Fast collapse but slow formation of secondary structure elements in the refolding transition of E. coli adenylate kinase

The various models proposed for protein folding transition differ in their order of appearance of the basic steps during this process. In this study, steady state and time-resolved dynamic non-radiative excitation energy transfer (FRET and trFRET) combined with site specific labeling experiments were applied in order to characterize the initial transient ensemble of Escherichia coli adenylate kinase (AK) molecules upon shifting conditions from those favoring denaturation to refolding and from folding to denaturing. Three sets of labeled AK mutants were prepared, which were designed to probe the equilibrium and transient distributions of intramolecular segmental end-to-end distances. A 176 residue section (residues 28-203), which spans most of the 214 residue molecule, and two short secondary structure chain segments including an α-helix (residues 169-188) and a predominantly β-strand region (residues 188-203), were labeled. Upon fast change of conditions from denaturing to folding, the end-to-end distance of the 176 residue chain section showed an immediate collapse to a mean value of 26 Å. Under the same conditions, the two short secondary structure elements did not respond to this shift within the first ten milliseconds, and retained the characteristics of a fully unfolded state. Within the first 10 ms after changes of the solvent from folding to denaturing, only minor changes were observed at the local environments of residues 203 and 169. The response of these same local environments to the shift of conditions from denaturing to folding occurred within the dead time of the mixing device. Thus, the response of the CORE domain of AK to fast transfer from folding to unfolding conditions is slow at all three conformational levels that were probed, and for at least a few milliseconds the ensemble of folded molecules is maintained under unfolding conditions. A different order of the changes was observed upon initiation of refolding. The AK molecules undergo fast collapse to an ensemble of compact structures where the local environment of surface probes seems to be native-like but the two labeled secondary structure elements remain unfolded.