Solvent dielectric effects on protein dynamics.

Electron paramagnetic resonance (EPR) spectroscopy and molecular dynamics (MD) simulations were used to investigate the dynamics of alpha-chymotrypsin in solvents ranging in dielectric constant from 72 to 1.9. EPR measurements showed that motions in the vicinity of two spin-labeled amino acids (Met-192 and Ser-195) decreased dramatically with decreasing solvent dielectric constant, a trend consistent with changes in the electrostatic force between charged residues of the protein. EPR results and MD simulations revealed a very similar functional dependence between rates of motion in the protein and the dielectric constant of the bulk solvent; however, predicted motions of protein atoms were markedly faster than measured motions of the spin labels. MD calculations for dielectric constants of 5 and 72 showed the greatest differences near the outer surface of the protein. In general, at the lower dielectric constant many atoms of the protein move more slowly, and many of the slowest residues are near the exterior. These results suggest that altered dynamics may contribute to the unusual properties--e.g., modified stereoselectivities--of enzymes in nearly dry organic solvents. IMAGES:

本研究采用电子顺磁共振（Electron Paramagnetic Resonance, EPR）波谱学与分子动力学（Molecular Dynamics, MD）模拟，对介电常数覆盖72至1.9范围的溶剂体系中α-胰凝乳蛋白酶的动力学行为展开探究。EPR测量结果表明，两处自旋标记氨基酸（Met-192与Ser-195）邻近区域的分子运动随溶剂介电常数降低而显著减弱，该趋势与蛋白质带电残基间静电力的变化规律一致。EPR实验结果与MD模拟均揭示，蛋白质内部运动速率与本体溶剂介电常数之间存在高度相似的函数依赖关系；然而模拟预测的蛋白质原子运动速率显著快于自旋标记位点的实测运动速率。针对介电常数为5与72的体系开展的MD计算显示，二者差异最显著的区域位于蛋白质外表面附近。总体而言，在较低介电常数环境中，蛋白质的多数原子运动更为迟缓，且运动最慢的诸多残基多分布于蛋白外表面附近。上述结果表明，动力学特性的改变可能是近乎干燥有机溶剂中酶类展现出异常特性（例如立体选择性改变）的潜在诱因之一。IMAGES: