Unfolding Process of the Secondary Structure in the Acid Denaturation of Streptomyces Subtilisin Inhibitor

Acid denaturation of Streptomyces subtilisin inhibitor was studied with CD and absorption spectroscopies. Difference CD spectra in the far-UV region showed that the α2-helix located at the peripheral region unfolds in the first step and the β-sheet located at the central region unfolds in the second step.The fractional contribution of step I and step II to the total difference CD were 21% and 79%, respectively. The α2-helix is considered to be as the most labile structure in SSI. The complete transformation of local structure around aromatic residues at pH 2 were detected by difference absorption, CD and fourth-derivative absorption spectra in the near-UV range. An apparent difference in the average number of protons bound per protein molecule during step II was determined to be 3.4±0.2.