Acid denaturation of
Streptomyces subtilisin inhibitor was studied with CD and absorption spectroscopies. Difference CD spectra in the far-UV region showed that the α
2-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 α
2-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.
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