Synthetic Approach to Study the Effects of β-Turn Residues on the Structure and Folding of Bovine Pancreatic Trypsin Inhibitor (BPTI)

Abstract

Previous work from our laboratory has established a system for accessing partially folded conformational ensembles of bovine pancreatic trypsin inhibitor (BPTI), based on the replacement of Cys 5, 30, 51, and 55 by α-amino-n-butyric acid (Abu), while retaining the disulfide between Cys 14 and 38 [1]. Several observations in the aforementioned [14–38]_Abu system suggest that the type I β-turn (A²⁵K²⁶A²⁷G²⁸) connecting the two core strands of the antiparallel β-sheet (residues 18–24 and 29–35) is the main site for initiation of folding of BPTI [2], For the present work, two analogues of [14–38]_Abu have been synthesized in which turn residues at positions 26, 27, and 28 were altered to maximize amino acid positional potentials for β-turns [3]. One analogue, P²⁶, D²⁷[14–38]_Abu, has at three positions (including the natural G28) residues with the highest potential for a type I β-turn. In a second analogue, N²⁶,G²⁷,K²⁸[14–38]_Abu, residues with the highest potential for a type I′ β-turn have been introduced. Type I′ β-turns are most common for β-hairpins [4].