A peptide model for proline isomerism in the unfolded state of staphylococcal nuclease

Nuclear magnetic resonance spectroscopy has been used to investigate a synthetic peptide (YVYKPNNTHE) corresponding to residues 113 to 122 of Staphylococcal nuclease. In the major folded state of the protein this region forms a type VI_a β-turn containing a cis Lys116-Pro117 peptide bond. There is, however, no evidence for any significant population of such a turn in the peptide in aqueous solution and the X-Pro bond is predominantly in the trans configuration. The peptide exhibits several well-resolved minor resonances due to the presence of a small fraction (4 ± 2%) of the cis-proline isomer. The ratio of cis to trans isomer populations was found to be independent of temperature between 5 °C and 70 °C, indicating that ΔH for the isomerism is close to zero. Using magnetization transfer techniques the rate of trans to cis interconversion was found to be 0.025(±0.013) s^-1 at 50 °C. The thermodynamics and kinetics of isomerism in the peptide are very similar to those estimated for the Lys116-Pro117 peptide bond in unfolded nuclease, suggesting that the cis-trans equilibrium in the unfolded protein is largely determined by the residues adjacent to Pro117 in the sequence. These results are consistent with previous suggestions that the cis-proline bond is stabilized late in the folding process and that the predominance of the cis form in folded nuclease is due to stabilizing interactions within the protein that give rise to a favourable enthalpy term.