The stabilizing effects of hydrophobic cores on peptide folding of bovine-pancreatic-trypsin-inhibitor folding-intermediate model.

A synthetic peptide model composed of alpha-helical and beta-sheet portions (P alpha P beta) is a crucial folding intermediate in the folding of bovine pancreatic trypsin inhibitor (BPTI), which contains 30 amino acid residues, and provides a good model for studying the folding structure. Using this model the roles of hydrophobic cores, such as non-polar amino acids and a short beta strand, in the folding structure stability were investigated by deleting the hydrophobic amino acids or substituting with Ala. As a first step, a mutant peptide, P alpha 6 P beta 1, was made by removing the four residues (NNFK46) from the N-terminus of P alpha which make a short beta strand in native BPTI, and each of the hydrophobic cores, Phe45 of P alpha and Tyr21 of P beta, were substituted by Ala. Without the short beta strand the peptide still folds in spite of its low solubility, suggesting that a simple alpha helix and central antiparallel beta sheet contain sufficient information to direct the folding of this region of molecule. The peptide without Tyr21 was much more unstable than P alpha 6 P beta 1, such that most of the folding structure collapsed even at 0 degree C, whereas without Phe45 the structure was more stable than P alpha 6 P beta 1. This indicates that the hydrophobic interactions of Tyr21 in P beta are more important in BPTI folding.