Anatomy of energetic changes accompanying urea-induced protein denaturation.
Because of its protein-denaturing ability, urea has played a pivotal role in the experimental and conceptual understanding of protein folding and unfolding. The measure of urea's ability to force a protein to unfold is given by the m value, an experimental quantity giving the free energy change for unfolding per molar urea. With the aid of Tanford's transfer model [Tanford C (1964) J Am Chem Soc 86:2050-2059], we use newly obtained group transfer free energies (GTFEs) of protein side-chain and backbone units from water to 1 M urea to account for the m value of urea, and the method reveals the anatomy of protein denaturation in terms of residue-level free energy contributions of groups newly exposed on denaturation. The GTFEs were obtained by accounting for solubility and activity coefficient ratios accompanying the transfer of glycine from water to 1 M urea. Contrary to the opinions of some researchers, the GTFEs show that urea does not denature proteins through favorable interactions with nonpolar side chains; what drives urea-induced protein unfolding is the large favorable interaction of urea with the peptide backbone. Although the m value is said to be proportional to surface area newly exposed on denaturation, only approximately 25% of the area favorably contributes to unfolding (because of newly exposed backbone units), with approximately 75% modestly opposing urea-induced denaturation (originating from side-chain exposure). Use of the transfer model and newly determined GTFEs achieves the long-sought goal of predicting urea-dependent cooperative protein unfolding energetics at the level of individual amino acid residues.
Auton
Matthew
M
Department of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, TX 77555-1052, USA.
Holthauzen
Luis Marcelo F
LM
Bolen
D Wayne
DW
D000596:Amino Acids / Q000737:chemistry; D001671:Biochemistry / Q000379:methods; D005998:Glycine / Q000737:chemistry; D008956:Models, Chemical; D008962:Models, Theoretical; D010455:Peptides / Q000737:chemistry; D011489:Protein Denaturation; D011506:Proteins / Q000737:chemistry; D012995:Solubility; D012997:Solvents / Q000737:chemistry; D013816:Thermodynamics; D014508:Urea / Q000737:chemistry*
D000596:Amino Acids; D010455:Peptides; D011506:Proteins; D012997:Solvents; D014508:Urea; D005998:Glycine