Electrostatic screening and backbone preferences of amino acid residues in urea-denatured ubiquitin

Local structures in denatured proteins may be important in guiding a polypeptide chain during the folding and misfolding processes. Existence of local structures in chemically denatured proteins is a highly controversial issue. NMR parameters [coupling constants (3)J(H-alpha, H-N) and chemical shifts] of chemically denatured proteins in general deviate little from their values in small peptides. These peptides were presumed to be completely unstructured; therefore, it was considered that chemically denatured proteins are random coils. But recent experimental studies show that small peptides adopt relatively stable structures in aqueous solutions. Small deviations of the NMR parameters from their values in small peptides may thus actually indicate the existence of local structures in chemically denatured proteins. Using NMR data and theoretical predictions we show here that fluctuating beta-strands exist in urea-denatured ubiquitin (8 M urea at pH 2). Residues in such beta-strands populate more frequently the left side of the broad beta region of phi-psi space. Urea-denatured ubiquitin contains no detectable beta-sheet secondary structures; nevertheless, the fluctuating beta-strands in urea-denatured ubiquitin coincide to the beta-strands in the native state. Formation of beta-strands is in accord with the electrostatic screening model of unfolded proteins. The free energy of a residue in an unfolded protein is in this model determined by the local backbone electrostatics and its screening by backbone solvation. These energy terms introduce strong electrostatic coupling between neighboring residues, which causes cooperative formation of beta-strands in denatured proteins. We propose that fluctuating beta-strands in denatured proteins may serve as initiation sites to form fibrils.