A pentapeptide model for an early folding step in the refolding of staphylococcal nuclease: The role of its turn propensity

Recently the folding of a staphylococcal nuclease (P117G) variant was examined with the hydrogen-deuterium (H-D) exchange technique. Many of the residues that showed significant protection are located in protection are located in β-sheet regions. About half the residues protected belong to an antiparallel β-hairpin structure (residues 21–35) in the native structure. The β-hairpin structure is formed by strands 2 and 3 of sheet 2 connected by the sequence²⁷ Y KGQP³¹ in a type I′ reverse turn conformation with a 4 → 1 hydrogen bonding between Q30 NH and Y27 C=O. We have targeted the conformational characterization of the peptide model Ac-YKGQP-NH₂ with ¹II two-dimensional nmr techniques in aqueous solution with a view to assessing its propensity to sample turn conformational forms and thus initiate the formation of β-hairpin structure. Based upon the observed dαn (i, i + 1), dαn (i, i + 3), and dnn (i, i + 1) nuclear Overhauser effect connectivities, temperature coefficients for amide protons and conformational analysis with quantum mechanical perturbative configuration interaction over localized orbitals method, we conclude that the model peptide samples turn conformational forms with reduced conformational entropy. We suggest that the turn can nucleate the formation of the β-hairpin structure in the refolding of nuclease. Observation of turn propensity for this sequence is consistent with the folding mechanism of the Greek key motif (present in Staphylococcal nuclease) proposed in the literature. © 1997 John Wiley & Sons, Inc.