Environment matters : the impact of urea and macromolecular crowding on proteins

Abstract

This work aims to analytically understand the impact of two diametric opposite environments on protein structure and dynamics and compared them to the most common solvent on earth: water. The first environment is a popular denaturing solution (urea 8M), which has served for years in protein-science laboratories to investigate protein stability; still many open questions regarding its mechanism of action remained unclear. The second environment instead moves towards a more physiological representation of proteins. The cell interior, in fact, is a crowded solution highly populated prevalently by proteins, but studies on protein structure and dynamics have lead so far to confusing or even opposite observations.

The lack of a consensus view in both phenomena possibly derives from the bias of the system under study. This work is an attempt of a comparative study using the most general systems: a diverse spectrum of proteins folds, different stages along the reaction path (early stages or end-point) and/or different protein force-fields. Our main objective was to derive common pattern and general rules valid at proteome level, focusing on three major aspects of proteins: the structure, the dynamic and the interactions with the solvent molecules. Molecular dynamics simulation appeared then as the most suitable tool because of its ability to i) analyze proteins at broad range of resolutions; ii) access the direct time-resolved dynamic of the system and iii) dissect the specific interactions that arise in the new settings.

Specifically, the case of urea-induced unfolding needs a system for which is possible to clearly identify folded and unfolded state – globular proteins are then the most suitable ones. We extracted general rules on the folded/unfolded transition by studying independently the two end-points of folded/unfolded reaction. We simulated the urea-induced unfolded state of a model protein, ubiquitin to understand the energetics stabilizing unfolded structures in urea. We found that the unfolded ubiquitin in 8M urea is fully extend and flexible and capturing efficiently urea molecules to the first solvation shell. Dispersion, rather than electrostatic, appear the main energetic contribution to explain the stabilization of the unfolded state.

We then simulated the early stages of urea-induced unfolding on a large dataset of folded proteins, which represent the major folds of globular proteins, aiming also to investigate the kinetic role of urea in triggering the protein unfolding. We found that partially unfolded proteins expose the apolar residues buried in the protein interior, mainly via cavitation. Similar to the unfolded state, it is the dispersion interactions that drive urea accumulation in the solvation shell but here urea molecules take advantage of microscopic unfolding events to penetrate the protein interior.

Macromolecular crowding instead is a phenomenon that universally affects all the proteins. We simulated a system that included as crowding agents proteins with different conformational landscapes (a globular protein, an intrinsically disordered proteins and a molten globule) arranged to reach cell-like concentrations. We conclude that the universal effect of crowding, valid for all the proteins types, is exerted via the aspecific interactions and favors open and moderately extended conformations with higher secondary structure content. This phenomenon counterbalances the volume-exclusion, which prevails at higher crowding concentrations. The impact of crowding is proportional to the degree of disorder of the protein and for folded protein crowding favors structural rearrangements while unfolded structures experience a stronger stabilization and a higher secondary structures content. The synthetic crowder PEG doesn’t reproduce any of these effects, arising concerns about its employment in study cell-like environments.