Dynamics of the conformational ensemble of partially folded bovine pancreatic trypsin inhibitor.

A single-disulfide variant of bovine pancreatic trypsin inhibitor (BPTI), [14-38]Abu, is a partially folded ensemble which includes two, and in one case three, conformations that interconvert slowly enough to exhibit separate cross-peaks in the amide region of homonuclear and heteronuclear NMR spectra. Each conformation is itself composed of many subconformations in rapid equilibrium. Partially folded BPTI undergoes local motions that are slow, noncooperative, independent fluctuations of short segments within the chain. Cooperative global unfolding of the ensemble is also observed. Heteronuclear NMR has been used to measure interconversion rate constants of partially folded conformational substates; the rate constants differ for each residue and vary over an order of magnitude. For local fluctuation, the forward rate constants for amide protons of the antiparallel beta-sheet are significantly smaller than the rest of the molecule, consistent with other indications that this is the most stable part of the partially folded protein. The reverse rate constants also vary; they are the highest for Ala 27 in the turn between the strands in the sheet and for Phe 33 in the antiparallel beta-sheet. Global unfolding interconversion rate constants vary over a 3-fold range, consistent with previously observed deviations from two-state behavior. Fast backbone dynamics, from T1, T2, and NOE relaxation parameters, are obtained for the slowly interchanging conformations in the partially folded ensemble. Clear differences are observed between the two conformations; one is more flexible and less compact than the other. In the more flexible and disordered partially folded conformation, intermediate exchange is detected for some backbone amides, namely, those in the central beta-sheet and the turn. These same sheet and turn residues are more ordered in the globally denatured ensemble as well. Our results suggest that the turn initiates formation of a partially folded ensemble in which the slow-exchange core is the most stable region and in which segmental fluctuations reflect multiple nuclei for folding of the rest of the molecule.