Urea-Mediated Protein Denaturation: A Consensus View

We have performed all-atom molecular dynamics simulations of three structurally similar small globular proteins in 8 M urea and compared the results with pure aqueous simulations. Protein denaturation is preceded by an initial loss of water from the first solvation shell and consequent in-flow of urea toward the protein. Urea reaches the first solvation shell of the protein mainly due to electrostatic interaction with a considerable contribution coming from the dispersion interaction. Urea shifts the equilibrium from the native to denatured ensemble by making the protein-protein contact less stable than protein-urea contact, which is just the reverse of the condition in pure water, where protein-protein contact is more stable than protein-water contact. We have also seen that water follows urea and reaches the protein interior at later stages of denaturation, while urea preferentially and efficiently solvates different parts of the protein. Solvation of the protein backbone via hydrogen bonding, favorable electrostatic interaction with hydrophilic residues, and dispersion interaction with hydrophobic residues are the key steps through which urea intrudes the core of the protein and denatures it. Why urea is preferred over water for binding to the protein backbone and how urea orients itself toward the protein backbone have been identified comprehensively. All the key components of intermolecular forces are found to play a significant part in urea-induced protein denaturation and also toward the stability of the denatured state ensemble. Changes in water network/structure and dynamical properties and higher degree of solvation of the hydrophobic residues validate the presence of "indirect mechanism" along with the "direct mechanism" and reinforce the effect of urea on protein.