Folding of the disulfide-bonded beta-sheet protein tendamistat: rapid two-state folding without hydrophobic collapse

We investigated the reversible folding and unfolding reactions of the small 74 amino acid residue protein tendamistat. The secondary structure of tendamistat contains only beta-sheets and loop regions and the protein contains two disulfide bonds. Fluorescence-detected refolding kinetics of tendamistat (disulfide bonds intact) comprise of a major rapid fast reaction (tau = 10 ms in water) and two minor slow reactions. In the fast reaction 80% of the unfolded molecules are converted to native protein. The two slow reactions are part of a parallel slow folding pathway. On this pathway the rate-limiting step in the formation of native molecules is cis to trans isomerization of at least one of the three trans Xaa-Pro peptide bonds. This reaction is catalyzed efficiently by the enzyme peptidyl-prolyl cis-trans isomerase. Comparison of kinetic data with equilibrium unfolding transitions shows that the fast folding pathway follows a two-state process without populated intermediate states. Additionally, various sensitive tests did not detect any rapid chain collapse during tendamistat folding prior to the acquisition of the native three-dimensional structure. These results show that pre-formed disulfide bonds do not prevent efficient and rapid protein folding.