Folding of a β-hairpin peptide derived from the N-terminus of ubiquitin: Conformational preferences of β-turn residues dictate non-native β-strand interactions

The role of the non-native β-turn sequence (NPDG) in nucleating the folding of a β-hairpin peptide derived from the N-terminus of ubiquitin, has been examined by NMR and CD spectroscopy. The NPDG sequence, while representing a common two-residue type I turn sequence in proteins, folds to give a G1-bulged type I turn in the context of a β-hairpin peptide, to the exclusion of other possible conformations. The turn conformation results in misalignment of the two β strands and a β hairpin with non-native side chain interactions. A truncated 12-residue analogue of the hairpin, in which the majority of residues in the N-terminal β strand have been deleted, shows some weak propensity to fold into a G-bulged type I turn conformation in the absence of interstrand stabilizing interactions. The NPDG turn sequence pays some of the entropic cost in initiating folding allowing interstrand interactions, which in this case arise from the non-native pairing of residue side chains, to stabilize a significant population of the folded state. Examination of the relative abundance of the Pro-Asp type I turn, with G in the +B1 position, vs. the type I G-bulged turn PXG, in a database of high resolution structures, reveals 48 instances of PXG bulged turns for which X = Asp is by far the most common residue with 20 occurrences. Strikingly, there are no examples of a type I PD turn with G at the +B1 position, in good agreement with our experimental observations that the PDG G-bulged turn is populated preferentially in solution.