The H/D-Exchange Kinetics of the Escherichia coli Co-Chaperonin GroES Studied by 2D NMR and DMSO-Quenched Exchange Methods

We studied hydrogen/deuterium-exchange reactions of peptide amide protons of GroES using two different techniques: (1) two-dimensional 1H–15N transverse-optimized NMR spectroscopy and (2) the dimethylsulfoxide-quenched hydrogen-exchange method combined with conventional 1H–15N heteronuclear single quantum coherence spectroscopy. By using these techniques together with direct heteronuclear single quantum coherence experiments, we quantitatively evaluated the exchange rates for 33 out of the 94 peptide amide protons of GroES and their protection factors, and for the remaining 61 residues, we obtained the lower limits of the exchange rates. The protection factors of the most highly protected amide protons were on the order of 106–107, and the values were comparable in magnitude to those observed in typical small globular proteins, but the number of the highly protected amide protons with a protection factor larger than 106 was only 10, significantly smaller than the numbers reported for the small globular proteins, indicating that significant portions of free heptameric GroES are flexible and natively unfolded. The highly protected amino acid residues with a protection factor larger than 105 were mainly located in three β-strands that form the hydrophobic core of GroES, while the residues in a mobile loop (residues 17–34) were not highly protected. The protection factors of the most highly protected amide protons were orders of magnitude larger than the value expected from the equilibrium unfolding parameters previously reported, strongly suggesting that the equilibrium unfolding of GroES is more complicated than a simple two-state or three-state mechanism and may involve more than a single intermediate.