Conformational Change of Delta-chymotrypsin Caused by Sodium Dodecyl Sulfate as Studied by Stopped-flow Circular Dichroic Method

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Abstract
The conformational change of delta-chymotrypsin was studied in a sodium dodecyl sulfate (SDS) solution containing 3.33 mmol/dm3 NaH2PO4 and 3.56 mmol/dm3 Na2HPO4 (pH 7.0, μ=0.014) by means of the circular dichroism (CD) and absorbance stopped-flow methods. The protein adopts a conformation with a higher helix content than the native state upon the addition of SDS. The rate of the conformational change strongly depends on the SDS concentration. The corresponding rate constant sharply increases in the range of 5–12 mmol/dm3 SDS, indicating that the higher the helix content, the faster the coil to α-helix transition. The nature of the conformational change was examined by parallel measurements of the CD, the difference spectra, and the binding isotherm. As a result, it was concluded that, at a certain SDS concentration above 5 mmol/dm3, the native conformation of the protein is directly transformed to another conformation without passing through any intermediate one which may be caused when the SDS concentration is below the one mentioned above.