Instituto de Nanosistemas , Universidad Nacional de General San Martín , Avenida 25 de Mayo y Francia , 1650 San Martín , Argentina.
Langmuir. 2018 Dec 26;34(51):15727-15738. doi: 10.1021/acs.langmuir.8b03411. Epub 2018 Dec 12.
The mechanisms of electrostatically driven adsorption of proteins on charged surfaces are studied with a new theoretical framework. The acid-base behavior, charge distribution, and electrostatic contributions to the thermodynamic properties of the proteins are modeled in the presence of a charged surface. The method is validated against experimental titration curves and apparent p Ks. The theory predicts that electrostatic interactions favor the adsorption of proteins at their isoelectric points on charged surfaces despite the fact that the protein has no net charge in solution. Two known mechanisms explain adsorption under these conditions: (i) charge regulation (the charge of the protein changes due to the presence of the surface) and (ii) charge patches (the protein orients to place charged amino acids near opposite surface charges). This work shows that both mechanisms contribute to adsorption at low ionic strengths, whereas only the charge-patch mechanism operates at high ionic strength. Interestingly, the contribution of charge regulation is insensitive to protein orientation under all conditions, which validates the use of constant-charge simulations to determine the most stable orientation of adsorbed proteins. The present study also shows that the charged surface can induce large shifts in the apparent p Ks of individual amino acids in adsorbed proteins. Our conclusions are valid for all proteins studied in this work (lysozyme, α-amylase, ribonuclease A, and β-lactoglobulin), as well as for proteins that are not isoelectric but have instead a net charge in solution of the same sign as the surface charge, i.e. the problem of protein adsorption on the "wrong side" of the isoelectric point.
采用新的理论框架研究了带电荷表面上静电驱动蛋白质吸附的机制。在带电荷表面存在的情况下，对蛋白质的酸碱行为、电荷分布和静电对热力学性质的贡献进行建模。该方法通过实验滴定曲线和表观 pK 值进行验证。理论预测，尽管蛋白质在溶液中没有净电荷，但静电相互作用有利于蛋白质在其等电点处吸附在带电荷的表面上。有两种已知的机制可以解释在这些条件下的吸附：（i）电荷调节（由于表面的存在，蛋白质的电荷发生变化）和（ii）电荷斑（蛋白质定向使带电荷的氨基酸靠近相反的表面电荷）。这项工作表明，这两种机制都有助于在低离子强度下的吸附，而只有电荷斑机制在高离子强度下起作用。有趣的是，在所有条件下，电荷调节的贡献对蛋白质的取向不敏感，这验证了使用恒电荷模拟来确定吸附蛋白质的最稳定取向的有效性。本研究还表明，带电荷的表面可以在吸附蛋白质中的单个氨基酸的表观 pK 值上产生大的偏移。我们的结论适用于这项工作中研究的所有蛋白质（溶菌酶、α-淀粉酶、核糖核酸酶 A 和β-乳球蛋白），以及那些不是等电点但在溶液中有与表面电荷相同符号的净电荷的蛋白质，即蛋白质在等电点“错误一侧”的吸附问题。
