Initial denaturing conditions influence the slow folding phase of acylphosphatase associated with proline isomerization

Abstract

The folding kinetics of human common-type acylphosphatase (cAcP) from its urea- and TFE-denatured states have been determined by stopped-flow fluorescence techniques. The refolding reaction from the highly unfolded state formed in urea is characterized by double exponential behavior that includes a slow phase associated with isomerism of the Gly53–Pro54 peptide bond. However, this slow phase is absent when refolding is initiated by dilution of the highly α-helical denatured state formed in the presence of 40% trifluoroethanol (TFE). NMR studies of a peptide fragment corresponding to residues Gly53–Gly69 of cAcP indicate that only the native-like trans isomer of the Gly–Pro peptide bond is significantly populated in the presence of TFE, whereas both the cis and trans isomers are found in an ∼1:9 ratio for the peptide bond in aqueous solution. Molecular modeling studies in conjunction with NMR experiments suggest that the trans isomer of the Gly53–Pro54 peptide bond is stabilized in TFE by the formation of a nonnative-like hydrogen bond between the CO group of Gly53 and the NH group of Lys57. These results therefore reveal that a specific nonnative interaction in the denatured state can increase significantly the overall efficiency of refolding.