Effects of negative charges of a model for bovine pancreatic trypsin inhibitor folding intermediate on the peptide folding.

A peptide model (called P alpha P beta) of bovine pancreatic trypsin inhibitor (BPTI) for studying the peptide folding structure was designed as a crucial folding intermediate. The P alpha P beta model folded but it was unstable at low pH. Four acidic groups in P alpha P beta: Glu-49, Asp-50, and C-termini at Phe-33 and Ala-58, were replaced individually with an Ala (in case of Glu and Asp) and an amide group (for C-terminal carboxyl groups) to study the effects of these negative charged groups on folding structure in terms of thermal stability and pH dependence. Substituting the Glu-49 or Asp-50 with Ala and blocking the C-terminus carboxyl group of Phe-33 in P beta destabilized the structure, but blocking the negative charge of C-terminus in Ala-58 of P alpha stabilized the structure at neutral pH. These results can be interpreted in terms of helix dipole momentum effects and/or salt bridge formation between Asp-50 and Arg-53, and of electrostatic interaction between positive and negative charges, which stabilized the structure. The melting temperature of model peptides (Tm) in low ionic strength buffer were lower than that in high ionic strength buffer except the C-terminus blocking of P alpha.