浓度范围内，都可以分析电离特性（全局和特定的氨基酸电荷和结合电容）和回转半径。没有拥挤剂，聚阳离子组蛋白5的滴定曲线是盐依赖性的，而聚两性电解质β-淀粉样42的滴定曲线几乎不受影响。发现在蛋白质结合电容（与电荷波动直接相关）较大的pH值下，盐浓度尤为重要。加入中性拥挤剂后，在histatin-5中观察到电荷调节，而对于β-淀粉样蛋白42，此作用很小。发现电荷调节的主要机理是由于排除体积而有效提高了离子强度。在带电拥挤剂的存在下，几乎在所有pH范围内，两种IDP的电荷都显着增加。在这种情况下，IDP电荷不仅通过有效离子强度的增加而改变，还通过其与带电拥挤者的直接静电相互作用而改变。而对于β-淀粉样蛋白42，这种作用很小。发现电荷调节的主要机理是由于排除体积而有效提高了离子强度。在带电拥挤剂的存在下，几乎在所有pH范围内，两种IDP的电荷都显着增加。在这种情况下，IDP电荷不仅通过有效离子强度的增加而改变，还通过其与带电拥挤者的直接静电相互作用而改变。而对于β-淀粉样蛋白42，这种作用很小。发现电荷调节的主要机理是由于排除体积而有效提高了离子强度。在带电拥挤剂的存在下，几乎在所有pH范围内，两种IDP的电荷都显着增加。在这种情况下，IDP电荷不仅通过有效离子强度的增加而改变，还通过其与带电拥挤者的直接静电相互作用而改变。