Robustness of the Long-Range Structure in Denatured Staphylococcal Nuclease to Changes in Amino Acid Sequence

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Abstract
A nativelike low-resolution structure has been shown to persist in the Δ131Δ denatured fragment of staphylococcal nuclease, even in the presence of 8 M urea. In this report, the physical−chemical basis of this structure is addressed by monitoring changes in structure reflected in residual dipolar couplings and diffusion coefficients as a function of changes in amino acid sequence. Ten large hydrophobic residues, previously shown to play dominant roles in the stability of the native state, are replaced with polar residues of similar shape. Modest increases in the Stokes radius determined by NMR methods result from replacement of five isoleucine/valine residues with threonine, one leucine with glutamine, and oxidation of four methionines to the sulfoxides. Yet in the presence of all ten hydrophobic to polar substitutions and 8 M urea, the NMR signature of a native−like topology is still largely intact. In addition, removal of 30 residues from either the N-terminus (which deletes a three-strand β meander) or C-terminus (a long extended segment and the final α helix) produces only very small changes in long-range structure. These data indicate that both the general shape of the denatured state and the angular relationships of individual bond angles to the axes describing the spatial distribution of the protein chain are insensitive to large changes in the amino acid sequence, a finding consistent with the conclusion that the long-range structure of denatured proteins is encoded primarily by local steric interactions between side chains and the polypeptide backbone.