Secondary structure of soybean trypsin inhibitor (Kunitz): a study by infrared spectroscopy

The infrared spectrum (amide I' region) of Kunitz soybean trypsin inhibitor (SBTI) was obtained in D2O solution and resolved into Gaussian components. A prominent broad band centered at 1643 cm-1 is shown on the unresolved spectrum, which is usually assigned to N-deuterated peptide groups in an unordered structure, since SBTI is known to be devoid of alpha-helix by CD and X-ray crystallographic studies. In addition, shoulders are evident at 1632 cm-1 and 1676 cm-1, which correspond probably to the v(pi, O) and v(O, pi) components assigned to an antiparallel-chain beta-pleated sheet structure. Parameters (maximum absorptivity, wavenumber at the maximum of the band, and half-width of the band at half-height) for the four Gaussian component bands (in which the amide I' band was resolved) are given. A crude estimation of 4% is obtained for antiparallel beta-sheet in SBTI, i.e., this protein would be practically devoid of such a beta-structure. Notwithstanding the fact that this result is apparently in agreement with the far-UV CD spectrum (data reported in the literature), the predominant conformation class found in SBTI has been demonstrated to be approximate beta-sheet structures, with a small amount of regular sheet (Sweet et al., (1974) Biochemistry 13: 4212-4228).