The properties of buried ion pairs are governed by the propensity of proteins to reorganize

Abstract

Charges are incompatible with the hydrophobic interior of proteins, yet proteins use buried charges, often in pairs or networks, to drive energy transduction processes, catalysis, pH-sensing, and ion transport. The structural adaptations necessary to accommodate interacting charges in the protein interior are not well understood. According to continuum electrostatic calculations, the Coulomb interaction between two buried charges cannot offset the highly unfavorable penalty of dehydrating two charges. This was investigated experimentally with two variants of staphylococcal nuclease (SNase) with Glu:Lys or Lys:Glu pairs introduce at internal i, i+4 positions on an -helix. Contrary to expectations from previous theoretical and experimental studies, the proteins tolerated the charged ion pairs in both orientations. Crystal structures and NMR spectroscopy studies showed that in both variants, side chains or backbone are reorganized. This leads to the exposure of at least one of the two buried groups to water. Comparison of these ion pairs with a highly stable buried ion pair in SNase shows that the location and the amplitude of structural reorganization can vary dramatically between ion pairs buried in the same general region of the protein. The propensity of the protein to populate alternative conformation states in which internal charges can contact water appears to be the factor that governs the magnitude of electrostatic effects in hydrophobic environments. The net effect of structural reorganization is to weaken the Coulomb interactions between charge pairs; however, the reorganized protein no longer has to pay the energetic penalty for burying charges. These results provide the framework necessary to understand the interplay between the dehydration of charges, Coulomb interactions and protein reorganization that tunes the functional properties of proteins.